ABSTRACT

A Review of hydrophyte evapotranspiration

RI. E. CRUNDWELL (1)

,1 literature revierv of fhe relative rate of hydrophyte evapotranspiration (ETh) to open mater eoaporation (EW) is presented. This liferature suggests that the EThf ETV ratio cari exceed unity. Furthermore, the magnitude of the EThfEW ratio is dependent upon the state of grorvth of the hydrophyte, species, climate and density. Mean annual ETh[EIV ratios for various species of hydrophyte according to climate cari be ohtained from tabulation of the relevant literature.

KEY wonos: Evaporation - Evapotranspirat.ion - Hydrophytes - Wetland.

RÉSUMÉ

L'ÉVAPOTRANSPIRATION DES HYDROPHYTES : UNE ANALYSE BIBLIOGRAPHIQUE

Les résultats publies concernant l’importance relative de l’évapotranspiration des hydrophytes (ETh) et de l’évaporation d’un plan d’eau libre (E?V) sont prèsentés et résumés. Il a.pparait que le rapport EThf ECV peut être supérieur à l’unité; ce rapport dépend en outre de la nature des hydrophytes, de leur développement, de la densité de la végétation et du climat.

Les divers rlsultats utilist% permettent de présenter des valeurs annuelles du rapport Ethf E\V en fonction du climat.

&fOTS CLÉS : Evaporation - Evapotranspiration - Hydrophyt.es - Marécages.

Hydrophytes cari be defined as plants of wet habitats (WARMING, 1895). Helophytes are hydro- phytes with at least 1 m of aerial growth. Rlany phreatophytes, plant,s whose roots are permanently in ground water, also cari be termed “hydrophytes”. In t,his review of hydrophyte evapotranspiration hoth helophytes and phreatophytes will be termed “hydrophytes” except in cit,ations where the authors original wording mil1 be used. A classification of the various species of hydrophytes reviewed below is presented in Table 1.

The literature on hydrophyte evapotranspiration is small. Indeed, LINACRE'S (pp. 343-344, 1976)

comment that,: “a thorough search of t,he (English) literature has demonstrated t,he paucity of knowledge on the physics of swamps (...) The process of evaporation from a reed field remains a matter of ignorance” is still appropriate. This “paucity of information” has been att,ribut,ed to the difficulty in carrying out rigourous experiments in a hostile, inaccessible area, that is remote from power supplies or lahoratories and is in nature rough, wet and unstable (INGRAM, 1983).

Within this literature, contrasting results of hydrophyte evapotranspiration bave been reported hy several aut,hors (EISENHLOR, 1966; PRIBAN

(1) Ljeparfmenf or Geoyraphy, lrniurrsify College London, Zti, Bedford Way, London, IVCIH OAP, Greaf Brifafn.

Rev. Hydrobiol. Irop. 19 (3-4): 215-232 (1986).

216 M. E. CRUNDWELL

RIld (.)NDOK, 1980). BLANEI- and YOUNG (I942), ~VCC)ONALD and HUGHES (1968) and SMIU (1975) a11 reported that hydrophyte evapotranspiration (ETh) cc.~uld exceed open wat,er evaporation (EW) untler ident.ical meteorological conditions. Conflicting results were obtained by RIIGAHID (1952), SH.JEFLO (lBG8) and LINACRE Côt al. (1970).

One of t,he aims of t.his review is t,o establish the magnitude of t-he rat.io between hydrop1~yt.e evapo- trenspirat.ion ETh and open water evaporat,ion (EW) ie. fhr ETh/EW rat.io. It is useful t,o know when studying the results of the various workers cited bel& that thr ETh/EW ratio for wheat and barley is 0.13 to 0.ï depending upon stage of growth (DO~HENB~S and PH~JITT, 1975).

In order to facilitate the comprehension of the debat,e surrounding hydrophyte evapotranspiration, this review will be divided int.o three sections:

1) work t,hat. supports t,he hypothesis that hydro- phyte e\-apot,ranspiration can exceed open water eyaporation,

II) work that. support,s the hypothesis t.hat hydrophyte evapotranspiration cannot exceed open water evaporation, and

III) a conrluding section that. evaluates 130th sets of results and at.t.empt.s to explain t,he variat.ion in t,heir conclusions.

1. HYDROPHYTE EVXPOTRANSPIRATION CA4N EXCEED OPEN WATER EVAPORATION

Therr havç: been various methodological approe- chrs to calcu1ot.e hydrophyte evapot.ranspirat.ion. These approaches cari be grouped into four broad headings: lysimeters, Rowen ratio/eddy flux, water balance and det,ached organ. Although sorne workers (GEL'RUKH, 196.4) bave used more t.han one metho- do~ogicd H~JprOaCh, it WHS decided to describe the work on hydrophyt-e evapotranspiration in t,hese four main headings.

Lysimeters

Lgsirnet-ers bave been defined (RODDA, DOWNING and LA~, 1976) as “a container that, is installed so that. its rim is leva1 with t.he ground”. The evapo- transpiration losses from the container are usually calculat~ed hy differences in weight over a set time period. Also detailed in t.his section are tank experi- mants and soi1 monolith experiments. Tank experi- rnents consist, of growing hydrophytes in tanks and noting t,he wat.er needed t.o top up the t,ank t.o a set. level. -Soi1 monoliths are undisturbed blocks of soil, whrre evapotranspirat.ion is meesured by either

not.ing tbe fall/rise of ground water level or hy some weighing t.echniyue. The similarity of the t.hree methods is obvious.

0~1s (1914) was an early worker to study the wat,er loss by hydrophyt,es compared to open water evaporation. He planted hydrophytes in large tanks which had been sunk int,o t.he margins of a lake to simulate nat,ural conditions for evàpotranspirat.ion. He calculated the hydrophyte/open wat,er (ETh/EW) ratio of evapotranspiration for six species: Potainoge- ton ~zoclosns ETh/EW ratio 3.1; Typha lufifolia, 2.5; Acwus cnlarnus, 2.0; Ponteclerin cordata, 1.2; Scirpus validix, 1.9; Nymphaea eclorata, 1 .O. According to 0~1s: “Water loss from the areas covered by normal densities of helophyt.es is several times greater than the evaporation loss from the same area of water.” (O~rs, 1914 citad by fiERN.4TOwIC.Z ?t ul., 1976, p. 276). 0~1s concluded that the cause of this inter- species variation in the ETh/EW rat,io were anato- mica1 structure and differences in wat.er management,.

STEARNY and BRYAN (1925) also grew helophytes in a t.ank set into the margins of Mud Lake, Idaho. They found t.hat. water loss from a tank of Scirprrs aczzlrrs exceeded t-he wat,er loss from a neighbouring pan of water bp as mu~h as two times. They concluded t.hat, (p. IOC)): “The total losses by evapora- t.ion and transpiration from t.he rnarsh and tule (Scirpus acrztrzs)-covered areas are c.onsiderably larger than t,lie loss by evaporation from open water surfaces.”

PRYTZ'S (1932) pioneering work in Jutland showed that. lysimet,er readings of hydrophyt,e evapotranspiration exceeded adjacent sunken pan values of open water evaporation by as much as 26 yo.

A most. comprehensice, rigorous and diut.urnous series of st.udies on tank hydrophyte evapotranspira- tion was initiat,ed by BLANEY et al. (1933). Between 1933 and 1965, BLANEY, in conjunction with various other authors, published several papers on t,he subject of hydrophyte evapotranspiration (BLANEY et cd., 1933; BLANEY et nl., 1938; BLhNEY ef ul., 1942; YOUNG and BLANEY, 1942; MI~CKEL and BLANEY, 1945; BLANEY and EWING, 1946; BLANEY and blUCKEL, 1955; BLANEY, 1959; BLANEY, 1961; BLANEY and HANSON, 1965). Al1 these papers published differing “c.onsumptive use” coefficients, for various hydrophyt.es and phreat.ophyt,es, depend ing upon species and climate. One experiment used to gain t,hese “consumptive use” coef’ficients was carried out at, San Louis Rey Valley in California (BLANEY and EWINC;, 1946). Tules (Scirpus acufus) were grown in 4 m2 t.anks, set up in in a swamp in t.he valley floors, for three years. The results were compared to an adjacent US Weather Bureau Class X pan. The hydrophyte/open water ratios ranged

A REVIEW OF HYDROPHYTE EVAPOTRANSPIRATION 217

from 1.05 to 1.55 depending upon the time of year. The overall conclusions of BLANEY'S work was t,hat t,he ETh-EW ratio c.ould range above and below unity, depending upon species, c,limate and time of year.

These conclusions are borne out by the following experiments. Scirpus aczzfzzs ratios ranged from 1.00 at Victorville and San Luis Rey, both in California (MUCKEL and BLANEY, 1945; BLANEY et af., 1933), t.o 1.26 obtained at Parma, Colorado (YOUNG and BLANEY, 1942). Sparfinia grown at. Santa Ana in California had a ratio of only 0.6 while Scirpzzs acutzzs also grown in California had a ratio of 1.0.

TURNER and HALPENNY (1941) also worked in t,he south west of the USA, calculating that the ETh/EW ratio of Tamaris and Baccharis ranged from 0.4-2.1 and 0.3-1.2 respectively. These ratios were derived from c.ircular pans compared t,o US Weather Bureau class A pan. The range in t.he rat.io was attributed to the state of growth. GATEWOOD et al. (1950) also calculated the water lost by Tumark and Baccharis in Arizona and found ratios to be Cl.S-2.0 and 0.5-l. 1 respectively.

TINRERGEN (1940), working in the head waters of t*he Roer, in the Belgium Plrdennes, compared hydrophyte evapot,ranspiration values, obtained from soi1 monolit,hs to sunken pans of open water; the ratio for Splragizzz7rl ranged from 0.S to 1.6. KuzNE~sov (1949) carrying out. 1ysimet;er experiments in Russia, stated that: “Water loss from a surface overgrown by helophytes is 1.5-2.5 times higher than from an open water surface, sometimes even tbree t.imes higher (depending on species and fheir density.)” (KUZNETSO~, 1949 cited by BERNATOWICZ ef al. 1976).

KIENDL (1953) also used lysimeters while working on Phragmifes australis, concluding that hydrophytOes ‘rob the water from water bodies’. KOVARIK (1958) showed that t.he ratio of water loss from Tgpha lafifolia was between 2 and 2.5 depending upon the stage of growth. EGGLE~MANN (1963 and 1964) worked upon tbe raised mires and swamps at the Konigsmoor in NW Germany. Using lysimeters with a surface area of 500 cI11~, EUGLESMANN calculated that. the hydrophyte/open water ratio of Sphagnum and Callzzna species ranged from 0.9 t,o 1.5, depending UpOII time Of year. BADEN and EGGLESMANN (1966) concluded that, “Raised bogs displayed a higher rate of evapotranspiration than comparable cat,ch- ments without. mire development.”

