J Grinùley

Estimation and mapping of evaporation

J. Grindley Meteorological Offce Bracknell, U. K.

SUMMARY: Most evaporation data in the United Kingdom are obtained using the Penman formula, which permits calculations of potential evaporation to be made. By making assumptions about‘ the way in which actual evaporation falls below the potential as soil moisture becomes limiting it is possible to calculate and map soil moisture deficit. Careful assessment of land use permits an assessment of actual evaporation and soil nioisturc deficit over any specific catchment area for use in water balance studies. A provisional map of potential evaporation has been prepared for England and Wales and a start has been made on the preparation of a map of actual evapora- iion using a 10 km network of grid intersections with an appropriate spectrum of land Lise for each tn tersection.

ESTIMA TI O N ET ETABLISSEMENT DE CAR TES R ~ S U M B : La plupart des données relatives à l’évaporation dans le Royaume Uni sont obtenues en utilisant la formule de Penman qui permet de faire des calculs dc l’évaporation potentielle. En faisant des hypothèses sur la manière dont l’évaporation actuelle est inférieure a l’évaporation potentielle quand l’humidité du sol apporte certaines limitations, il est possible de calculer et de cartographier le déficit en humidité du sol. Une détermination soigncuse de l’utilisation du sol permet unc évaluatjon dc l’évaporation actuelle et du déficit d’humidité du sol sur l’étendue d’un bassin spécifique, pour leur utilisation dans des études de bilans d’eau. Une carte de l’évapo- ration potentielle a été préparée pour l’Angleterre et le pays de Galles et on a commencé la pré- paration d’une carte de l’évaporation actuelle en utilisant un réseau avec des mailles de 10 km. avec un spectre approprié d’utilisations du sol pour chaque intersection.

DE L’E VA POR A TIO N

ESTíMACíÓN Y ELABORACJÓN DE MAPAS RELATIVOS A LA EVAPORACIÓN

RESUMEN: La mayor parte de los datos relativos a la evaporación se obtienen, en el Reino Unido, mediante la utilización de la fórmula dc Penman, que permite calcular la evaporaciun potencial. Estableciendo hipótesis sobre la forma en que la evaporación actual desciendc por debajo del potencial a medida que la liuniedad se limita, resulta posible calcular y elaborar del déficit de la humedad del suelo. Una evaluación cuidadosa del suelo permite calcular ia evaporación real y definir CI déficit de la humedad del suelo para una cuenca de captación determinada, lo que sirve para llevar a cabo los estudios del balance hidrológico. Se ha prcparado un mapa provisional de la evaporación potencial para Inglaterra y el país de Gales y se ha iniciado la preparación de otro para la evaporación real, utilizando una red de 10 kilómetros dc intersecciones en forma de cuadrícula, con una imagen adecuada de Ia utilización de la tierra para cada una de las interseccion. es.

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Estimation anil mapping of evaporation

1. JNTRODUCTION

Although a number of evaporation tanks and a few lysimeters are maintained in the United Kingdom, the main source of information concerning evaporation is based on estimates using the well-known Penman formula. This formula has received wide acclaim since its first publication [i] as one of the most soundly based methods of calculating evaporation using readily available meteorological data. Basically the Penman formula provides estimates of potential evapotranspiration, the amount of water which would be transpired by a vegetation cover when water is at all times freely available to the root system of the vegetation. By making certain assumptions it is possible to estimate the actual amount of evaporation which occurs when soil moisture limitation does impose a restriction on potential evaporation and, as an important corollory to this, to calculate and map soil moisture deficit.

2. THE CALCULATION OF POTENTIAL EVAPOTRANSPIRATION

The merit of Penman's method for calculating potential evapotranspiration lies in the combination of two of the classical approaches to the estimation of evaporation, the energy budget and the aerodynamic. By combining these two approaches, the require- ment for the measurement of the temperature of lhe evaporating surface, a measurement which is often difficult and rarely carried out on a routine basis, is eliminated. Although the formula is soundly based physically some empiricism is inherent in the

derivation of incoming and outgoing radiation and particularly in the aerodynamic terni. Much of the empiricism in the radiation terms can be eliminated if measurements of global or net radiation are available. The version of the formula used in the hydrometeorological branch of the United

Kingdom Meteorological Office is, with one important amendment, that published by Penman in 1963 [2]. The calculations are carried out by computer and the formula is programmed to calculate evapotranspiration from a vegetated surface, the albedo of which is taken to be 0.25. This albedo is generally considered representative of grass, most agricultural crops in most phases of their development and deciduous woodland in leaf (but not conifers). N o account is taken in the calculations of zero plane displace- ment or roughness length. The formula used is:

AH + YE, A+Y

E =

where d is the slope of the saturation vapour pressure curve at air temperature, y is the hygrometric constant (taken as 0.49 in the program, where temperature is expressed in "C and vapour pressure in nim Hg).

