Physical aspects and determination of evaporation in deserts applying remote sensing techniques

In the deserts of the world groundwater losses by evaporation are shown to be a very important and almost unknown quantity in the water balance of aquifers present.After establishing a new combination formula to calculate actual evaporation from within the soil, the evaporational soil physical aspects are discussed in detail. It is shown that under specified conditions vapour flow can take place against the direction of heat flow. A new, fundamental, definition of evaporation sites is given and its bearing on a model of heat and moisture flow is discussed. A theory of heat exchange at the soil - air interface is presented in terms of the apparent soil thermal admittance. Equations to relate the latter soil property to soil layering and to the frequency of the surface temperature wave are given. A theory of evaporation in terms of multi-dimensional geometry is proposed and it is shown how to derive approximate formulae to calculate actual evaporation by making use of satellite data only. For more accurate results point ground reference measurements have been used to calibrate the satellite-based calculations.After describing a particular desert area in Libya, the accuracy of a number of satellite data of that area is discussed on the basis of ground reference measurements and numerical correction procedures.Finally, the theory is combined with satellite and point ground reference data to calculate the different terms of the surface energy balance and the actual evaporation for some 36,000 km ²of the Libyan desert.