There has been a considerable amount of work by authors from t.he USSR on hydrophyt,e evapo- transpiration from ponds, lakes, reservoirs and bogs. ROMANOV (1953) compared lysimeter and Bowen ratio measurements of hydrophyte evapo- t-ranspiration t.0 values of open water evaporation

Rw. Hydrobiol. frop. 19 (3-4): 215-232 (1986’).

derived from the Bowen ratio method. ROMANOV'S hydrophyte/open water ratios were between 1.1 and 1.4. He then went on to develop an empirical equation to predict bog evapotranspiration set out below:

E = Hz + C Where E is evaporation, R is the radiation balance of the bog, z and c are empirical coefficients.

KUZNETSOV (1954 and 1959) cited by KONSTAN- TINOV (1963) stated that partly submerged aquatic vegetation c.ould exceed open wat.er evaporation during the summer, by up t,o 10-20 y/;, but in winter open wat.er evaporation was higher. BRASLAVSKII and VIKULINA (1963) cite other work by KUZNETSOV that took place at the Valdai reservoir. Hydrophytes grown in 0.3 1x12 tanks over two years gave ETh/EW ratios of between 1.28 and 2.02 depending upon species type. BRASLAVSKII and VIIWLINA (1963 p. 113) stated that the reason for these ratios was, “because of the intensive transpiration of moisture by the vegetation.”

GEL'BUKH (1964) calculated hydrophyte evapo- transpiration rates from 0.3 m3 “transpiration evaporimeters”, comparing the results gained to a GGI-3000 pan, in northern Kazakhstan. He concluded that-, “the evaporation from growths of aquatic vegetation cari considerably exceed evapora- tien from an open water body”. He t,hen went on to postulate that this was becausc, flrst., the plants could utilize solar radiation better than a water surface; second, t.hat plants were more susceptible t.o the effects of advect,ion; and third that plants had a higher interchange of convectional heat from the air than open water. GEL'BUKH (1964) also linked hydrophyte evapotranspiration to t.he leaf area and thus the density of growth and then calculat:ed the effec.t that, hydrophytes would bave on several Russian lakes. These results are detailed in the section on water balance methodologies.

BAVINA (1967) compared lysimeter readings (using a Russian B-1000 lysimeter) to BORISOV'S (BORIS~~ 1965) est.imat,e of potential evapot,ranspirat,ion for the area where BAVINA was working, deriving an ETh/EW ratio of bel ween 1.1 and 1.4, for Spkz~711z7~

In a concluding/reviewing article BULAVKO (1971) cited lysimeter work that gave a hydrophyte/open wat,er ratio of bet.ween 1.2 and 1.4. !.&LAVKO stated that, “The evaporation from a water-logged marsh is often 20-40 YJ, more than frorn a free water surface.”

PENFOLD and EARLE (194s) experimented on t.he water loss of water hyacinths in Louisiana, finding the hydrophyte/open water ratio to be 3.2 for ‘tightly packed (i.e. high densit.y) 1 rn” tanks of water hyacinths (Eichhornia crassipes). TIMMER

218 BI. E. GRIINDWELL

ad WELDON (1967) and ROGERS and DAVIS (1972) bath worked in Florida, deriving ETh/EW ratios for water hyacinth from tank experiments of 3.7 and 5.3 respectively. Van der WEERT and KAMERLING (19ïa) carried out tank experiments in Bolland on wat,er hyecint-h evnpotranspiration obtaining an ETh/EW ratio of only 1.44-l ..48 substantially lower than t.he American work had indicated. Van der WEERT and KAMERLING (p. 212) stressed that “For wat.er plants the “trop” characteristics deter- mine to a large esfent t,he transpiration rate. Evaporation of swamps therefore depends on the species growing in t,he swamp... The climatological conditions seem t,o be very dominat~ing.”

BRENZNY et al. (1973) also calculated the wat.er 10~s from watrr hyacint.h as well as from 5 other species of hydrophytes. Working in Hajastban, India, they usrd 3600 cm2 cernent tanks to grow the hydrophytes, comparing the results to identical t.anks with no hydrophytes. The ratios they obtained varied from 0.92 to 2.5 depending upon species type and time of year. For instance. Typha augusii- folin rat&J ranged from 1.64 in summer to 1.38 in win ter. In the corresponding time period, water hyacinth’s ratio ranged from 1.02 to 1.36.

BAY (1966, 196s) calculated the evapotranspiration of peat bogs in Minnesota. Comparing t,he results gained from 3 bottomless lysimeters, containing mainly Sphagnum, to US Weather Bureau Glass A pan, and THORNTHWAITE and HAUDE'S measure of evaporation, BAY derived a hydrophyt.e/open wat,er rat.io of 0.9 t.o 1.45 depending upon the time of yeer. N~caocs and BROWN (1986) experiment,ed upon cores transplanted from Minnesota peat. bogs to an experimentnl station, and discovered that evapot.ranspirat.ion from Sphngnum was t.wice that from a similnr sized ‘tub’ of water. STURUES (1968) found t.hat evapotranspiration from a Iysimeter situatad in a Wyoming sedge bog was 27 O/” greaier than evaporation from a 1.8 m diameter pan.

SIYHIEMIJL rt ~1. (1970) also experimented upon the t?vapotrarispiration of k~~~hagnnm using lysimeters of the Porov a11d IVITSKII design, calculating the ETh/EW ratio to be 1.4. CLYMO (1973) calculated the evapotranspirat.ion loss of various species of Sphagnrzm contained in small beakers in a laboratory, deriving hydrophyte/open water ratios of between 1.5 and 3, depenciing upon species type.

&lC&)NALD ad HIJGHES (1968) found that. Typhn latifoliu grown in 10 m2 t.anks in t,he fringes of Lake Mit-ty near Yuma, Arizona, had an ETh/EW rat-m of 0.9 in winter and 1.56 in summer. PRIBAN and ONDOK (1978, 1980) experimented on a 1 m2 tank of C:ulmagrosii.six cutwscem in the Trebon fishpond in Czerhoslovakia, deriving an ETh/EW ratio t.hat Iny between 1.1 to 1.5, depending upon

t.he season. Gavencia& (1972) experimented upon Phragmites australis in southern Czechoslovakia and found that the maximum rate of evapotranspira- tien was as high as 27.8 mm per day. BERNATOWICZ et ul. (1976) used small lysimeters (c.alled phyto- meters) situated in Lake Mikolajskie, to calculate t,he water lost by Phragmitrs aastralis, Scirprzs lacustris, Typhu angustifolia and T!yphu latifolia over a three year period. The ratios they gained were 2.1, 3.0, X2! and 3.4 respect,ively.

NEWSON and GILMAN (1983), working in Great Brit.ain, discovered that, hydrophyte evapotranspira- tion from Phrngmites austrulis and Cladium mnrissus communities in the Anglesey wetlands exceeded open water evaporation by about 1.2 to 2.0 times The hydrophyte evapotranspiration values were derived from ‘measuring cylinders’ and ‘buc.ket tanks’, whilst. t.he open water evaporation was calculated using Penman’s formulae.

WILLIAMS (1986), also working in Great Britain, calc.ulated t.he 10s~ of wat,er from Carcs riparia. Comparing the results gained from two adjacent. floating IJS Glass A pans, WILLIAMS derived ratios for high st.at,es of growth of 1.17 and 1.49 over the two year st,udy period.

Detached organ This methodology is based on the cutting and

then periodic. weighmg of leaves from hydrophytes. In many c.ases only transpirat,ion rates bave been obtained and there were no direct comparisons to open water evaporation. These experiments are discussed in ‘Pond Littoral Systems’, edited by DYKYJOVA and KVET (1978). The experiments where the results were compared to open water evaporation are discussed below.

TUSCHL (1970) weighed water loss from single leaves of Phragmitrs australis from a lake in Austria and found that hydropl1yt.e evapotranspiration exceeded open weter evaporation. Czechoslovakian workers suoh as KROWLIKOWSKA, KVET and RYCHNOVSKA calculated t.ranspiration losses for a variety of hydrophytes, coming to t,he conclusion that hydrophyte evapotranspiration was eyual to, if not above open water evaporation (HENJY, 1969; KROWLIKOWSKA, 1971; KVET, 1973; RYCHNOVSKA, 1972; RYCHNOVSKA et nl., 1972; RYCHNOVSKA and SMID, 1973). NEUHAUSAL (1975) reporting on earlier work carried out. in the Afties, concluded that. hydrophyte transpiration rates, exceeded Piche atmometer values by up t.o 1.18, depending upon species type.

Water balance approach This approaçh calculates the effec.t. that hydro-

phytes have on t.he overall water budgets of lakes

A REVIEW OF HYDROPHYTE EVAPOTRANSPIRATION 219

or swamps by either calculating the water loss from lakes with and without hydrophytes, or by applying evaporation empirical formulae with a hydropliyte ‘trop coefficient’ to the water budget of various lakes.

BLANEY and MUCKEL (1955) used the “consumptive use” coefficients derived from their earlier work to calculate the effec.t of Scirpus acutus and Typha Iatifolia on the water balance of San Francisco bay. They calculat,ed that, Scirpus acuius and Typha latifolia would lose subst,antially more water than open water evaporation, from 1.16 to 1.4, depending upon location within the bay.

GEL'BUKN (1964) used the results from his lysime- ter and density st,udies to calculate the effect.s of hydrophytes on 11 Russian lakes. The ratios he derived ranged from 0.9 t.o 2.2. This range in rat,ios WRS mainly caused by the differences in densities of hydrophytes growing in t.he respect.ive lakes, t.he size of the lake, and the annual average windspeed across the lake, for GEL'BUKH calculated his open water evaporation values from a mass transfer methodology devised by Zau~ov (1949).

BERNATOWIECZ et al. (1976) used their results described above to calculate the effect that emergent hydrophytes would have on the water balance of four Polish lakes. Using a formula developed by NOVIKOVA (1963), they found that in t,hree out of the four cases presented, the presence of hydrophytes would mean more water loss than if there were no hydrophytes present.

BENTON ef al. (1978) calculated the effect of water hyac.inth upon Texas reservoirs. Using PENFOLD and EARLE'S earlier work, BENTON ef al. calculated that. on a variety of Texas reservoirs the “water Ioss by a mature plant is about three times as much as evapot,ranspiration from an eyuivalent area of open water”.

HUTHERFORD and BYERS (1973) have been cited by NICOLS and BROWN (1980) as stating that. evapotranspiration from a New England bog exceeds open water evaporat,ion by 30 %.

BALEK and PERRY (1973) compared evapotranspi- ration from st.ands of Brachystegia woodland, in the Kafue basin in Zambia, calculated by a water balance approach to Penman’s estimate of evapora- tion. They obtained average ETh/EW ratios for the three year st.udy period of bet.ween 0.3 and 1.35 depending upon season.

DOLAN et al. (1984) calculated t.he evapotranspira- tion of a freshwater wetland in Florida using a quasi water balance t.echnique. UGlizing a groundwat,er measuring technique pioneered by DAVIS and DEWIEST (1966), they calculated t,hat t.he evapo- t.ranspiration from Saggittaria lancifolia could exceed open water evaporation by a factor of 2.5. The

Reo. Hydrobiol. Pop. 19 (3-d): 215-252 (1.986).

