eratures cooler, then wind deflection would have a greater effect on rates of heat flux than on temperature per se. Soil Moisture and Humidity Wind speed has a major influence on evaporation rate (Penman 1948, Hellwig 1973), especially in arid regions (Krishnan and Kushwaha 1973). Quantitatively, Van Eimern et at, (1964) found evaporation from flat moist containers to be proportional to the square root of the wind speed. Thus, a decrease in wind speed and turbulence should result in reduced evaporation rates, and concomitant increases in soil moisture. This has been observed experimentally in numerous shelter-type studies (Jensen 1954, Van Eimern et at, 1964, Marshall 1974, Rosenberg 1974). Reduction of evaporation by wind deflection in arid regions may be proportionally greater than the reduction of wind velocity. This is because a major beneficial aspect of wind deflection with respect to moisture conservation is the reduction of the advection of sensible heat from surrounding unsheltered areas (Miller et at, 1974). This is probably an important reason why planting of shelterbelts with the resultant wind reduction is considered more effective in arid and semi-arid regions (Siddoway 1969, Rosenberg 1974), and in regions with continental climates (Van Eimern et at. 1964, Marshall 1967) . The relevance of these studies to the microclimate of heliostat and collector fields is apparent. If soil moisture is higher under collectors than in the surrounding desert, then significant reductions in wind speed by the collectors could significantly reduce advective losses of moisture to the surrounding drier desert. With increases in soil moisture will be increases in humidity and vapor pressure of the air under the collectors, as has been observed experimentally in almost all shelter studies (Rosenberg 1974). Increase in these variables, together with soil moisture increase, should result in a much more mesic microclimate relative to the open desert.
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