H = 0.75 R, (0.18 + 0.55;) - 0.95 Ta4 (0.10 + 0.90 ij (0.56 - 0.092JG). Here: R, is the anionnt of short wave radiation reaching the outside of the earth's atmo-

sphere expressed in mrn water equivalent; n/N is the ratio of observed hours of sunshine to possible number of hours; aTa4 is the theoretical black body radiation at mean air temperature T, (expressed in

degrees absolute) and e, is actual vapour pressure at mean air temperature Ta.

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J. Griiidlry

The coefñcieiit 0.95 is the important amendilient rcferred to and is intended to allow for vegetation not radiating as a perfcct black body [3]. Ea is given by the equalion Eo = 0.35 (e,-e,,) (I + ü/iOO) where e, is the saturation

vapour pressure at the mean air temperature, ed is the actual vapour pressure and U is the run of wind in miles per day. Beginning with I970 data, estiinates of potential evapotranspiration have been published

in the United Kingdom for about 80 stations which have meteorological elements measur- ed four times a day averaged to give mean values of each element for the month. Values are also calculated but not published for stations which measure the required meteorolo- gical elements less frequently than four times a day. The question of the extent to which time and frequency of measurement affect calculated values of evapotranspiration is a subject for investigation at the moment.

3. THE MAPPING OF POTENTIAL EVAPOTRANSPIRATION

The number of stations for which estimates of long-period average annual potential evapotranspiration are available is quite inadequate to permit a valid detailed map of the distribution of average annual potential evapotranspiration over the United Kingdom. This is particularly true for upland areas. The number of stations for which evapotrans- piration data are available is about 150 of which 75 per cent lie below 100 metres. There are only five stations situated at a height greater than 500 metres, the highest being at 847 metres. Nevertheless, for water balance studies and assessment of yield from catchment areas

it is necessary to make soine assessment of areal average evapotranspiration. To this end an altempt has been made to construct a map showing the distribution of average annual evapotranspiration over Britain, which, whatever its limitations, will permit a quantitative estimate of evaporation over an area. Such a map has been prepared, so far for England and Wales only, on a scale of 1 :625 000. The averages are mainly based on periods of not less than seven years within the period 1954-1966. The basis of the construction of the map was to apply multiple regression analysis

of evapotranspiration on altitude and any other factor which appeared significant, commonly grid northing of the United Kingdom National Grid. Separate analyses were carried out for River Authority areas (major river basins

such as the Thames, Great Ouse) and estimates of potential evapotranspiration were made, using appropriate regression equations, for each 10 km intersection grid. At boundaries between different areas agreement between estimates obtained for a common grid intersection by different regression equations was generally good, although in hilly areas discrepancies of up to 15 per cent were occasionally discerned. For about 4- of the country, a simple height relationship was used to estimate evapotranspiration for cach grid intersection. The relationship, obtained from reference4, allowed for a variation of .35 inch evapotranspiration per LOO feet in altitude, the quantity to be added or sub- tracted to the value for a mean county altitude depending on the difference between the mean county altitude and that of the grid intersection. The map (a simplified version of which is shown at figure i) must be considered as

tcntative at the moment, particularly for altitudes above 200 metres where little factual data exist and extrapolation is wide. It is susceptible to considerable modification and it is hoped improvement in accuracy as data from. more stations, partic~ilarly at greatcr altitudes, become available. Neverthelcss, it is hoped that for heights below 200 metres thc accuracy is within 10 per cent.

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Esrimaiion und mopping of euuporuiion

O F y 460

FIGURE I. Auernge unnuul porentiul eunporniion in rnilliineters for u surfuce with nlbeùu 0.25. Isopletlis ur 350, 400, 460, 500, 530, und 560 m m interuuls

4. THE CALCULATION OF SOIL MOISTURE DEFICIT

Soil moisture deficits are considered to have been set up when evapotranspiration exceeds precipitation and vegetation has to draw on reserves of moisture in the soil to satisfy transpiration requirements. Such deficits may occur in winter but sustained deficits do not usually arise until late spring. In drier areas of Britain, they commonly persist until autumn or early winter and occasionally, in exceptional cases, throughout the following winter. Often, the sustained period of soil moisture deficit starts in mid-month in spring and

allowance needs to be made for this by estimating evapotranspiration for the dry period.