ETh/EW ratio they gained depended upon state of growth. They then linked the exact magnitude of the ETh/EW ratio to an indicator of state of gr0wt.h: above ground biomass.

Bowen ratio/Eddy correlation

This methodology measures t.he vapour/t,empera- ture profile over hydrophytes and then compares the results gained t.o measures of open water evaporation.

The Bowen ratio met.hodology was extensively used by Russian mire workers such as ROMANOV (1953), BAVINA (1967) and BELOTSERKOWKAYA et al. (1969). R.ates of hydrophyte evapotranspiration rather than comparisons with open water evaporation were made SO no ETh/EW rat,io’s were obtained.

SMID (1975) in Czechoslovakia, compared hydro- phyte evapotranspiration to some form of open water evaporation. Working with mixed stands of Phragmites australis with an undergrowth of Curez riparia, at the Nesyt fishponds in South RIoravia, SMID found that the hydrophyte evapotranspiration calculated from the Bowen rat.io method was 0.8-1.5 times higher than from a moored pan of water situated in a nearby lake, depending upon species type and st.ate of growt.11. PRIBAN and ONDOK (19SO) also used a Bowen ratio method to calculate the evapotranspiration from a fishpond in Trebon. They concluded that using this approach, “The evaporation from the Treborn marsh must approach the potential evapotranspiration.”

Conclusions

Table II is the tabulat,ion of the work described above int.o climate, species and then stage of growth. Figure 1 is the means of each major species for each major climate. From table II and figure 1, few flrm conclusions on hydrophyt,e evapotranspiration cari be drawn, primarily due t-o the lack of detailed experiments. Many pieces of work cited above cari net. be tabulat,ed because t.he authors did not describe their work fully. GEL'BUKH (1964) is a good example of t,his, as he described the hydrophytes under investigation as “reeds”. From table I this could be construed to mean Phragmztes, but, as no latin name was given t.he result cari not be tabulated.

However, despite t.hese problems, some t.entative conc.lusions cari he drawn. From table II it is clear that. a dominating factor on the ETh/EW ratio is state of growth. It, is important to note t.hat in many of the pieces of work shown in table II, the ETh/EW ratio is below 1.0 for low stat,es of growth and above 1.0 for high states of growth. This fact

BI. E. CRUNDWELL

2.5 1

0

0 0

@ B Dry Climates

0 C Humid Meso-Thermal

CI C Humid Micro Thermal

0 0

GENUS

I;ig. 1. - Mean ETh/EW ratios for selrcted hydrophytes in various climates Rapports moyrns de l’t~vapofran.spirution d’un marais par rapport à l’&aporalion d’une nappe d’eau libre pour dizws types d’hydrophyies

dans différents climafs

will becorne more important. when the results presented above are compared to those which st,at.e that the ETh/EW ratio is below 1.0. In fact differen- ces in tha ETh/EW ratio due t.o st,age of growth cari have a larger effect than that of climate. For instance, t.he difference in the ETh/EW ratio for stage of gr0wt.h in Tgphn is 0.5 in climate types A and C while it. is 1.1 in climate t-ype B. The differeme in climate for Typha has only the range of 1.2 to 2.1.

Many workers (BLANEY et al. 1942, Van der WEERT and KAMERLING 1974) have stated that c.limate has an irnlJclrtimt. effect on the ETh/EW ratio, yet from figure 1 it appears that this effect is not constant.

A dry climate (B) would expect. to have a higher ratio than a humid meso-t*hermal climate (C) yet figure 1 shows this not to be t,he case. The anomalies in the effec.t of climate shown in figure 1 could be due parUy t.o the classification, as meso-t,hermal climates (C) groups a Mediterranean climate with a British climate. TO overcome t.his problem, more classifications of climates could have been used, but there are not enough studies for each climate tSo obtain a meaningful average. It could also be due to the fact that. while hydrophyte transpiration stays more or less the same from chmate to climate, the open wat,er evaporation decreases in the humid

Rrv. Hydrobiol. frop. 19 (s-4) : 21.~2.32 (1986).

A REVIEW OF HYDROPHYTE EVAPOTRANSPIRATION 221

TABLE 1

Characterist.ics and habits of hydrophytes described in the reviem Caracférisfiques et comporiemeni des hydrophyies décrits dans le texte

LATIN NAME COMMON NAME HABIT/HABITAT

Cyperuspapyrus Phragmitesaustralis Scirpùs lacustris Scirpus validus Typha latifolia Typha angustifolla Typha orientalis

Papyrus Reeds Bulrush Soft-Stern bulrush Raedmacecat-tails Narrow leaf reedmace Reedmacecat-tails

Tall emergantmacrophyte ” <I u I< ” ”

Equisetum Scirpusacutus Scirpusolneyi Scirpusamericanus

Home-tail Tules ” ”

Leaflessemergent macrophyte u I, ”

Calamagrotis canescens Carexriparia

Purplesmall-raed Great pond sedge

Leafyemergent macrophyte n

Baccharis Tamarixgallica

Baccharis Saltcedar

Woodyshrub ><

Cyperus rotundus Eichhornia crassipes lpomaea aquatica Lemna miner Nymphaea Pistia stratiotes Potamogeton nodosus Trape natans

Purplenutsedga Water hyacinth Swampmorning-glory Common duckweed Waterlily Waterlettuce American pondweed Waterchestnut

Floating macrophyta II ” ” I, <, ” ”

Sphagnum Bog moss

Thuja occidentalis Eastarn white cedar

All nomenclature followa Tutin et al. 19941990

Bog macrophyte

Conifar

meso-thermal climate (C), t,he net. effect being an increase in the ETh/EW ratio obtained.

0~1s (1914) and CLYMO (1970) are among many authors who identified species as having an import.ant effect upon t,he ETh/EW ratio. Information gained from figure 1 t.ends to support this view as such genus as Eichhornia and Typha have a higher ratio t.han Tamarix or Baccharis.

Work by BAVINA (1967) and PRIBAN and ONDOK (1980) has shown the variante in results gained by different methodologies as shown in table III.

It, has proved impossible to subst.antiat,e this with reference t.o other workers results as there are not enough studies in the same climate with t,he same species using different, methodologies to obtain a meaningful average. Yet if t.he varying methodologies are scrutinized it is apparent that the choice of methodology affect.s the ETh/EW rat,io.

KIESELLBACH (1916) lists t.he main sources of error in using tank/lysimet.er experiments. Hydro-

phyte tank experiments add several other potent.ial errors t,o those listed by Kiesellbach. Firstly, if the tanks are taken out of sifu and t.hereby isolated, t,he resulting ETh/EW rat,io Will be artificially high due to the effects of advection. This is clearly shown by t.he results of BLANEY et ~1. (1933) who measured the water loss of a t,ank of Scirpus both inside and outside a swamp in California. The resulting ETh/EW ratios gained from either tank differed by a factor of four. BLANEY et al. (1933) stressed that “Consump- tive use of water by tules or cat-tails grown in tanks in exposed locations is not closely indicative of the truc use by these plants growing in their natural environment.. . use of water by swamp growth transplant,ed to exposed locations is inordinat.ely high.”

Indeed several aut,hors (notably LINACRE, 1976) have stressed t.hat advection would affect a11 tank experiments, whether grown in sifu or not. NEWSON (pers. Comm. 1986) stated that the biggest problem

Rev. Hydrobiol. trop. 19 (3-4): 215-232 (1686).

222 M. E. CRUNDWELL

TABLE II Studirs of individual species of hydrophytc which showed the ETh/EW ratio could ùe great,er than unity

Étude des espkes d’hydrophyfes dont le rapport ETh/EW montre une yrnndeur supérieure à l’unifé

CLIMATE SPECIES REF$;F&NCE ST&;E$; ANNUAL AVERAGE WORKER

High Low ETh/EW ET /EW

A-Tropical rainy

I-Tropical rainforest

2-Tropical savanna

B-Dryclimates l-steppe

2-Desert

C-Humid Meso- 1-Mediterr- thermal anean

BHumid sub- tropical

Scirpus Scirpus Scirpus Eichhomia Eichhornia Eichhomia

acutus acutus acutus crassipes crassipes crasSpes

3-Marine west Calamagrostis canescens toast Calamaarostis wnescens

Eichhoriia Eichhomia Phragmites Phragmites Phragmites Potamogeton Scirpus Scirpus Scirpus Sphagnum Sphagnum Sphagnum Sphagnum

DHumid micro- thermal

Brachystegia cvpe~~ Eichhomia Ipomaea Pistia Trapa T@a

Scirpus Sphagnum Bacchads Eaccharis Scirpus Tamarix Tamarix Twha Twha T@a TyPha

rotundus crassipes aquatiw stratiotes

anguçtifoia

acutus

acutus galliw galiiw latifolia latifolia latifola latifoia

Carex Carex Equisetum Equisetum Sphagnum Sphagnum T@a Typha

crassipes australis australis australis nodosus ohevi

validis

angustifolia latifolia latifolia latifoia

Penman PEt 1.35 3.6m’tanks 2.4 I< 1.35 ” 1.24 ” 1.08 n 1.03 u 1.65

0.30 *.x*x

1.02 1 .Ol 0.90 0.83 1.27

0.80 2.4 1.15 1.10 1.00 0.95 1.40

USClassApan 2.0 0.7 1.35 US ClassA pan **** **xx 1.27 N 1.25 0.4 0.98 ” 1.1 0.5 0.86 u **xx x*x* 1.26 ” 2.0 0.36 1.02 3, 2.1 0.5 1.30 ” 1.6 0.5 1.20 u 1.8 0.85 1.35 ” 1.62 0.77 1.18 ” 1.68 1.12 1.40

USClassApan 1.5 « **xx II 1.2

1.1 x*,x

2.0 **xx **** x*x*

1.25 1.24 1.35 3.2 3.7 5.3 1.55 1 .oo 2.0 1.48 1.2 2.1 1.6 3.1 1.04 3.0 2.5 1.2 1.14 1.1 2.25

Small potin Lab. xx*x *ii** **** *xx* *x*x l ***

1.1 X*l x*x*

x*x* **xx

USClassApan 1.69 3m*floating pan 1.8 0.3mzphytometer **** Penman PEt 2.0 **** ****

USClassA pan 1.65 0.3m2 phytometer***” XXXX *xi*

Haude PEt 1.5 USClassApan 1.45 ThornthwaitePEt 1.4 Small beaker 2.5 :n lab 0.3m2phytometer **** **xx **xx 0.3m2phytometer2.5 ” **xx

