With the exception of Shreve (1931), evaporation studies indicate shading significantly reduces water loss from arid soils. Hellwig (1973) concluded that solar radiation input far outweighs air temperature and relative humidity as a factor affecting evaporation from arid sandy soils, while Krishnan and Thanvi (1972) found the number of hours of daylight to be an important variable in predicting evaporation. Transpiration studies further support the role of radiation in water loss, as Abd El Rahman and Batanouny (1965) found shaded vegetation to exhibit less water loss per gram of tissue than plants in the open when both populations were well watered. With respect to arrays of solar collectors, Black and Veatch (1977) and Sears et at. (1977) predict that soil moisture under the array fields will be significantly higher than in the adjacent open desert. Furthermore, additional water may be available between the collectors for short periods following rain storms due to precipitation runoff (Meinel and Meinel 1972) and possibly due to water used to clean the heliostats (Black and Veatch 1977) or photovoltaic collectors (Sears et at. 1977) The amount of interspace soil moisture compared to that under the collectors is unknown and should be determined. In conclusion , removal of a majority of the incident solar radiation from an arid ecosystem should tend to prolong favorable moisture conditions, and reciprocally shorten the duration of adverse (i.e. drought) conditions. Plant and Animal Response The desert Southwest is considered a harsh environment for plant and animal life in which the major limiting factor is moisture. Most of the growing season is characterized by low soil moisture and high evaporitivity (Noy-Meir 1973). If shading by solar collectors, through interception of a large proportion of incoming solar radiation, increases soil moisture due to suppression of evaporation rate, profound changes could possibly occur on the desert floor underneath the collector arrays. In the open, water-limited desert, plants generally have adaptations which increase the efficiency of water use. Morphological adaptations such
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