Forest Hydrology and Watershed Management -HydrologieForestiere et Amenagement des Bassins Hydrologiques(Proceedings of the Vancouver Symposium, August 1987; Actesdu Colloque de Vancouver, Aout 1987):IAHS-AISHPubl.no.167,1987.

Use of Iysimeter measurements for estimationof the evapotranspiration of pine forest byBagrov's method

A. HELBIG

Humboldt-Universitat zu Berlin, Sektion Physik,

Bereich Meteorologie und Geophysik, Berlin,DDR-1l62

ABSTRACT With a nonweighing lysimeter (surface area660 m2, soil depth 4 m) near Magdeburg, GDR, the com-ponents of water balance runoff RK and precipitation PKwere measured for a pine forest (Pinus silvestris, 440trees) since 1974, and yearly mean values of the evapo-transpiration ETK were estimated. With advancing yearsthe evapotranspiration ETK increased from 293.6 mm year-l(3 years old) to 528.8 mm year-l (14 years old).Simultaneously the evapotranspiration ETG for grassvegetation were measured by means of 12 weighing lysi-meters. Up to the age of seven for the pine forestETK<ETG was observed. Combined with the computation ofthe potential evapotranspiration ETp after Turc/Ivanov,the "effectiveness parameter nK" after Bagrov based onquotients of smoothed monthly sums PK/ETp and ETK/ETp wasestimated. Knowledge of these nK values allowed thecalculation of yearly evapotranspiration sums of pineforests from P and ETp data taking into considerationtheir age.

L'utilisation de mesures Iysimetriques dans I'estimationde l'evapotranspiration d'une pinede seion Ia methode deBagrovRESUME La precipitation PK et l'ecoulement RK furentmesures, et l'evapotranspiration moyenne annuelle ETK futestimee dans une pinede (Pinus silvestris, 440 tiges)pres de Magdeburg, RFA, au moyen d'un lysimetre sanspesee (669 m2 d'aire, 4 m de profondeur de sol). Avecles annees, ETK est passe de 293.6 mm (3 ans) a 528.8 mm(14 ans). En meme temps, l'evapotranspiration de lacouverture herbacee, ETG, a ete mesuree au moyen de 12lysimetres a pesee. Jusqu'a l'age de sept ans, on aobserve ETK<ETG' Le "parametre d'efficacite" nk, d'apresBagrov, fut estime a partir de l'evapotranspirationpotentiells ETp, d'apres Turc/Ivanov, et des rapportsmensuels PK/ETp et ETK/ETp. Ces valeurs de nk permettentIe caiculde I'evapotranspirationannueIIede pinedes a

partir de leur age, et des donnees de P et de ETp.357

358 A.Helbig

BAGROV'S RELATIONSHIP AND EVAPOTRANSPIRATION

The regional evapotranspiration is determined by soil structure andvegetation characteristics as well as by climatological conditionssuch as the energy supply and precipitation which limit the evapora-tion process. For a catchment area and a sufficiently long timeinterval the following boundary conditions are valid:

ET + P if ETp + ro (1)

ET + ETp if P + ro (2)

with ETp potential evapotranspiration and P precipitation.Bagrov (1953) found a relationship for calculating the mean

regional evapotranspiration from precipitation sum and potentialevapotranspiration which fulfills the boundary conditions (1) and(2) as well as taking into account the influence of soil and vegeta-tion by means of an "effectiveness parameter nO:

d ET / d P = 1 - (ET/ETp)n (3)

The numerical integration

JET -- -- n

p = a ET / 1 - (ET/ET )0 ~ (4)

yields the solutions of equation 4 for any n. The results are drawnin Fig.l for some values of n (after Glugla et al., 1982).

1.0

1.0 2.0 3.0 {O

Ef

n;,

0.5

p

rr;

FIG.1 Relationship between evapotranspiration ET,potential evapotranspiration ETp, precipitation P and

effectiveness parameter n after Bagrov.

Lysimeter measurements of forest evapotranspiration 359

Where both the yearly sums of potential evapotranspiration andprecipitation and the effectiveness parameter n are known, theactual evapotranspiration is calculated using this information asinput data in Fig.l.

In GDR the Bagrov method has been widely applied in water yieldestimations (Glugla et ai., 1976; Glugla & Enderlein, 1975). If theinfluences form soil type and vegetational cover are constant intime, than the parameter n approaches fixed values as e.g. n = 0.3

for unvegetated sandy soil, and n = 0.9 for agricultural plants(Glugla et ai., 1982). In contrast, for forest areas n depends onthe age of stands.

In GDR 27.4% of the territory is covered by forests which modifythe water balance over wide scales of time and space. Forestry act-ivities such as clear-cutting and reforestation cause significantregional changes energy and water balance and of the biomass. Theseresult in altered values of the effectiveness parameter n.