0.3m’phyometer 1.28 I, 1.45 ” 1.34 II 1.53 0.16mZ’tub’ ****

**xx **xx **xx *x*x *x*x

1.1 **xx **ii*

0.3m’phytometerl.4 I# 1.75 ” 1.38

2.0 **xx **** 1.28 0.8 1111

1.2 *s**

0.53 *xx*

0.9 0.9 0.6 2.0

**** 3.2 **xx 2.0 2.0 2.25 **xx 3.4

1.16 1.18 1.36 1.19 2.0 1.25 1.60 1.23

1.15

1.40

1.35 1.27

0.90 1.26

1.15

1.28

1.27

4.06

1.75

1.65

2.75

1.40

2.7

1.28

1.42

BalekandPerry1973 Breznyetall973 ,, ” » ,I *

Stearnsand Bryan 1925 Sturges 1968 TumerandHalpenny1941 Gatewoodetal. 1960 Young and Blaney 1942 TumerandHalpenny1941 Gatewoodetal. 1950 McDonald and Hughes 1968 Blanayetal.1938 Blaneyetal. 1938 Blaneyetal. 1942

Muckel and Blaney 1945 BlaneyandEwing1946 Blaneyand Muckell955 Penfold and Earle 1948 Timmerand Weldon 1967 Rodgersand Davis 1972 Pribanand,Ondok1980(tank) Priban and Ondok 1980 (B’wen) Dtis 1914 VD’Weeti&Kamerling1974 Smid 1975 Bernatowieczetal. 1976 Newson and Gilman 1983 otis1914 Blaneyetal.1933 Bematowienetal. 1976 Dtis 1914 Egglesmann 1963 Bay 1966 Schmiedll970 Clymo 1970

Bernatowiczetal. 1976 ais 1914 Kovarik1958 Bernatowieaetal. 1976

Kuznetsov 1952 ” 1953 Kuznetsov 1953 ” 1962 NicolsandBrownl980 Bavina 1967 Kuznetsov 1963 “1952

N.B. i) Only tank experiments carried out in situ included ii) Mean values obtained from at least one full year of results iii) **.** - Missing or data net available

Rw. Hydrohiol. trop. 19 (J-4): 21%2.Z (1986).

A REVIEW OF HYDROPHYTE EvAPOTRAN~PIRt~TlON 223

TABLE III A comparison of daily hydrophyte evapotranspiration for similar specks mith different methodologies (mm)

Comparaison de l’évapo~ranspirafion journalière d’espéces similaires sel&i diffkrenfes mtlihodologies (mm)

Bavina (1967)

Priban and Ondok(1980)

Heat balance method Tankmeasurements Heat balance method Tankmeasurements

Mw June July Aug. sept. oct. 3.9 26 0.3

Al: 4.7 ::; 3:2 1:: 0.8

::7 32 2.4 1.7

AS 5:1 3.3 2.6 ::5

in calculating hydrophyte evapotranspiration in the Anglesey wetland st,udy was to overcome the effects of advection. Advection plays an important. role in t.he process of hydrophyte evapotranspiration and affect.s tank methodologies in particular by the oasis and clothes line effects.

The oasis effect arisea due to horizontal differences in the wetness of an area. Incoming dry .warm air supplements the net. radiation, provldmg more energy for evapotranspiration to occur. The oasis effect would, therefore, affect areas of hydrophyt,es along a narrow strip, such as a river bed or the edges of a larger swamp.

The c.lothes line effect increases evapotranspiration by increasing the ventilation of air through a canopy, increasing the turbulent exchange and t.hereby increasing evapotranspiration. The clothes line effect. would afrect a11 ta11 hydrophytes and a11 helophytes. A clear example of t,he combined effects of advection has been detailed by DAYENPORT and HUDSON (1967). Working on cott,on fields in the US, they found that evapotranspiration decreased signifi- cantly as progression was made into the cotton fie&. Advection Will, therefore affect tbe results gained from workers using the tank methodology.

However, LINACRE (1976) states: “Advection affect,s a11 swamp evaporation except, in the middle of a large swamp, where the air is remote from the influenc.e of the surroundings.” It is apparent that hydrophytes growing in nat,ural conditions, in most, though not all, cases will be affected by advection and Lhis therefore is an inherent part of their evaporative characteristics. GEL'BUKH (1964) cer- tainly believed that advection was an inherent, part, of hydrophyte evapotranspiration. The conclusions that, cari be drawn from the above discussion on advect,ion are that it, is an import,ant factor on isolafed tank met.hodologies but as advection is an inhererzf part of most. hydrophyte evapotranspirat.ion as long as the t.anks are sited in sifu the results gained Will be the act,ual values of hydrophyte evapotranspiration.

Secondly, if the pan of water is located within the swamp, there will be rest.rict.ed airflow over t,he pan, reducing the evaporation and therefore raising t.he ETh/EW ratio. Yet if the pan of wnter is situated

Rev. Hydrobiol. irop. 19 (3-d): 215-2.32 (1986).

outside of the swamp, the pan is then subjected t.o the ef??ec.ts of advection described above.

Rowen rntio/heat t.ransfer/eddy correlation techni- ques are also prone to inaccuracies. These are because the c.omplex series of measurements are usually only taken for part.s of the day, and there are great problems in data capture and analysis. ddvection (which LINACRE (1976) stated affected a11 swamps except very large ones) could conceivably create air instability, which Will cert.ainly affect. Bowen rat,io measurements.

There are only a few water balance studies that primarily compute ETh/EW ratios; most use ETh/EW rat.ios gained from other met.hodologies, albeit their own work, and t,Jlen apply t.hose ETh/EW ratios t.o water ba1anc.e studies. Thus the main errors in the water balance methodologies are the errors in ot.her methodologies that, have already been described.

Sir@e leaf and detached organ methods sut’fer from two principle errors; flrstly that, the water lest from a tut leaf.is un-natural. Secondly that not a11 the leaves on a plant Will t.ranspire at the same rat.e. It is not easy eit-her to make leaf area estimates of large areas, SO that applying ratios gained from single leaves is extremely hazardous.

KUZNETSOV (1949), GEL'BUKH (1964) and BERNA- TOWICZ et al. (1976) bave a11 stressed that the densit.y of the hydrophytes bas an important effect upon the ETh/EW ratio. It is hard to yuantify this from studying table 1 and figure 1. RANTZ (1968) published coeff+ients relat& to BLANEY et aZ.‘s consumpt.ive use coeflleients t.hat were dependent upon t.he density of the hydrophyte under study. RANTZ suggested a simple linear dec.rease in the ETh/EW ratio wit.h dec.reasing densities. However GEL'BUKH (1964) postulated that. the ef’fect, of density was more complex than this, st.ating (p. 369), “The increased density increases the shading of the plants which, in turn, reduces t,he rate of t.ranspira- tien and of total evaporation.” Density, therefore, will be an important. fac.tor on hydr0phyt.e evapo- t.ranspiration, yet t.he precise effect is unclear.

An indicator of thr exact magnitude of the ETh/EW ratio was identifîed by DOLAN et al. (1984): above ground biomass. Tt is clear that this factor is

224 M. E. CRUNDWELL

intluenred by state of growth, species type and density: three of the five factors that influence the HTh/EW ratio. Measurements of above ground hiomass could tlius potent.ially be important. in calculating hydrophyt,e evapot,ranspiration. This is because if a relationship is experimentally derived which links above ground biomass to the exac.t. magnitude of t.he ETh/EW rat.io, t.his relationship could then be applied to values of above ground hiomass gaineci from large areas of freely growing hydrophytes. Accurate measurements of evapo- transpiration from this area of hydrophytea coulcl be Lhen be made by rnultiplying st.andard met.eoro- logical values of evapot,ransplrat.ion by the ETh/EW ratio inferred from above ground biomass values. GEL'RUKH (1964) utilized t.his methodology by linking t-he magnitude of the ETh/EW ratio to t.he leaf area (which is highly correlated to above ground 1:biorrrass) in bis expeiiments in Kazakhstan.

Partial conclusions (1)

Thr. fnllowing conclusions cari be drawn :

1) There is very little long-terni systematic. work upo11 hyclrophyte evapotranspiration. This entails t.hat any conclusions cari only be tentatively put forward.

2) The ETh/EW rAio cari be greater t,han unity, and this ratio is affeot,ed by the following factors:

a. Stnt e nf qrowt-h

h. Species

c. C1imat.e

ti. Rlet~liodological appr0ac.h

e. Denaity

3) Hydrophyte evapotranspiration is affected by advection which in most cases is an inherent. part of hydrophyte evapotranspiration.

4) Thr exact magnitude of the ETh/EW ratio for a particular speçies in a part.icular climate cari 1~ gained from measurements of the above ground hioniass or thr leaf area index.

II. QPEN WATER EVAPORATION ESCEEDS HYDROPT-IYTE EVAPOTRANSPIRATION

There is less work that- suggests 1lydrophyt.e evapot.ranspirat.ion cannot exceed open water evapo- ration. This work Will, however be çovered in the same manner as the earlier section.

Retr. Hgdrobiol. frop. 14 (5-J) : 21.5-282 (1986).

Lysimeters

MIGAHID (1952 quoted by PENMAN, 1963) was an early worker to conclude that hydrophyte evapoLranspirat,ion, measured in a t,ank was lower than open water evaporation. In the Sud region of the Nile, papyrus was grown in 10 m3 tanks set in the middle of a large swamp. Evapotranspiration from this t,ank was compared to evaporation measu- rements of a t,ank of water, situated in a nearby, open lagoon, taken the following year. The results for the two years in question gave a hydrophyte/ open water ratio of 0.9. ~IIGAHID is also c.ited as saying, “WaLer lest by evaporation from a free water surface inside a swamp was about. 20 o/. of the loss from a free water surface in the open.” (MIGAHID cited by GIUB et nl. 1956 if2 LINACRE 1976, p. 338).

Indeed, RIJKS (1969) cites other work by MIGHAD where p1ant.s left, undisturbed for six years yielded a hydrophytelopen water ratio of 0.55.

GIBB et al. (1956), working near the Shambe in Sudan, compared tank values of hydr0phyt.e evapo- transpiration to values of open water evaporation obLained from Oliver’s and Thornthwaite’s evapora- tion formulae. The hydrophyte/open water evapora- tion ratio was found to be 0.6.

JOHANSSON (1974) working in Bomosse, sout,h Sweden, compared B-1000 lysimeter readings of Sphagnum, to estimates of open water evaporation derived from LIS Weather Bureau Glass h pan, GGI-3000 pan and Penman’s empirical formula. Johansson’s ETh/EW ratio ranged from 0.64 to 0.85.

BREZNY et al. (1973) cite work by SEYBOLD (1930) which showed that a water surface covered by Lemna miner decreased the evapotranspiration 10s~.

One of the more recent pieces of work t.o be published on tank hydrophyte evapot,ranspiration being lower t.han open wat,er evaporation was that of COOLEY and Tuso (1980) and ANDERSON and Iuso (1985). Kesponding t,o the work of BENTON et al. (1978) on the wat.er loss of wat,er hyacinths, COOLEY and Inso published data on evapotranspira- tien from Nymphaea that had been collected in Lhe month of Riay 1968. Two sunken evaporat‘ion pans were measured daily, une with Nymphaen caver of about 18 só, t.he other t.otally clear. The hydro- phyt.e/open water rat,io Cooley and Idso derived was 0.9.

