Solar collectors will also affect the thermal (long-wave) radiation emitted by the ground to the sky. In a normal open desert system, intense heating of the ground surface during the day is followed by a high rate of reradiation at night from the ground to the ’’cold" sky (Sellers 1965), resulting in rapid cooling of the ground surface. If 10 to 40% or more of the sky is blocked by solar collectors, then approximately the same percentage of long-wave reradiation would be intercepted. If the collectors are in a stow (inverted) position at night (Hildebrandt and Vant-Hull 1977), then the thermal radiation intercepted will be redirected back toward the ground. Therefore, solar collector arrays will substantially decrease incoming solar radiation in the daytime, as well as reduce outgoing thermal radiation in both the day and night. The result will be reduced net radiation under the collectors on a daily basis, and a substantial reduction in average hourly radiation flux. Several related studies support these predictions. Bajza et at. found an irrigated landscape (i.e., with trees and large bushes) to have lower incoming radiation at the ground surface than a desert landscape during the day, as well as having lower long-wave emitted radiation at night due to less sky exposure (as well as cooler surface and subsurface temperatures). Net radiation on the irrigated landscape never fell below zero, while on the desert landscape it was very high during the day, and fell well below zero at night. Results by Lowe and Hinds (1971) are similar. They found a palo verde tree in winter in the Arizona desert to reduce incoming radiation by 50% compared to the open, while also reducing effective outgoing radiation at night by the same amount. The result was a maximum diurnal net radiation flux under the tree of just 44% of that observed in the open. Patten (1975) found similar reductions of incident radiation by palo verde trees in the Sonoran Desert near Phoenix. Of potential importance is the amount of radiation a given area of ground will receive as bands of sun and shadow move across the desert surface. When an area is in shadow (i.e., shielded from direct beam radiation), the area will still be receiving diffuse radiation. How much will depend upon
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