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Adjustment is made for the rapidly increasing rates of avcrage evapotranspiration which occur in spring months. A typical exaniple of the building up of soil moisture deficits would be:

Month Rainfall Evapotranspiration Accumulated soil milliinetrei moisture deficiii

April 15th-30tli Nil 30 30 May 50 79 59 June 75 96 80

In assessing soil moisture deficits one needs to take account of the fact that vegetation has increasing difficulty in extracting moisture from the soil (because of increasing soil moislure tension) as accumulated potential evapotranspiration becomes greater than accumulated rainfall. The question of the manner and extent to which actual soil moisture deficit falls below

the potential is extremely controversial and has been discussed extensively in the liter ature. Some models postulate a divergence between potential and actual evaporation (and hence soil moisture deficit) at a rate proportional to the remaining sojl moisture as soon as a deficit is set up; other models propose a linear divergence, sometimes with threshold values at which the rate of divergence changes sharply and others suppose evaporation to continue at near potential rate until near wilting when actual evaporation drops to nil. Baier’ has a useful discussion of various models. The model used in the hydrological branch of the United Kingdom Meteorological

Office is that proposed by Penman6 where the concept of root constant is introduced. The root constant defines a specified amount of soil moisture (expressed in mm equivalent depth) which can be extracted from the soil without difficulty by a given vegetation on a given soil. A further 25 mm of moisture can be extracted with increasing difficulty and extraction thereafter becomes minimal. Typical figures for a root constant of 75 mm are:

Potential 100 125 150 175 250mm Actual 99 109 113 115 121mm

In the model used in the United Kingdom Meteorological Office for the preparation of soil moisture deficit maps it is assumed that each station is representative of a typical cross-section of catchment area and a system of variable storage basin accounting is adopted. It is assumed that 50 per cent of the area is covered by short-rooted vegetation (grass etc.) which can draw up to 75 nun of moisture from the soil before actual evapo- transpiration (and hence soil moisture deficit) starts falling below the potential, 30 per cent of the area is covered by long-rooted vegetation (trees, etc.) which can draw freely on 200 mm of soil moisture, and 20 per cent or the area is riparian where the permanent ground water is so near the surface that moisture is always freely available to rooting systems and evapotranspiration is never restricted. This, essentially, is the model adapted by Penman in his study of the water balance of the Stour Catchment [7]. Adopting the procedure for divergence between actual and potential evaporation

outlined in reference [6], the modified procedure for estiinating soil moisture deficits over a catchment area becomes:

~ 2 0 0 C,, Catchment area

C, Month Raiiifall (R) Evapotrans- R-E pirution (E) millimïlres

June 75 96 -21 80 80 80 64 July 13 I o0 - 87 167 167 I14 107 August 2 90 - 88 255 237 I22 132

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E.stimution und mapping of evuporrition

____ 3 4 5 6

ESTIMATED SOIL MOISTURE DEFICIT

0900 GMT 29..Qç.r~har.I969 .....

Areas with no sail moisture deficit are shaded. Remaining areas are bounded

FIGURE 2.

Here C, indicates the accumulated potential soil moisture deficit; C,,, the accumulated deficit over the zone with long-rooted vegetation and C,, the accumulated deficit over the zone with short rooted vegetation. The “areal” soil moisture deficit represents the average of the deficit over the C,, zone (50 per cent of the total area), the deficit over the Czo0 zone (30 per cent of the area) and zero deficit over the riparian zone (?O per cent of the area). W h e n rainfall exceeds evapotranspiration the soil moisture deficits are reduced by the

difference between the two amounts. For a number of years, end of month values were obtained by using inontlily totals of rainfall and evapotranspiration. It becaine apparent

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that such a procedure may seriously underestimate the accumulated potential soil niois- ture deficit if, for example, lhe bulk of a month's rainfall should fall in the last few days of the month following a very dry period in the earlier part of the month. Estimates are now made on a daily basis therefore. Additional complications arise with this system of daily accounting, particularly when the actual rate of evapotranspiration has fallen below the potential and alternating wet and dry spells occur. Such complications are dealt with in reference [8] where the estimation of soil moisture deficits is discussed more fully. It should be noted that excess rainfall over evapotranspiration will make an almost immediate contributioii to run-off over the riparian zone (C,,) and that the zone with

0900 GMT .12.Nnvernber..1969 ....-

Areas with no soil moisture deficit are shaded. Remaining areas are bounded by -.a, ~.55,'.2.,IB.4.jnch.!.ffl~ ._.__.._ _..__

1

9

7

I

7

FIGURE 3.