ANDERSON and IDSO (1985) ext.ended this work and calculated the water 10~s from four species of hydrophytes grown in a variety of pans. This was because t,hey believed that the reason that some workers found the ETh/EW ratio to be above unity was due t.o differences in the evaporative surface areas of the t(anks of hydrophytes and the tanks of open water. They wanted to show that

A REVIEW OF HYDROPHYTE EVAPOTRANSPIRATION 225

there was a strong correlat.ion bet.ween the ETh/EW rat,io nnd the vegetation/open wat‘er surface area ratio. The ETh/EW ratios gained ranged from 0.85 to 1.6 depending upon species type and height of the vegetation.

Water balance

NOVIKOVA (1963) worked on t.he effects of the evapotranspiration losses of Tgpha angustifolia and Phragmites australis on the wat,er balance of t.he Kengirdam reservoir in Kuzakh, flnding the ratio of hydrophyt.e/open water to be 0.71-0.73. He then developed an empirical formula to calc.ulate lake evaporation with the presence of hydrophytes shown below:

T-l-e EhtE-

E

where Eht is total loss of water of lake as quotient (T + e) is water loss from t.he area overgrown by hydrophytes, and E is the evaporation from the open area of t.he lake.

EI~ENHLOR (1966) and SHJEFLO (196s) calculated the water loss by evapotranspiration from veget.at.ed and unvegetated prairie potholes in N. DAKOTA, USA, using a wat,er balance approach and a mass- transfer approaçh devised by HARBECK (1962). They concluded that the hydrophyt,e/open water ratio for a mixt,ure of ‘white top’ and ‘hardst.em bullrush’ was between 0.7 and 0.86. Eisenhlor argued that hydrophytes would reduce evaporation in two ways:

1) by shelt,ering the water surface from the wind and

2) by shading t,he water surface, thereby reducing the incident. solar radiation.

EIYENHLOR (p. 452) concluded that; “The presence of hydrophytes reduces t.he evaporation from a free wat,er surface significantly.”

Bowen ratio/Eddy correlation methods

RIJKS (1969) worked in a papyrus swamp in East Africa. Measuring hydrophyte evapotranspira- tien using the Bowen rat,io method, he compared this to open water values of evaporation gained from Penman’s 1948 empirical formulae. The hydrophytel open water ratios gained by RIJI~S ranged from between 0.38 and 0.81.

LINAÇRE et al. (1970) used an Eddy correlation method devised hy DYER et al. (1967) to compare calculations of evapotranspirat,ion from V@a orienfalis and Typha dogingensis to those of a lake some 16 km away, in t,he Barren Box Swamp in

SW Australia. Taking readings for about 3 heurs per day and for four days in February, LINACRE ef al. found the hydrophyte/open water ratio to be between 0.3 t.o 0.9, depending on prevailing weather condit.ions. This ratio was att,ributed to the lower albedo of clear water surface of the lake, the shelter given by the reeds in the swamp to the water surface and the interna1 resistance to water movement of the reeds themselves. LINACRE (p. 385) concluded that, “It is likely that, the growth of reeds in a lake or other water body will reduce rather than increase the water 10~s.”

The evaporative charac,teristic,s of t,he saturat,ed tundra around Hudson Bay, Canada were examined by ROU~E et al. (1977). Comparing hydrophyte evapotranspiration calculat.ed by the equilibrium technique, ROUSE et al. found that the hydrophytel open water ratio was 0.9-1.0.

31~~~0 (1979) used a Bowen ratio met.hod t.o calculate the evapotranspiration losses from a wooded swamp in southern Ontario, obtaining a hydrophyt,e/open water ratio of 0.76 to 1.04, over the flve day study period.

Partial conclusions (II)

The results cari be tabulated in the same manner as section 1 (table IV). Table IV shows that t.he ETh/EW ratio is never greater than unity. From the study of ~IIGAHIU (1952) and hIUNR0 (1979) t,hat stage of growth ii; an important factor in determining the exact, magnitude of the ETh/EW ratio.

The following conclusions cari be drawn:

1) Hydrophytes lose less water than a correspon- ding area of open water,

2) This ETh/EW ratio depends mainly upon the state of growth and

3) Although c.limate and species are probably important factors in determining the ETh/EW ratio, the results presented above do not justify this assumption.

III. THE EVALUATION OF HYDROPHYTE EVAPOTRANSPIRATION RESEARCH

The first major point in evaluating why there is a conflict in the conclusions drawn by the authors in 1 and II is that most of the work in 1 had ETh/EW ratios of below and above unit,y. Those in II generally did nota.

TO fully comprehend why this has occurred, section III will be split into two part,s: firstly, to

Rev. Hydrobiol. trop. 19 (3-4): 218-232 (19SO).

226 IV. E. CRUNDWELL

‘~AI(LE Iv

Studies of individual species of hydrophytc which showed the ETh/EW ratio was lower then unity Étude des espèces d’hydrophyfes dont le r~zpport ETh/EIV monfre une granderzr inft’riezzre à f’unifë

CLIMATE

A-Tropical rainy

l-Tropical rainforest

Z-Tropical savanna

SPECIES REFERENCE STATE OF GROWTH ANNUAL WORKER OF EW High Low EThiEW

Eichhornia Eichhornia

Nymphaea Nymphaea cIpe:us

T@a

crassipes cmssipes

patw~s PaPYNs

orientalis

Varioussized ponds 0.88 .*yx* 0.86 Anderson and Idso 1985 ” 1.44 **** 1.44 ,,

346mssunken pan 0.95 **** 0.95 Cooley and Idso 1980 Varioussized ponds 0.85 **** 0.85 Andersen and Idso 1985 Penman 10m2pan PEt **** 1.98 0.8 0.6 0.95 0.6 Migahid Rijks 1969 1952

Mastransfer *xx* 0.6 0.6 Linacreetal. 1970

S-Dryclimates l-Steppe

Z-Desert

C-Humid Meso- I-Mediterr- thermal anean climates

Z-Humidsub- tropical

SMarinewest Sphagnum GGI3000 0.8 0.6 0.7 Johansson 1974 toast

D-Humid micro- thermal

l-Humid Continental

Thuja T@a Twha

occidentalis Equilibrium mode1 1.0 0.7 0.85 Munro 1979 MassTransfer 0.8 0.7 0.75 Eisenhlorl966&Shjeflo 1966 X*.X* **xx **ii* 0.5 Novikova 1963

N.B. i) Gnly tank experiments carried out in situ included ii) **** indicates missing or data not available

assess whether hydrophyte evapotranspiration cari theoret~ically exceed open wat.er evaporation. Secondly, to explain why the two set.s of workers c.ame to different conclusions.

Can hydrophyte evapotrauspiration theoretically exceed open water evaporation?

There are three main theories whereby it is possible that. hydrophyte evapot,rnnspiration cari exceed open water evaporat.ion: Increases in surface area, lower areodynamic resistances and finally high transpiration c«efGents.

INCREAYES IN SURFACE ARE.~

The basic premise is that. as hydrophytes grow out. of a wat,er surface, an increase in t.he evaporative surface area results, due to the foilage of the hydro- phytes.

IL~) (1979) estimated that out of the ETh/EW ratio of 3.7 for water hyacinth derived from TIM~ER and WELUON (1967), 85 o/. was due to the differences in the total evaporative area of the hydrophyte compared to the open water. This factor is thus

R~U. H~drohiol. irop. 10 (J-4): 215-232 (1986).

clearly important in determining not only whether hydrophyte evapotranspiration cari exceed open water evaporation, but also the exact, magnitude of tbe ETh/EW ratio. Thus the increase in surface area factor WI1 depend upon spec,ies type, density and state of growt,h.

LOW AERODYNAMIC RESISTANCES

MONTEITH (1967) and VAN BAVEL (1968)published evaporation formulae with a terrn known as the ‘aerodynamic resistance’. This factor was dependent upon the nature of the evaporat.ive surfac.e and was a surrogate value for the amount of turbulent exc.hange.

VAN BALTEL suggested t.hat the evaporation rat.e is proportional t.o t.he sum of net. radiation added to the daily range of ambient temperatures divided by the aerodynnmic resistance of t.he evaporative surface. Because of the differences in t,he resistances of hydrophytes and open water it. is theoretically possible for hydrophyt8e evapotranspiration to exceed open water evaporat,ion. For example, using values obtained from LINACRE'S 1970 work it apears that at. a t,emperature of 20°C and a wind speed of 2ms-1

A REVIEW OF HYDROPHYTE ET'APOTRANSPIRATION 22.7

the ratio of hydrophyte evapotranspiration to open water evaporation is 1.3. LINACRE (1970) studied the evapotranspiration of Ty@z and from table 1 it is clear that this rat,io of 1.3 derived from VAN BAVEL'S equation is rery similar t,o the results of ~~~DONALD and HUGHES (1968) who were working in a similar climate.

If the equation for predicting the aerodynamic resistance developed by THOM and OLIVER (1977) is studied it is seen that the exact magnitude of the aerodynamic resist.ance is dependent upon height as shown below:

Ra = 4.72. [Ln(Z/Zotla

1 + 0.54T.J

Where Ra is aerodynamic resistance, LJ windspeed at height h, z is distance above Surface>s zero plane displacement and Zo is 0.1 t.he height of t,he vegeta- tion.

Thus Ra is a function of species t.ype as well as state of growth, for bot11 of these factors will influence the height of the plant. Therefore, the aerodynamic resistance factor cari mean that the ETh/EW ratio is above unity, t.hough this Will depend upon species type and state of growt,h. The similarit-y between this process and the clothes line effect discussed in 1 is abvious. This merely reinforces t,he belief that advection is an inherent part of hydropl1yt.e evapotranspiration.

HIGH TRANSPIRATION COEFFICIENTS

LOFTFIELD (1921) was an early worker to comment that hydrophytes had very little control on their stomatal openings. LOFTFIELD (p. 299) stated: “Such plants as Scirpus validzzs, Eqzzisefzzm hiernale and Equisetzzrn palustre, showed the stomata conti- nously wide open, and this seems to be their usual state.” FIRBAS (1931) cited by INGRAM (1983, p. 83) concurred with LOFTFIELD'S earlier results, stating, “Their [hydrophytes] stomata remain fairly wide open even on sunny days at noon, and showed maximum stomatal apertures in du11 weather.”

More recent c.onfirmat)ion of this 1ac.k of control of stomata has corne in the work of PENMAN (1963), IDSO (1968) and VAN BAVEL (1968). Al1 believed that hydrophytes behaved as passive wic.ks, responding to the atmospheric demand for water. As hydrophy- tes, by definition, live in areas where water supplies are not a limiting factor t.hey must be able to transpire at a potential rat.e. Thus the transpiration coefflc.ients as used in NOVIKOVA’S work (detailed in II) cari be high, thereby suggesting that, hydro- phytes cari indeed lose more water than open water areas.

