Hinds 1967, Hinds and Rickard 1968, Patten 1975, Bajza et at. 1977), Lowe and Hinds (1971) have also shown reductions of up to 13°C in the winter from shading of deciduous desert trees. Soil temperatures are reduced in shaded areas during the day (Shreve 1931, Hinds and Rickard 1968, Patten 1975), and possibly at night as well due to the time lag of soil temperature fluxes in relation to air and surface temperature (Shreve 1931, Hadley 1970). Further supporting the concept of reduced soil temperatures in the shade is a study by Abd El Rahman and Batanouny (1966) who found soil temperatures in an arid watershed to be more highly correlated with incoming radiation than with air temperature. A further ameliorating effect of shading is the fact that shaded areas tend to have less diurnal (and possibly seasonal) variation than do exposed areas (Patten and Smith 1974, Bajza et at. 1977). In conclusion, of the four temperature variables discussed (air temperature, radiant temperature, surface temperature, and soil temperature), air temperature is the only variable which should not be substantially reduced due to shading by solar collector arrays, based on studies of natural desert systems. Interestingly, air temperature is probably the temperature variable of least importance with regard to physiology, behavior, and distribution of desert organisms. Also, shading of the desert surface has a more significant influence on diurnal variability of environmental temperatures than on absolute or mean values, further indicating the ameliorating effect of shading on temperature. Soil Moisture Studies in arid and semi-arid regions generally indicate shading by vegetation to cause a decrease in evaporation rate and thus increase soil moisture relative to open areas (Shreve 1931, Abd El Rahman and Batanouny 1965,1966, Bajza et at. 1977). Although Shreve (1931) found evaporation rates in the open and in the shade of a palo verde tree to be similar in all months of the year, he did observe the shaded microhabitat to exhibit higher soil moisture content following heavy summer rains as well as following light rains in the cooler months of the year.
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