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Estitmiion and niupping of evuporotion

short-rooted vegetation (C75) in general will have deficits madc good (and hence be contributing to run-off) before zone C,,, . The United Kingdom Meteorological Office has been preparing regular estimates of

soil moisture deficits in map form and as tabular data ror River Authority areas for a number of years. This information, accompanied by a verbal description relating soil moisture variations to changes in weather patterns, has been distributed, usually twice a month, to interested autorities since September 1962. A sequence of maps is shown in figures 3-4. The maps are intended as an aid to authorities responsible for flood warning and to engineers interested in the delay which might be expected before appreciable contribution to surface and ground water reserves occurs.

FIGURE 4.

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5. THE DERJ.VATION OF ACTUAL EVAPORATION FROM SOIL MOISTURE DEFICIT

Actual evaporation can be derived quite simply for a month or any other specified period by subtracting accumulated soil moisture deficit at the beginning of the period from that at the end and adding to the difïerence any rainfall which has occurred during the period. Thus, for the example in the previous section, actual evaporation would be:

Month R PE R-E CP czoo c7 5 Area

mm mm mm SMD AE SMD AE SMD AE SMD AE

June 75 96 -21 80 96 80 96 80 96 64 96 July 13 100 -87 167 100 167 100 114 47 107 74 August 2 90 -88 255 90 237 72 122 10 132 45

Here R signifies rainfall; PE potential evaporation; SMD soil moisture deficit and A E actual evaporation. Actual evaporation over the short-rooted zone (C,,) is only 47 mm in July compared with a potential of 100 mm, and in August 10 mm compared with a potential of 90 mm. The areal actual evaporation is obtained by multiplying the value over the riparian zone, Cp, (always the potential rate) by 0.2, the value over the C,,, zone by 0.3, the value over the C,, zone by 0.5 and summing the products.

6. SOIL M O I S T U R E DEFICIT AND ACTUAL EVAPORATION OVER SPECIFIC CATCH MENT AREAS

From the outset it was realised that the generalised catchment model with three storage capacities, used in the preparation of the soil moisture deficit maps, could not be applied to specific catchment areas where widely different land use might apply. Accordingly a more detailed model has been developed which takes into account a

much wider spectrum of vegetation type and associated root constants. Advice was obtained from Lhe agricultural branch of the United Kingdom Meteorological Office on themaximum amount of water likely to be extracted before wilting became permanent. These amounts for a wide variety of crops are shown in table 1. These maximum deficits have been equated with root constants as follows:

Maximum soil moisture deticit Root constant mm m m

250 200 150 125 1 O0 50

200 I40 97 75 56 13

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Estimation und mapping of evuporution

TABLE 1. Land Utilization Survey

Crops Maximum Actual SMD millimetres

Wheat Barley Oats Mixed Corn Rye Potatoes, first early varieties Potatoes, maincrop and second earlies Beans and Peas Turnips, Swedes and Fodder Beet Mangolds Rape (or Cole) Kale Cabbage, Savoys and Kohl Rabi Mustard Other Crops Sugar Beet (for Sugar) Hops Orchards, grown commercially Orchards, not grown commercially Small fruit, grown commercially Vegetables; Hardy Nursery Stock; Flowers; Crops under Glass Small Fruit and Vegetahles, not grown commercially Bare Fallow Lucerne Clover, Sainfoin Temporary Grasses Permanent Grass Rough grazing Permanent woodland

200 200 200 200 200 1 O0 150 1 O0 150 150 125 125 125 125 125 150 200 225 200 150 I O0 I O0 25 150 1 O0 1 O0 125 50 250 but 125 on poor land

The amount by which actual evaporation falls below the potential for each root constant has been expressed in tabular form following the model in reference [6]. The root constants refer strictly to vegetation type and make no reference to soil type

except in so far as the soil type is reflected in the vegetation which it carries. Where soils are known to be poor allowance can be made by reducing the root constant. Certain seasonal restrictions are imposed for different vegetation cover and other

forms of land use are also taken into account as follows:

1. Bare ground, fallow. Evaporation is assumed to take place at the potential rate until 25 mm of moisture has been extracted when evaporation ceases until more rain has fallen.