Rev. Hydrobiol. trop. 19 (3-d): 215-2.32 (1986).

It is seen that through the three main processes described above it is theoretic.ally possible for hydrophyte evapotranspiration t,o exceed open water evaporation. Therefore, why did the workers detailed in II corne t.o varying conclusions to those detailed in I? The next section will detail why.

An evaluation of the varying conclusions

The variante in the conclusions drawn by the authors in 1 and 11 cari be explained by three main factors: poor experimental design resulting in incorrect conclusions, tbe drawing of sweeping conclusions from a limited amount of work. finally, the current ‘accepted’ geographical thinking.

POOR EXPERIMENTAL DESIGN

The best example of poor experimental design is that, of MIGAHID (1952). Widely quoted as the first to prove that hydrophyt,e evapotranspiration was lower than open water evaporation, the experi- ment, was published in 1952 and was not widely known unt,il it was cited by PENhfAN in 1963. The fatal flaw in the experiment,al design was that MIGAHID compared tank values of hydrophyt,e evapotranspiration gained in 1947 to open water values gained in 1948. When the results are examined, the ‘real’ ratio is 1.14 as shown in table V. The value of 0.98 was only gained by çlaiming that 1947 was a abnormal year and that the value of hydro- phyte evapotranspiration should be lowered accord- ingly. PENMAN then went. on to quote other work by MIGAHID that compared tanks of hydrophytes to open water tanks. The ratio then was 6.0. This second result of RIIGAHID, published on the same page as the other experiment seems to have rec.eived scant att.ention shown by those who cite t,he experi- ment.

Another example of poor experimental design is that of the USGS work of the potholes of the Dakota prairies (EISENHLOR, 1966 and SHJEFLO, 1968). The error in this work was the division of the evapo- transpiration term into its two components: evapora- tion and t,ranspirat,ion. Each component was calcula- ted separately, and t.hen added together to produce a value of evapotranspiration for each of the potholes studied. This experiment:al design leads to errors for no account. of t,he transpiration of wat,er drawn from the roots is made in the calculations. This would lead to an under-estimation of the transpira- tion term, and a consequent under-estimation of the total hydrophyte evapot,ranspirat.ion.

The errors in this work are clearly shown when the seepage losses of the potholes are studied (SHJEFLO, 1968). A straight, comparison of clear pothole to

2.28 AI. IX. CRUNDWELL

Evaporation from swamp vcphtion and open mater: Sutlan L?vaporafion d’un marais el d’un plan d’eau libre arr Soudan

Papyrus Year

7,947 I< I>

« II ,I <,

lgm II I<

Open water Evapy;tion Month Ye?J

ml v) EAK&or;ty

Y

s7 3 XX? ” 715 5:3 6.3 5.4

4.0

7.1 4.7 5:;

$7 Jan. 6.2 Feb. ?.9@ 51

Mar. « 6:l G:2 Apr. If 6.5

Mean (12) 6.5

Omit Dec.- 6.0

After Migahid 1952,from Penman 1963 ~63

5.7

6.1

vegetated pothole gives a result, of 0.89 for t,he ETh/EW ratio. Yet vegetated pot,holes alrvays had a higher average üeepage rate than clear ones, on average 33 7; higher. This means that if seepage is taken int.o arcount. the true ETh/EW ratio is about 1.14, much closer to the results gained for other hyclrophytes in t.he same climate (see table II).

of reeds in a lake or other water body will reduce rather t.h:rn inc.rease wat,er loss”, due to the fact that LINACRE et al. only measured “10~ state of growth evapotranspiration.”

Thus it is seen that two of the results that proved hydrophyte evapotranspiration was below t.hat of open water were obtained from poor experimental des@. Yet there are several others who came to t,he same conc.lusions as MIGAHID and EISENHLOH/ SHJEFLO. Some of t-hese are explained by the next sect-ion.

Exactly the same criticisms cari be levelled at t,he work of RIJKS (1969) who calculated the water 10~s of an “old st,and of papyrus with a fair proportion of brown and dried out heads”. COOLEY and IDSO

(1980) also only calc.ulated the water loss for one mont,h and ‘fiddled’ the results from a true rat,io of 0.98 obtained from the act.ual result t,o a result of 0.85 by subtracting from the original ETh/EW ratio a fac.tor of reflectivity.

The above discussion is not meant to imply that the experiments detailed in 1 a11 bave good methodo- logirs. Rather it is the author’s intention to show that since it., is çlear t,hat hydrophyte evapotranspirn- tion cari exceed open water evaporation theoreticnlly, the variante in the aonclusions between 1 and II must be due to somet~hing suc11 as the fact,ors mentioned above.

MUNRO (1979) had a flawless experimental design, but tbe results t,hat he gained from a wooded swamp cont.aining mainly Cederus could net, apply to a11 other hydrophytes. TO draw this latter conclusion as COOLEY and IDSO did in 1980 is obviously incorrect.

Finally, a last possible cause in the differenc.e between the t.wo sets of workers could be the date at which the work was published. This is elaborated upon below.

EXPERIMENTS WITH LIMITEE) APPLICABILITY CURRF,NT GEOGRAPHICAL THINKING

These are t.he experiment.s detailed in II which drew conclusions far beyond the scope that their work actually allowed. LINACRE ef al. is a prime example of this. Although they uved a correct. experimental procedure t.heir experiments were for only 4 days at the erzd of fhe grorving season. As stated in 1, one of the key factors in det.ermining the ETh/EW ratio was state of growth. MCDONALD and HUGHES (1968) working on the same species and in a similar climate found the average ratio of ETb/EW in the low season of growth to be 0.5. This is very similar to the average of 0.6 derived by LINACIIE et al. Tt is thus dubious to claim (LINACRE et tzl. 1970, p. 385) that, “it is likely that, growth

If a11 the work listed in 1 and II are tabulated then grouped into year of publication and result, the theory that hydrophyte evapotranspiration was lower than open water evaporation started in the late 1940’s to early 196O’s- the time that PENMAN was presenting his views upon potent,ial evapo- transpiration. This idea of a theoretical maximum evapotranspirational loss calculat,ed from meteoro- logical variables seemed to mean t,hat it was not possible for hydrophytes to exceed it. Yet not only has this idea been shown to be untrue, but the whole concept of potential evapotranspiration is under attack, from the theories of causal evapotrans- piration developed by PRIESTLY and TAYLOR (197%)

Rev. Hfldrobiol. trop. 19 (J-4): 215-232 (7956).

A REVIEW OF HYUROPHYTE EVAPOTR,@JSI’IRATION 229

and from the idea of complement,ary relationship areal evapotranspirat,ion proposed by NORTON (1983).

IV. CONCLUSIONS

From the preceding sections, the following conclu- sions cnn be drawn:

1) Hydrophyte evapot.ranspiration cari on oc.casion esceed open wnt,er evaporation. This is due to the inc.reases of surface area, the differences in the aerodynamic: resistances of wat,er bodies and hydro- phyt,es, and the large transpirational coefficients that. hydrophyt es bave.

2) The conflict report,ed by several authors was due to poor experiment,al design, the drawing of conclusions from inadequate dat.a sets and the reluc.tance of some authors to acc.ept a value of hydr0phyt.e evapotranspiration above that of Penman’s potential evapotranspiration.

3) The exact magnitude of the ETh/EW rat,io dcpends upon the state of growth, species type, climate and density.

4) Above ground biomass and leaf area index cari be useful indicators of t,he ETh/EW rat,io.

It. is import.ant to compare theoe conclusions with those of the published review articles on hydrophyt,e evapotranspiration. This is dealt with below.

Review papers on hydrophyte evapotranspiration

There bave been t.hree review papers published on hydrophyte evapotranspiration (LINACRE, 1976; TDSO, 1951; INGRAM, 1983). LINACRE, described by INGRAM (1983) as carrying out a ‘careful’ review, posed (p. 332) the fundamental question: “The main concern has been to determine the evaporation rat,es of swamps for comparison with the ral.e of evaporation from a lalre in a similar environnient”, but avoided answering it by concluding (p. 344): “Compared with regional climate and local advection, the presence of swamp vegetation and its type have relatively minor infh1enc.e.s on evaporaCon rates, at least while the vegetation is growing.”

This is in cont,rast to LINACRE ut al. work of 1970 cited in II. Yet., despite this apparent neutrality on t.he exac.t magnitude of ETh/EW ratio expressed in the literature review, LINACRE (1976) was at pains to find fault in the work t,hat, gave a hydro- phytelopen water ratio greater than 1. LINACRE correctly noted t,hat most. of t,he tank experiments were grown in the fringes of swamps or lakes, and that. this would lead to misleading results due to the clothes line and/or oasis effects. These effect.s

and t,he errors that. c.ould arise have been dealt with in I and need no further comment here. LINACRE (1976) made few conclusions but it appears t.hat, he did recognize climate as an important factor controlling hydrophyte evapotranspiration.

The next review paper to be published was that of Idso in 1981. It was considerably shorter in lenght than that of LINACRE (1976), and started by quoting (IDSO, 1981, p. 47): “The presence of vegetation does indeed increase the evaporative wat.er loss [on an open water surface]“.

IDSO then used results gained by MUNRO (1979) and COOLEY and IDSO (1980), det,ailed in II, to introduce new coefficients for stomatal resistances into LINACRE’S 1970 equations SO t,hat the hydro- phytelopen water ratio was less t,han 1. IDSO concluded that for an extensive body of water the ETh/EW rat,io will be sbout unity, though for smaller bodies the ETh/EW ratio Will be great,er than unity. He also recognized the effect, of state of growth on the ETh/EW rat.io.

The last review to be published was that of INGRAM (1953). It, covered mainly bog and fen evapotranspiration. It is the most extensive of t.he three reviews. He concluded (p. 98) that: “Actual evapotranspiration from bogs is approximately equal to polential evapotranspiration, while on t.he limited evidence that from fens is greater.”

As far as swamps were conc.erned, INGRAM (p. 9% 99) concluded that: “The evidence presented SO far tends to make one cautious of accepting LINACRE’S 1976 conclusion that ta11 helophytes reduc.e lake evaporation (...) In their evaporative behaviour, reed swamps and allied systems therefore differ profoundly from the majority of true mires (...) and one is justified in regard@ the ta11 helophyte swamps as special cases.” INURAM (p. 99) then t,entativeIy put. forward the conclusion from the study of ths literature presented in the preceeding review t.hat, “(Hydrophyt.e/open water rat,io for t.all helophytes) falls bet.ween 1 and 1.4...in summer advection may cause a tempornry rise above 2.5”.

From the sections above, t,he conclusions drawn in III. 2 are backed up t.o some extent by the three reviews. Where there was a major diagreement, as in LINACRE’S 1976 critic of tank experiment.s, a justification of t.he resu1t.s was presented.