WATER BALANCE MEASUREMENTS WITH LYSIMETERS

In order to estimate variations in water balance, lysimeter measure-ments from the period 1974-1985 which have been carried out with anonweighing lysimeter were analyzed to ascertain the impact of agrowing pine forest on water balance. Studies using 12 weighinglysimeters, covered by grass vegetation, were analyzed to assess thewater balance in clear-cutting periods.

The lysimeter devices are situated north of Magdeburg in the"Letzlinger Heide".

The nonweighing lysimeter has a closed soil volume with 660 m2surface area and 4 m soil depth. In the centre of lysimeter asampling device has been installed to catch the percolating waterunder the soil layer (Glugla et ai., 1982). In spring 1970, one-year-old pines (Pinus siivestris) were planted around the lysimeter,and in spring 1973 two-year-old pines were planted in the lysimeterarea. The pine stands now cover an area of 5.2 hectares. In theperiod 1976-1980, 575 trees were growing in the lysimeter areainitially; this number gradually reduced to 549. In conforming withcommon practice and growth stage, the number of trees was reduced by20% to 440 trees in June 1980. In 1985 the mean height of trees was5.20 m, the mean stem diameter was 6.1 cm at a height of 1.3 m.Today there is no groundflora in the lysimeter area, in contrast tothe surroundings which have a dense grass cover.

The 2 m thick soil monoliths of the 12 weighing lysimeters are1 m2 surface area each, and are covered by grass, which is typicalof clear-cut areas. At the beginning of the research period thegrass cover consisted of Dechampsia fiexuosa which became mixed withCaiamagrostis fiexuosa. Both grass species are common in the vicin-ity of lysimeters. Since 1980, heath has grown in one of the lysi-meters. If not declared otherwise the following data are meanvalues of all 12 lysimeters. More details about the devices werepublished previously by Kortum & Helbig (1980).

At the research site only precipitation is measured; other meteo-rological data are.taken from the nearest weather station(Magdeburg) of the GDR Meteorological Office.

360 A.Helbig

The potential evapotranspiration needed in Bagrov's method iscomputed using the relationship after Turc (Dyck et al., 1980) ifthe daily mean air temperature ~5°C, otherwise using formula ofIvanov (Wendling & Muller, 1984), using daily meteorological data.

The daily global radiation K+ is estimated from relative sunshineduration (SD) in the following manner (Schonermark et al., 1973):

K+ = K+R . (0.21 + 0.61 . SD) (5)

with K+R global radiation in a Rayleigh-atmosphere.All sums of potential evapotranspiration computed in this way

were increased by 10% (Budyko-correction).Measured directly at the nonweighing lysimeter are the precipita-

tion PK (raingauges at 0 m height to eliminate wind errors) and thewater percolated RK through the soil layer. At the weighing ly-simeters, as well as the corresponding values PG and RG' the changein soil moisture storage ~SG and the evapotranspiration ETG (Table1) are obtained.

TABLE 1 Yearly sums (mm) of potential evapotranspiration and waterbalance components for pine stand (index K) and grass vegetation(index G)

Year ETG

197419751976197719781979198019811982198319841985

x

ETp

608.9660.6675.0573.5568.2580.9594.2595.7703.7666.6568.7617.5

617.8

PK

630.7443.5427.3703.5577.3727.0624.2762.4398.6615.0591.5569.8

589.2

RK

277.7341.9167.3239.3156.1267.2134.6141.8122.598.071.132.6

162.5

PG

640.1472.1450.7754.2590.7740.7679.3768.6427.7606.8627.7560.1

609.9

RG

251.4163.4151.8354.3234.1419.0330.8396.1192.3274.7341.6238.1

279.0

Since the general water balance equation

ET = P - R - ~

!:!.sG

29.7

-49.3

3.2

39.2

26.4

-17.4

5.515.3

-61.2

37.8

0.9

5.7

3.0

359.0357.9295.6360.7330.2339.1343.0357.2296.5294.3285.2316.2

327.9

(6)

is not soluble for the nonweighing lysimeter because ~SK data aremissing, the change in storage has been approximated from ~G ofweighinglysimetersusing~K = 2 . ~G' In this way the yearlysums of pine forest evapotranspiration ETK are obtained in suffi-cient accuracy (Table 2).

The quotients from ETKcumulative sums, indicatestands. Beginning at thespiration exceeds that of

and evapotranspiration ETG' related toc1early the effect of the growing pine

age of seven the pine forest evapotran-

the grass cover progressively (Table 2).

ESTIMATION OF EFFECTIVENESS PARAMETERS

In order to estimate the effectiveness parameter n for grass andheath (index H) as well as for pine forest, the time series ofmonthly sums PK' RK' ETG and ETH has been submitted to a low-pass-filtering procedure with a triangle-type filter (interval 37months).

Assumingthat for this averageinterval~SK ~ 0, the smoothedmonthly sums of pine forest evapotranspiration ETK as differencesbetween the filtered data PK and RK are obtained.