2. Riparian area. Evapotranspiration always takes place at the potential rate.

3. Open water. Evaporation is considered to be 20 per cent higher than the potential evapotranspiration from a vegetated surface.

4. Urban areas. These are taken to be the areas coloured grey on the United Kingdom I :63 360 Ordnance Survey maps. It is assumed that 25 per cent of the area so obtained is water-proofed (pavements, roofs, etc.), the remaining 75 per cent consisting of gardens, parks, open ground etc. Evaporation from the water-proofed zone is consid- ered to occur only on days of rainfall and is taken to be the rainfall or the potential evaporation whichever is smaller.

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5. After harvest (convential data for United Kingdom, 1st August) it is assumed that evaporation from cereal areas takes placc as froni fallow ground.

6. For certain crops when ground cover is incomplete in the spring it is assumed that evaporation takes place at 4- the potential rate until cover is complete (conventional data Ist May). The proviso is made that up to 75 nim may evaporate at the potential rate .:as with bare ground).

7. Rough grazing. An upper limit of 50 mm extractable water is imposed.

8. A major weakness of the model is that no proviso is made for direct surface run-off. For much of lowland Britain the assumption of no surface run-off may give rise to no serious, error, except in intense thundery rain, but for mountainous Britain the assumption is clearly not tenable. For such areas an attempt has been made to allow for direct surface run-off by measuring the areal extent of precipitous (gradient greater than 1 in 3), vegetationless slopes; the information is obtained from Ordnance Survey maps. For such slopes it is assumed that all rainfall runs off and no evaporation occurs.

Typical examples of distribution of land use for (u) an area containing a good-deal of urbanisation in northwest England and (b) a predominantly farming area in East Anglia are:

Land use type Root constant Per ccnt of total area mm (4 íb)

Cereais, etc. Rootcrops, etc. Permanent grass, etc. Temporary grass, etc. Rough grazing Woodland Fallow Urban (waterproofed area) Riparian O p e n water

140 97 75 56 13 200

14.7 3.3 39.4 16.7 10.7 1.7 0.4 7.2 4. I 1.8

60.8 10.7 10.2 9.8 1 .o 2.6 0.9 0. I 3.9 0

100.0 100.0

In carrying out water balance studies of catchment areas, the qproach has been to use cclumpeá” estimates of rainfall aiid evaporation over the catchment area. Areal rainfall has beeil estimated on a daily or mcnthly basis by using a network of stations covering the ai-ca aad preparing arcal gcneraJ values by (o) ariíhmetic mean; (E) Thiessen weighting: (e) e?cpressing rainfall at each station as a percentage of station long-period coverage, meaning percentage values an¿ applying percentage mean to thc areal 1.sng-period average to obtaiii a general quantitative value for the area; (dj cartographicülly. Method (e) has been the one most generally used in the U iiited Kingdom Meteorological OfTice. Estimates of areal general potential evaporation ûre prepared by íìrst of all assessing

a long-period avcrage cartographically, using the distribution of i:;ûpleths of potential evaporation from the niap discussed in Section 3 and illustrated in figurc I. Areal potential evaporation for a given month is obtaincd by a method analogous to method íc) for obtaining areal gcncral rainfall i.e. a network of evaporation stations is used lo repre- sent the catchment area, potential evaporation at each station is expressed as a percentagc

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Estiinution unci mapping of evuporniioii

of station long-period average, percentage values are ineaned and the mean value applied to the areal average to obtain a general quantitative value. Potential evaporation is not calculated for periods shorter than a month. Where estimates of daily evaporation are required long period estimates of daily evaporation for an annual average of 500 mm obtained from empirical curve are increased in the ratio of calculated monthly potential ’ evaporation for the month to long period monthly evaporation. Estimates for any particular day may be in error using this method but errors are not likely to be accumu- lative and in any case must cancel out by end of month. With the data for rainfall and potential evaporation so obtained estimates of general actual evaporation over an area are obtained using the methods set out above with appropriate land use apportionments.