1 would like to thank my superviser, Dr. G. E. HOLLIS for ail his help. Dr. A. C. STRVENSON deserves special mention for t.he prompt and heIpfu1 sug.g&ions that he gave during the propcration of this manuscript.. The work has been carried out. as part of NERF studentship award GT4/84/AAPS/32.

Mrznzzscrit ncccpié pur le Comité de Rédaction le 12 fëvrier 1987

330 M. E. CRUNDWELL

REFERENCES

~~NDERP<JN (M.) and Ir~so (S. B.), 19S5. - Evaporative rate5 of floating and <n~rrgent, aquatic vegetation. 17th Conf. Agr. 6 For. Mrt. Scotsdale, Arizona, USA hlay 19% : 49-52.

B~IIEK (W.) and EGGLESMANK (R.), 1966. - Diskussions- beiirag. Dfsch. Gewasserkdl. Miit., 10 : 22-24.

BALEK (J.) and PERRY (J. E.), 1973. - Hydrology of sea- sonally inundated African headwat,er swamps. J. Hydrology, 19 : 227-249.

BAwNA (L. G.1, 1967. - Refnemcnt of paramet.ers for calculating evaporation from bogs on the basis of observations at bog stations. Trudy GGI, 145 : 69-96.

BAY (R. R.), 1966. - Evaluation of a evapotranspirometer for pcat bogs. Wafer Resources Research, 2 : 437-442.

BAY (R. R.), 1968. - Evapotranspiration from two peatland water shrds. In 14th Gen. Assem. Int. Un. Geod. Geophys. Berri.. Sept.-Oct. 1967 : 300-307.

BELOTSERKOVSKAIA (G. A.), LARGIN (1. F.) and Ronr~~ov (V. V.), 1969. - Investigation of surface and intornal evaporation on highmoor bogs, Trzzdy GGI, 177 : 16-3%

BE~TON (A. R.), J-MES (W. P.) and ROU~E (J. W.), 1978. - Evupotranspiration from water byacinth. Wuter Resources Rewarch Bull., 14 : 919-930.

BERNATO~TECZ (Sj, LESZCYN~KI (S.) and TY~ZNSI~~ (S.), 1976. - Ths influence of transpiration by emergent. plants on the water balance of lakes. ;Iquafic Bof., 2 : 17R-2%

BLMEY (H. F.1, 1957. - Evaporation study at Silver Lake in thr Mojave Desert, California. Trar!s. dm. Geophya. Lin., 3X : 209-215.

BLANE~ (H. F.), 1959. - hIont.hly coefficients for consumptive use of water formula and use of wat.er by phreatophytes. hlet. Meeting Tel Aviv May 6-9th 1959 : 690-699.

BLAKEY (H. F.j, 1961. - Consumptive use and waterwaste by phreatophytos. Am. Soc. Civil Eng. Proc-, 87 : 37-46.

BLANEY (H. F.) anl CHIULILE (W. D.), 1950. - Determining water requirements in irrigated areas from climato- l»giCal and irrigation data. Soi1 Cons. Serv. TP-96.

BLANEY (H. F.), EWING (P. A.), ISRAELSEN (0. W.), Ronn-ER (C.1 and SC~BEY (F. C.), 193% - TT\rdter utilization, Upper Rio Grande Basin. National Resources Com- mit.tee, Pt. 3, Feb. 19% : 19-87.

BLANEY (II. F.) and E’IVINC~ (P. A.), 1946. - Irrigat.ion prnctices and cOnSumptive use Of Wdbr in %liINS Valley, California. Calif. Sf. D~O. of Water Resourc.?s Bull., 52 : 61-66.

BLANEY (H. F.) and HANSON (E. G.), 1965. - Consumptive LIS~ and water requiremcnm in NP~ Mesico. Tech. Rep. N. Alex. Eny., 32 : 43 p.

BLANEY (II. F.) and X’I~RIN (1~. V.), 1942. - Evaporation and consumptive use of water empirical formulas. Trnns. Am. Geophys. lrn., 23 : 76-83.

Rev. Hydrobiol. trop. IY (3-1): ZlJ-~32 (1986).

BLANEY (H. F.), MORIN (K. V.) and Cnrnnrx (W. D.), 1942. - Consumptive use and water requirements, Report of the part.icipating agencies, Pecos river joint inves- tigation. Nat.ional Hesources Planning Board, June 1942 : 170-230.

BLANE~ (II. F.j and MUCKEL (D. C.), 1955. - Evaporation and evapotranspiration investigations in thc San Fransisco Bay area. Trans. Am. Geophys. Un., 36 : X13-820.

BI.ANEY (H. F.), TA~LIJR (C. A.) and YOZIN~ (A. A.), 1933. - Water losses under naturnl conditions in wet areas of Sout.hern California. Sf. Depf. of Pub. Wrks. Div. of IValer Res. Bull., 44 : 126 p.

B~RI~O~ (A. A,), 1965. - Climates of t.he USSR. Oliver and Boyd, Edinburgh 255 p.

BRASL~~SKII (A. P.j and VIRULINA (Z. A.), 1963. - Eva- poration norms from mater reservoirs. Tsrael program for scientitlc Translations (A. 1. Chebotarev, Ed.) .Jrrnsalem 1963 : 100-135.

BKEZNT (Cl,), MEHTA (1.j and SHARNA (R. K.), 1973. - Studies on evapotranspiration of some aquatic weeds. Wced Science, 21 : 197-201.

BULAKVO (A. G.), 1971. - The hydrology of marshes and marsh-ridden lands. Nature Res., 7 : 12-14.

CHRISTIANSEN (J. E.] and Low (J. B.), 1970. - Water requircmrnts of waterfo~l mdrshlands in Northern Dakota. 19th Utah diu. Fish und Game, Wildlife mnna- gemenf Inf. Bur. of Sport Fisheries and Wildlife.

COOL~~ (K. R.) and Inso (S. B.), 1980. - Effects of Lily pads on ovaporntion. IVrrfer Resources Reseurch, 16 : 605-606.

CLYMO (R. S.), 1973. - The growth of Sphagnum : some effects of environment. J. Ecolog~~, 61 : S-19-869.

DAVENPORT (D.C.), HUDSON (J. P.), 1967. - Changes in evaporxt.ion rates along a 17 km transect in the Sudan Gezira. Agriculfural Mefeorology, 4 : 339-352.

Davrs (S. N.) and DBWIEST (R. J. RI.), 1966. - Hydrogeology. Wiloy, New York.

DOL.&N (T. J.), HERM~%NN (A. J.), BAYLEY (S. E.) and ZOLTEI~ (J.), 198-t. - Evapotranspiration of a Florida, USA., wetland. J. Hydrology, 74 : 353-371.

DOORENBOS (J.) and PRUITT (W. O.), 1975. - Crop water requirements FAO Irrigation 6 Drainage paper 24, Rome, 144 p.

UYER (A. J.), HICI~S (B. B.) and K~irna (K. M.), 1967. - The Fluxat.ron. A ravised approach t.o the measurement Of eddy fluxes in the lower atmosphere. Qzrarl. J. Roy Mef. Soc., 93 : 501-508.

DYKYJOVA (D.) and KVET (.J.), 1978. - Pond Littoral Eçosystems. Springer-Verlag, Berlin, 164 p.

EGGLESMANN (R.), 1963. - Die potent.ielle nnd aktuelle Evaporation eines Seeklimahochmoors. IASH PubZ., 63 : 88-97.

A RETIEW OF HYDROPHYTE EVAPOTRANSPIRATION 231

EG~LESDIANN (R.), 1964. - Die Verdunst.ung der Hochmoore und derrn hydrographischer Einfluss. Dfsch. Gemasserkdl diiff., 8 : 138-147.

EISENHL~R (W. S.), 1966. - Water loss from a natural pond through transpiration of hydrophytes. ?k-ufer Resources Research, 2 : 443-453.

FIRBAS (F.), 1931. - Untersuchungen iibrr den Wasserhous- hdlt der Hochmoorpflanzen. Pringsheims Jahsb. Miss. Bof., 74 : 459-696.

G~TEWJ~D (J. S.), ROBIKSIJN (T. W.), COLBY (B. R.) and HALPENNY (L. C.), 1950. - Use of water hy bottom- land veget.at.ion in Lo\ver Safford Valley Arizona. USGS Il’afer szzpply paper, 1103, 245 1).

GEL’~WW (T. M.), 1964. - Evaporation from reed iields in water bodies. Trudy GGI, 92 : 152-17-1.

GESSNER (F.), 1959. - Hydrobotanik Bd. II, VEB Deutscher Verlag, Berlin, TO1 p.

GIBB (A ), 1956. - Water resources survey of Uganda : Applied to irrigat.ion and swamp reclamation. Szzpple- menfnry report on srvamp hyclrology : 9-12.

GILMAN (K.) and NEWSOPT (Ri. D ), 1983. - The Anglesey Wetland Study. Pul)l. Insf. Hydr. Wallingford, 113 p.

HALL (P. R.), RUTHERFORD (R. J.) and BYERS (G. L.), 1972. - The influence of a New England wctland on water quantity and quality. LJni. New Hanfs. Wafer Res. Centre Res. Depf Reporf, 4, 51 p.

RARBECI~ (G. E.), 1962. - A practical tleld technique for messuring resevior evaporation utlizing mass transfer t.heory. USGS Prof. Paper, 272.-E, 105 p.

HENJY (S.), 1969. - Coleanthus subt.ilis. Bohemoslou, 4 : 345-399.

tDS0 (S. B.), 1968. - Comments on a paper by R. Lee. Il’ater Resources Research, 4 : 665-666.

IDSO (S. B.), 1979. - Discussion on paper by Benton et al. Wafer Resozzrces Rasenrch Bull., 15 : 1466-1467.

IDSO (S. B.), 1981. - Relative rates of evaporative water losses from open snd. vegctation covercd wator bodies. Wafer Resources Researciz Bull., 17 : 46-48.

INGRM (H. A. P.), 1983. - Hydrology. In Mires : Smamp, Bog, Frn and Moor Ecosystems of t.he world. A. Gore (Ed.) Vol. 4, Elsevier, Amsterdam : 67-155.

JWANSSON (I.), 1974. - Hydrologiska undersokningar inom myrkomplexet Komosse. Rap. K. Tee. Hogsk. Self. Lnntmat. Insf. Kulturtek, 3 : I-161.

KIENDL (J.j, 1953. - Zum Wasserhalt des Phragmitetum Communis und Glycerirtum aquat.iacac. Berichte der deuischen Bof. Ges., 66 : 246-263.

KONSTANTINOV (A. R.), 1963. - Regime of ovaporation from the surface of ponds and reservoire. Trans. Ukrainiczn Sci. Hyd. Insf., 39 : 78-X9.

KROWLI~O~SKA (J.), 1971. - Transpiration of reed. Pol. Arch. Hydrobiol., 18 : 347-358.

KUZX~TSO~ (V. I.), 1949, cited by BERNATOWECZ (S.), LESZCYNS~I (S.) and TYCZ~SI<A (S.), 1976. - The intluenc~c of transpiration by emergent plants on the water balance of lakes. dqtzafic Bof., 2 : 275-288.

