H

02000-

A LAND/OCEAN

A LAND/OCEAN

02000-

l500 l000 500

Figure 4.5 Zonal distribution of mean evaporation (mm/year): (A) annually for the ocean and land surfaces, and (B) over the oceans for December to February and June to August.

l500 l000 500

Figure 4.5 Zonal distribution of mean evaporation (mm/year): (A) annually for the ocean and land surfaces, and (B) over the oceans for December to February and June to August.

Sources: After Peixoto and Oort (1983). From Variations in the Global Water Budget, ed. A. Street-Perrott, M. Beran and R. Ratcliffe (1983), Fig. 22. Copyright © D. Reidel, Dordrecht, by kind permission of Kluwer Academic Publishers. Also partly from Sellers (1965).

Figure 4.6 Mean evaporation (mm) for January and July. Source: After M.I. Budyko, Heat Budget Atlas of the Earth (1958).

the wind is generally associated with the advection of unsaturated air, which will absorb the available moisture.

Water loss from plant surfaces, chiefly leaves, is a complex process termed transpiration. It occurs when the vapour pressure in the leaf cells is greater than the atmospheric vapour pressure. It is vital as a life function in that it causes a rise of plant nutrients from the soil and cools the leaves. The cells of the plant roots can exert an osmotic tension of up to about 15 atmospheres upon the water films between the adjacent soil particles. As these soil water films shrink, however, the tension within them increases. If the tension of the soil films exceeds the osmotic root tension, the continuity of the plant's water uptake is broken and wilting occurs. Transpiration is controlled by the atmospheric factors that determine evaporation as well as by plant factors such as the stage of plant growth, leaf area and leaf temperature, and also by the amount of soil moisture (see Chapter 12C). It occurs mainly during the day, when the stomata (small pores in the leaves), through which transpiration takes place, are open. This opening is determined primarily by light intensity. Transpiration naturally varies greatly with season, and during the winter months in mid-latitudes conifers lose only 10 to 18 per cent of their total annual transpiration losses and deciduous trees less than 4 per cent.

In practice, it is difficult to separate water evaporated from the soil, intercepted moisture remaining on vegetation surfaces after precipitation and subsequently evaporated, and transpiration. For this reason, evaporation, or the compound term evapotranspiration, may be used to refer to the total loss. Over land, annual evaporation is 52 per cent due to transpiration, 28 per cent soil evaporation and 20 per cent interception.

Evapotranspiration losses from natural surfaces cannot be measured directly. There are, however, various indirect methods of assessment, as well as theoretical formulae. One method of estimation is based on the moisture balance equation at the surface:

This can be applied to a gauged river catchment, where precipitation and runoff are measured, or to a block of soil. In the latter case we measure the percolation through an enclosed block of soil with a vegetation cover (usually grass) and record the rainfall upon it. The block, termed a lysimeter, is weighed regularly so that weight changes unaccounted for by rainfall or runoff can be ascribed to evapotranspiration losses, provided the grass is kept short! The technique allows the determination of daily evapotranspiration amounts. If the soil block is regularly 'irrigated' so that the vegetation cover is always yielding the maximum possible evapotranspiration, the water loss is called the potential evapotranspiration (or PE). More generally, PE may be defined as the water loss corresponding to the available energy. Potential evapotranspiration forms the basis for the climate classification developed by C. W. Thornthwaite (see Appendix 1).

In regions where snow cover is long-lasting, evaporation/sublimation from the snowpack can be estimated by lysimeters sunk into the snow that are weighed regularly.

A meteorological solution to the calculation of evaporation uses sensitive instruments to measure the net effect of eddies of air transporting moisture upward and downward near the surface. In this 'eddy correlation' technique, the vertical component of wind and the atmospheric moisture content are measured simultaneously at the same level (say, 1.5 m) every few seconds. The product of each pair of measurements is then averaged over some time interval to determine the evaporation (or condensation). This method requires delicate rapid-response instruments, so it cannot be used in very windy conditions.

Theoretical methods for determining evaporation rates have followed two lines of approach. The first relates average monthly evaporation (E) from large water bodies to the mean wind speed (u) and the mean vapour pressure difference between the water surface and the air (e - e ) in the form:

where K is an empirical constant. This is termed the aerodynamic approach because it takes account of the factors responsible for removing vapour from the water surface. The second method is based on the energy budget. The net balance of solar and terrestrial radiation at the surface (Rn) is used for evaporation (E) and the transfer of heat to the atmosphere (H). A small proportion also heats the soil by day, but since nearly all of this is lost at night it can be disregarded. Thus:

n where L is the latent heat of evaporation (2.5 X 106 J kg-1). Rn can be measured with a net radiometer and the ratio H/LE = B, referred to as Bowen's ratio, can be estimated from measurements of temperature and vapour content at two levels near the surface. B ranges from <0.1 for water to >10 for a desert surface. The use of this ratio assumes that the vertical transfers of heat and water vapour by turbulence take place with equal efficiency. Evaporation is then determined from an expression of the form:

The most satisfactory climatological method devised so far combines the energy budget and aerodynamic approaches. In this way, H.L. Penman succeeded in expressing evaporation losses in terms of four meteorological elements that are measured regularly, at least in Europe and North America. These are net radiation (or an estimate based on duration of sunshine), mean air temperature, mean air humidity and mean wind speed (which limit the losses of heat and vapour from the surface).

The relative roles of these factors are illustrated by the global pattern of evaporation (see Figure 4.6). Losses decrease sharply in high latitudes, where there is little available energy. In middle and lower latitudes there are appreciable differences between land and sea. Rates are naturally high over the oceans in view of the unlimited availability of water, and on a seasonal basis the maximum rates occur in January over the western Pacific and Atlantic, where cold continental air blows across warm ocean currents. On an annual basis, maximum oceanic losses occur about 15 to 20°N

and 10 to 20°S, in the belts of the constant trade winds (see Figures 4.5B and 4.6). The highest annual losses, estimated to be about 2000 mm, are in the western Pacific and central Indian Ocean near 15°S (cf. Figure 3.30); 2460 MJ m-2 yr-1 (78 W m-2 over the year) are equivalent to an evaporation of 900 mm of water. There is a subsidiary equatorial minimum over the oceans, as a result of the lower wind speeds in the doldrum belt and the proximity of the vapour pressure in the air to its saturation value. The land maximum occurs more or less at the equator due to the relatively high solar radiation receipts and the large transpiration losses from the luxuriant vegetation of this region. The secondary maximum over land in mid-latitudes is related to the strong prevailing westerly winds.

The annual evaporation over Britain, calculated by Penman's formula, ranges from about 380 mm in Scotland to 500 mm in parts of south and southeast England. Since this loss is concentrated in the period May to September, there may be seasonal water deficits of 120 to 150 mm in these parts of the country necessitating considerable use of irrigation water by farmers. The annual moisture budget can also be determined approximately by a bookkeeping method devised by C.E. Thornthwaite, where potential evapotranspiration ins 6

mm 150

mm 150

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