Finally, the effectiveness parameters nK, nG and nH for pineforest, grass and heath, respectively, are derived by means of thediagram constructed by Glugla & Tiemer (1970) (Fig.l), and fromquotients ET/ETp, P/ETp, computed from filtered data, for eachvegetation type. The results are shown in Fig.2.

The parameter nK is closely correlated with the age of trees,whereas nG values for grass are scattered between 0.5 and 0.6 and nHfor heath increased up 1.4. The trend of increasing nK indicatesthe effect of impact in 1982 very well. Starting at a value nearthose for bare sandy soil, the effectiveness parameter had reachedthe first maximum with nK = 4.0 for the original density of pinestands. For the present situation (0.67 trees m-2, 6700 trees per

hectare and age 14) nR = 4 is typical.In addilion to the previous computations, the effectiveness

parameters nK and nG are estimated from three-year running mean

Lysimeter measurements of forest evapotranspiration 361

TABLE 2 Yearly evapotranspiration sums {mm} of a growingpine stand and grass vegetation and smoothed values ofeffectiveness parameter nK, nG respectively

Year Age ETK ETG LETKfLETG nK nG

1974 3 293.6 359.0 0.82 -1975 4 300.0 357.9 0.83 0.60 0.831976 5 253.6 295.6 0.84 0.69 0.781977 6 385.8 360.7 0.90 0.74 0.681978 7 368.4 330.2 0.94 1.23 0.631979 8 494.6 339.1 1.03 1.70 0.661980 9 478.6 343.0 1.08 2.80 0.581981 10 590.0 357.2 1.15 2.50 0.611982 11 398.5 296.5 1.17 2.20 0.551983 12 441.4 294.3 1.20 2.40 0.531984 13 518.6 285.2 1.25 2.90 0.521985 14 525.8 316.2 1.28

362 A.Helbig

yearly sums of water balance components in the same manner as de-scribed above and given in Table 2.

n

1976 1978 '1980 '1982 198' . Jahr

'.0

3.0

2.0

1.5

1.0

0.8

0.60.5

0.'

FIG.2 Effectiveness parameter n for pine forest (.J,grass vegetation (oj and heath (+J as function of time.

These data on effectiveness parameter n after Bagrov permitcalculation of the evapotranspiration sums of pine forests of vary-ing ages and of clear-cut areas where grass grows, based on meanyearly sums of precipitation and potential evapotranspiration fromthe region concerned.

ACKNOWLEDGEMENTS The work was carried out in cooperation with theWater Management Authority Magdeburg, GDR.

REFERENCES

Bagrov, N.A. (1953) 0 srednem mnogoletnem isparenii s poverchnostisusi (On multi-year average of evapotranspiration from landsurface). Met. Gidrol. No. 10, 20-25.

Dyck, S. (ed.) (1980) Angewandte Hydrologie (Applied Hydrology) vol.2nd ed, VerI. Wilhelm Ernst & Sohn, Berlin, Munchen.

Glugla, G. & Tiemer, K. (1970) Ausarbeitung eines Berech-nungsverfahrens von Mittelwerten der Grundwasserneubildung nachder Formel von BAGROV unter Verwendung der EDV (Treatise oncomputation of mean values of groundwater recharge after BAGROV'sformula by help of electronic computers) Report no. 10/70 rfWBerlin, GDR, unpubl.

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Lysimeter measurements of forest evapotranspiration 363

Glugla, G. & Enderlein, R. (1975) Zur Bestimmung des innerjahrlichenGanges der Grundwasserneubildung (On the estimation of theinnerannual variation of groundwater recharge).Wasserwirtschaft/Wassertechnik 25(12), 404-408.

Glugla, G., Enderlein, R. & Eyrich, A. (1976) Das programm RASTER -ein effektives Verfahren zur Berechnung der Grundwasserneubildungim Lockergestein (programm RASTER - an effective method for

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Glugla, G., Fischer, D., H6hne, U., Kortfim,F. & Helbig, A. (1982)Lysimeteruntersuchungen in der Letzlinger Heide - wichtigerBeitrag zur Bestimmung der Wasserressourcen bewaldeter Gebiete(Lysimeter investigations in the Letzlinger Heide - an importantcontribution for estimation of water resources on woody areas).Wasserwirtschaft/Wassertechnik 9, 319-322.

Kortfim,F. & Helbig, A. (1980) Zur Auswertung von Lysimetermessungen(On the interpretation of 1ysimeter measurements). Z. Met.30(2), 107-111.

Sch6nermark, E.v., Wuchold, H. & Freydank, E. (1973) MethodischeUntersuchungen der Berechnung der Globalstrahlung ffirdas Gebietder DDR (Methodical investigations on computations of globalradiation for the territory of GDR). Z. Met. 23(9-10), 255-267.

Wendling, U. & MUller, J. (1984) Entwicklung eines Verfahrens zurrechnerischen Abschatzung der Verdunstung im Winter (Treatise ofa method for numerical estimation of the evapotranspiration inwinter). Z. Met. 34(2), 82-85.