Kev. Hydrobiol. irop. 10 (3-8): .?lS-2.32 (1.986).

KUZNETSOV (V. I.), 195% - Cited in ‘Evaporation norms from water reservoirs, by A. P. Braslavskii and Z. A. Vikulinn. Israel program for scientiflc translations, Jerusalem, 1963 : 100-135.

KUZNETSOV (V. I.), 1953. - Cited in ‘Evaporation norms from mater reservoirs’. By A. P. Braslsvskii and Z. A. Vikulina. Israel program for scientific translations, Jerusalem 1963 : 100-135.

KUZXETWV (V. I.), 1954. - Effcct of water vegetation on evaporation. TrzzdZy GGI, 46 : 45-89.

KUZNET~~V (V. I.j, 1959. - 011 evaporation from overgrown reservoirs. Truns. .37U Ail-[Inion Hydr. Con~., Gir- dromcteoizdnt, Leningrad, USSR, 3.

KVET (J ), 1973. - Transpiration of S. Moravian phragmites communis. In Littoral of the Ncsyt Fishpond. (Ed. Kvet) Studie CSAV 15/1973 Publ. Academia Phraha.

KVET (J.), 1975. - Transpiration in seven plant species colonizing a ilshpond shore. &oZ. Plant, 17 : 434-442.

LEICIC (E.), 1929. - Zur îrage der Wasserbilanz von Hoch- mooren. Miff. Natzzrzviss. ver. Neu- Borpnmmern, 52-56 : 146-174.

LINACRE (E. T.), HICKS (B. B.), SAINTY (G. R.) and GR~U~E (G.), 1970. - Thr evaporat.ion from a swamp. AgriczzZ- fur& Mefeorology, 7 : 375-396.

LINACRE (E. T.), 1976. - Swamps. In Vegetation and the htmosphcrc, J. L. Monteith (Ed.) Academic Press, London : 329-347.

LOFTFIELD (J. V. G.), 192.1. - The behaviour of stomata. Curnegie Insf. R-a&. Pubi. 314.

R’ICDONALU (Cl. C.) antl HUC;HBS (G. H.j, 1968. - Studies of consumptive use of mater by phreat.opl1yte.s and hydrophytcs ncar Yuma, Arizona. LISCGS Prof. Paper, 486-F, 2.L 1,.

MI~AHID (1952). - Çited in PENMAN (H. L.), 1963. - Vegetation and Hydrology. CommonrveaZfh Bzzr. SoiZ Sri. Tech. Comm., 53, 124 p.

MORTON (F. I.), 1983. - Operational estimates of areal evapotrsnspiration and their signiflcance to the science and practico of hydrology. J. Hytfrologg, 66 : l-76.

MUCKEL (D.) and RLANEY (H. F.)? 1945. - Utilization of the waters of t.he lower San Luis Rcy Valley, San Diego county California. Calif. DirJ. of Irriy., SoiZ Cons serv.

MUNRO (D. S.), 1979. - Daytime enrrgy exchanges and evaporation from a wooded swamp. Xofer Resources Research, 15 : 1259-1265.

NEUHAUSL (R.), 1975. - Hochmoore am Teich Velkr Darka. Academia, Prague 267 p.

NICOLS (D. S.) andB~omiv (J. RI.), 10X0. - Evaporation from a sphagnum mass surface. J. Hydrology, 48 : 289-302.

Novtrcov.4 (Y. V.j, lc)G3. - Transpiration of hydrophytes and their part. in the total evaporat.ive discharge of water from the Krngirdam resrrvoir. Tr. Inst. Bof. <‘Mad. Nuuk. lin:. WR, 16 : 118-135.

PENFOLD (W. T.) and EARL~ (T. T.), 1948. - The biology of the water hyacinth. Ecoloyical Monographs, 18 : 447-472.

“32 31. E. CRUNUWELL

PENMAN tH. L.), 1963. - Vrgrtntion and IIydrology-. f:ommonurzc~lfh. Hur. Soi1 Pei. Tech. Comm., 53, 1?1 p.

PRIBAN (K.) and ONDOK (J. P.), 1978. - Microclimate and rvap»transpiration in two wetland grassland commu- nities. Folia Geobofanica ef Phyfofaxonomica., 13 : 113-128.

PHIH.~N (K.) an<l ONDOK (J. P.1, 1980. - Thc daily and seasonal course of evapotrarispirat.ion from a Centri european sedpr-grass marsh. J. Ecology, 6X : 517-559.

PRIESTL.EY (Cl. H. B.), TAYLOH (R. J.), 1972. - On the assessmcnt of surface heat. tlux and evaporation ueing large scale parameters. i\ConfhZy Weafher Reuiem, 100 : Yl-92.

PRYTZ (li.), l%%. - Drr Kreislauf des Wassers auf unhe- ruhrtem Hochmoor. Ingeniorz~idena~. SIîr. Ser. A., 33 : I-l’%.

RANTZ tS. E.), 1968. - A suggestcd method for estimating evapotranspiration by native phreatophytes. IJSGS Prof. Paper 600-D.

RIJRR (D. A.), 1969. - Evaporation from a Papyrus swamp. Qzzaf. .r. Roy. Nef. Soc., 95 : O-13-649.

R~BINWN (T. W.), 1952. - Phreatophytcs ancl their relation to xat.er in western IUnitcd Statee. Trcms. .Irn. Geophysical Z’n., 33 : 57-66.

Hono 1-J. C.), DOWNING (R. A.) and LA~ (F. M.), 1976. - Systematic Hy~lrolo~y. Newnrs-Buttrrworths, Londtm 376 p.

f?OGEH!S CH. FI.\ ancl Dr\vrs (D. E.I, 1972. - Nutrient removal hy mater hyacinth. Weed Science, 20 : 4’23-428.

13OMANOV (1’. v.1, 195.7. - Sludy of rvaporation from Sphagnunr bogs. Trzzdy GGI, 39.

ROhfANOV (y. y.), 1966. - Evaporation from boas in the European territory of the LJSSR. N. Kaner (Translater) Heimann (Editor). Israel Programme for scientitic translations, .Jerusatem, 183 p.

ROM,~XC-IV (V. V.1, 1968. - Hgdrophysics of bogs. N. Kaner tTranslator1 Heimann (Editor). Israel Proqamme for seientific translations, Jerusalrm, 299 p.

ROU~E tW. R.1, MILL~ (1’. F.) and STEWART (R. B.1, 1977. -, Evaporation in high latitudes. IVnfcr Resources Research, 13 : 909-9 1-t.

RLITHERF~RD {R. J.) and BYERS (G. L.), 1973. - Conservinq wetland lvater by supprcssing evaporat.ion. Canudion Ayric~~ffnraf Engineering, 15 : 9-l 1.

RYCHNIJVSKA (W. I, 19’7% - Transpiration of meadon communities. In Ecosystem Study on Grassland Biome in C:zcch«slnval<ia. Czcchoslovak IBP/PP-PT Report 2 : 37-13.

RYC~V~VSI~A (M.), KVET (J.), GLOSER (J.) aritl JARRLOVA (J), 1972. - Plant water relations in three zones of Grass- lands. Acfn. Sci. Nnt. Brno., 6 : I-38.

RY~RNWSI<A IM.I and Sarre cP.1, 1973. - Preliminary eva111~~tio1~ in txo Phra,amitcs st.antls. In Ecosystem

s111dy on \Vetland Biomo in Czechoslovakia. C&c. JBP/PP-PT Report 3 : 111-119.

ScrrrEmL (H.), %Hum (31.) and \VAIWE (R.j, 1970. - Wasserhaushull und Klima ainer kultivierten und unberuhrten Hochmoorflnche am Alpenrand. Gchriffelzr. Kuruf. Kulf. Hambrzry., CO : 1-171.

SRJEFLO (J. B.), 1968. - Ev;~potrnnspiration and the mater budgcl of prairie potholcs in N. Dakota. USGS Enfer Snpply paper .jS5-B, 49 p.

S~IID (rJ.1, 1975. - Evaporation from a reed swamp. ..J. Ecology, 63 : 299-309.

STEARNS (H. T.) and BRYAN (L. L.j, 1925. - Geology and water rcsources of thr Mud lake Basin, Idaho. LTSGS IVafer supply paper SGO-D, 160 p.

STURGES {II,. A.), 1968. - Evaporation at. a Wyoming mount.ain Log. ..J. Soi1 ruater Conserzrafion, 23 : 23-25.

THATCHER iK. M.r, 1921. - The eifect of peat on the trans- piration and gromt,h of se~~ral plants. J. Ecology, 8 : 39-59.

THOM (A. S.j and OLIVER (H. R.), 1977. - On Pcnman’s equation for estimating rqional evapotranspiration. Quart. J. Royal Mef. Soc., 103 : 345-347.

TIMMER (C. E.) and WmnoN (L. W.), 1967. - Evapotranspi- ration and pollution of wat.cr by water hyacint.h. Hyacinfh Control J., 6 : 34-37.

TINBER~EN (L.), l%O. - Observations sur l’cvaporation do la vé#tntion d’une tourbière dans les Haut.es-Fagnes de Belt$que. dlem. Suc. R. Sci. Liège, 4 : 21-76.

TZIRNER (S. F.j antl HALPESNT (L. C.), 19-11. - Ground mater inventory in the upper Gila river valley, New Mexico and Arizona. Trans. *lm. Geophys. Un., 22 : 738-744.

TLJSGHL (P.), 1970. - L)ie Transpiration van Phragmites communis Trin. im geschlossenen Bestand des Neu- siedler Sces. Wiss. Arb. Buryenfd., 64 : 126-186.

v.4~ BA~EL tC. I-I. M.), 196% - Further t.o the hydrolofic importance of transpiration contra1 by st0mnt.a. Wafer Resources Rrscnrch, .1 : 1387-1388.

\‘AN UER \YEERT (n.1 and K.~UERLING (G. E.), 1971. - Evapotranspiration of wat.er hyacinth. ,J. Hydrofoyy, 22 : mi -212.

WARMINCI (.J. E. B.I, 1895. - Plantesamfund, grundtrack af drn okologriske plantrgeografi. Philipsen, Copenhagen, Dennmrk.

WILLIAM~ (W. h.1, 19%. - Evaporation from a wreded watcr surface. IJnpublished Pli. D. Thesis, Dept. of Civil Eng. Univ. Nrmcastle Upon Tyw, Enpland, 317 p.

YOUNG (A. A.) anal BLANEY (II. F.,, 1942. - Use of water by native vcgrtalion. St, Cd. Pub. Wrks. Div. Wofer Hcs. Uull. 50, 154 p.

Z;zYI<OV, 1949. - Cited in GEL.‘BTJKH (T. M.i, 1964. - Evaporation from rred fJelds in water bodies. Tradg GGI, 92 : 152-17-1.

KW. Hydrohinl. frop. 19 (S-1): ‘>I.i-Z:>“> (1986).