O 1 miles IO 20

SCALE I:b25,000 With National Grid

I I I I I I I I I

O

- Rivers -.- Boundary of River Authority

FIGURE 5. Mersey und Weauer River Authority. Average unnual potential eucrporulion (mt r i )

7. MAPPING ACTUAL EVAPORATION

The calculation of evaporation using “lumped” data masks a considerable areal variation in rainfall and evapotranspiration, the variation moreover acting in an opposite sense with rainfall in general increasing with altitude and evapotranspiration decreasing. A more desirable procedure would be to carry out calculations on a point basis using a

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J. Grindley

O 2 0 SCALE 1:625.000 W t h National Grid I miles

io

I I I I I I I I I I 40 I

- Rivers -.- Boundarg of River Authority FIGURE 6. Mersey und Weuuer River Authority. Averuge unnuul ucfucrl euaporation (nini)

close grid network, each point having its appropriate land use value or spectrum of values. Isopleths of actual evaporation would then be drawn and the point values integrated (by planimeter) to obtain an areal value of actual evaporation. Such a proceûure has been carried out for one River Authority area and a map of

actual evaporation constructed for points at the intersection of a 10 km grid network. Average annual rainfall at each point was obtained from survey maps and average annual potential evaporation from the larger scale version of the map illustrated at figure 1. A spectrum of land use was made for an area of 4 km2 surrounding each major (10 km) grid intersection. The distri bution of derived potential and actual evaporation arc shown in figures 5 and 6 respectively. The areal estimate of annual evaporation over a ten-year period using lumped data for the catchment was 488mm and that using integrated point values was 480 mm. Average annual potential evaporation was 531 mm. It is hoped ultimately to extend the map of long-period actual evaporation over the

whole country. When such a map is available average annual actual evaporation at each 10 km grid intersection can be substracted from long period rainfall to provide an esti- mate of annual run-off at the point. A map of the derived distribution of annual run-off can then be prepared.

21 2

The prediction of actual euuporation it2 semi-arid arem

REFER ENCES

1. PENMAN, H.L. (1948): Natural evaporation from open water, bare soil and grass, Proc. R.

2. PENMAN, H.L. (1963): Woburn irrigation 1951-1959: (I) Purpose, design and weather, J. Agric.

3. BUDYKO, M. I. (1956): The heat balance of the earth’s surface. Leningrad, Gidrotneteoizduf. 4. Ministry of Agriculture, Fisheries and Foods, (I 967): Technicol Bulletin No. 16, Potential transpiration, London, Her Majesty’s Stationery Office.

5. BAIER, W. (1967): Relationships between soil moisture actual and potential evapotranspiration, Proc. of Hydrology Syitiposiirm No. 6 heldat Utliuersity of Snskatcheiuun on 15 und 16 Noueniber 1967.

6. PENMAN, H.L. (1949): The dependence of transpiration on weather and soil conditions, Jnl. Soil Sci. Oxford, 1, pp. 74-89.

7. PENMAN, H.L. (1950): The water balance of the Stour catchment area, Jnl. Innstn. Waf. Etigrs., London, 4, pp. 457-469.

8. GRINDIXY, J. (1967): The estimation of soil moisture deficits, Mel. Mug. London, vol. 96, pp. 97-108.

Soc., London, A, 193, pp. 120-145.

Sci. Cambridge, 58, pp. 343-348.

The prediction of actual evaporation in semi-arid areas

J. V. Sutcliffe, Institute of Hydrology, Wallingford and C. H. Swan, Sir Alexander Gibb and Partners, London

SUMMARY: A study of the world water balaiice based on rainfall and potential evaporation alone will not lead to a prediction of actual evaporation in semi-arid areas. O n the other hand, it would be an immense undertaking to measure the total runoff from a region to deduce empirically actual evaporation by comparison with rainfall. It is, therefore, essential also to consider the role of the soil moisture regime in dividing rainfall between actual evaporation and runoff. The dangers of the simple comparison of rainfall and potential evaporation are illustrated by

comparing the two sidcs of the Alborz range in Iran. The southern slopes receive moderate winter precipitation but because of this seasonal distribution the actual evaporation is low compared with he high annual potential evaporation; the runoff is relatively high. The northern slopes facing the Caspian Sea receive summer rainfall as well but the actual evaporation is high and the runoff low. The clue to this apparently anomalous situation is the forest on the Caspian slopes which transpires at the potential rate where the perennial rainfall allows it to survive. A study which took account of the limited soil moisture storage and the seasonal precipitation pattern would predict this result at least in qualitative terms. While global and regional rainfall maps, together with maps of potcntial evaporation, are an

essential first step towards a water balance, any realistic study must take account of the existing information on regional runoff, soil depths, and vegetation.

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