Various Evaporation Rates

Lake Evaporation Rate Eo

So far we have mainly considered the simplest case, that of evaporation from an extensive water surface, i.e. the lake evaporation rateEo. It is useful to consider this as the surface has a more or less standard roughness, albedo and wetness, and reservoirs are important to us. But it is difficult to measure Eo accurately. Also, there is the complication that evaporation from the upwind edge of a large lake moistens the air, so that evaporation downwind is reduced.

Evaporation from a choppy sea is complicated by the effects of dissolved salt and spray-droplet curvature, mentioned in Section 4.3, and the effect of a surface roughened by waves.

Pan Evaporation Rate Ep

The pan evaporation rate Ep is what is measured with an evaporimeter, such as the Class-A pan (Figure 4.6). It is simple to measure, but is not the same as Eo. The ratio Eo/Ep is called the pan coefficient. It is normally less than unity, i.e. a pan loses more water per unit area than a lake, because a pan gains extra heat through the base and sides during the day, so that the water's vapour pressure is increased. The ratio for a ClassA pan varies widely, according to the weather and season, e.g. measurements at Lake Eucumbene in the Snowy Mountains in Australia gave monthly mean values of 0.6 in summer and 1.8 in winter. Annual mean values for eight reservoirs in Australia ranged from 0.63 to 0.94, so it is not possible to infer Eo accurately from Ep. Despite this, a coefficient of 0.7 is often taken as typical for the Class-A pan.

Potential Evaporation Rate Et

The potential evaporation rate applies to a wellwatered surface, as measured by a drainage lysimeter, for example (Section 4.4). It combines evaporation from the ground with evaporation from the plants. The latter is part of the process of transpiration, the flow of liquid water from soil to roots, through the plant ('transpiration' means 'breathing through'), and then evaporation from within the leaves into the atmosphere. The combination of soil evaporation and plant evaporation is sometimes called evapotranspiration, but this is clumsy, unnecessary and often wrongly taken to imply that evaporation from vegetation is somehow different from other evaporation. To distinguish 'plant evaporation', 'soil evaporation' and 'crop evaporation', it is simpler to label them accordingly.

The ratio between a crop's potential evaporation and adjacent lake evaporation, Et/ Eo, depends on the respective albedo and surface-roughness values of crop and water. For well-watered grass, it is typically about 1.2. Comparing this with the pan coefficient for a Class-A evaporimeter (which implies that Ep/Eo equals 1/0.7, i.e. 1.4), shows that potential evaporation for a crop such as grass is roughly similar to nearby pan evaporation Ep.

Actual Evaporation Rate Ea

In practice, the rate of actual evaporation taking place in a field is more important than either Eo Ep or E. But it is the most difficult to measure. So it is often deduced from evaporimeter measurements Ep, using values of the crop factor, E/ E calculated from previous measurements of Ep and Ea with the same crop at that same stage (Figure 4.9). Unfortunately, the climate, soil and crop management now are likely to differ from those in the previous measurements, so that the crop factor they yielded is hardly applicable,

Figure 4.9 Seasonal variations of the crop factors Ea/E per cent for four crops in South Africa.

and therefore the actual evaporation is inferred only very approximately.

One feature of actual evaporation is its dependence on the soil's wetness. There are two stages involved, illustrated in Figure 4.10. Initally, drying proceeds at a rate which depends on the climate, and equals the potential rate E. After that, Ea equals the rate at which the soil can deliver moisture to the roots, Em. The latter depends on the relative moisture content M of the soil within the layer containing the roots, where M is expressed as a fraction of unity. A value of zero means that the soil contains no moisture available to the plants, so they wilt. (The soil still contains some moisture, but it is held too tightly between the particles to be available to the roots.) A value of unity for M corresponds to soil at 'field capacity', which is soil that has been saturated and then allowed to drain for a day. The difference between these extreme conditions defines the 'maximum available moisture', which amounts to 111 mm depth of water in a metre depth of heavy clay soil, but 155 mm for a sandy loam, for instance.

Figure 4.10 shows Em as equal to 16 M2 mm/d, determined experimentally. So Ea equals whichever of Et and Em is the less, given the particular soil wetness M.

The actual evaporation rate from vegetation is affected also by the rainfall and dew (Section 4.7) held by the canopy. This can amount to a layer a millimetre deep on vegetation with a large total leaf area. Such a layer is a significant fraction of a typical day's evaporation. This intercepted water on leaves evaporates rapidly, presumably at the potential rate Et. On the other hand, evaporation from the soil depends on how much it is shaded by the vegetation. The combined evaporation from intercepted water and soil can sometimes equal that from the crop itself.

In broader terms, the actual evaporation Ea is slightly less than the precipitation P in any region where P is less than the potential evaporation rate E. Then the difference (P-Ea) approximates the runoff (Chapter 10). On the other hand, Ea approximates Et in humid climates.

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