by increasing evaporation rate and desiccating plants. Thus, windbreak systems have been found to be useful in arid regions as a means of limiting the process of desertification (Van Eimern et at. 1964, Petrov 1966, Stepanov 1971) . Although windbreaks are designed and function to decrease wind velocity (Van Eimern et at. 1964, Rosenberg 1974), changes in turbulence due to windbreaks are not as easy to control or predict. Many studies have indicated turbulence to be reduced in the lee of wind shelters (Brown and Rosenberg 1971, Miller et at. 1973, Tuller 1973, Rosenberg 1974), while other studies have observed, despite a decrease in velocity, an increase in turbulence (Rosenberg 1966a, Van Hylekama 1970, Hanson and Ravzi 1977). Rosenberg (1966a) observed turbulence to vary diurnally being more unstable in sheltered areas during the day, but more stable at night. Also indicating diurnal variation, Bajza et at. (1977) found an irrigated landscape with irregularly spaced wind obstructions to have many more hours of calm than an open desert landscape, but also exhibiting higher maximum velocities and turbulence due to funneling of winds during periods of peak wind speed. Prediction of changes in wind speed and turbulence will thus be an extremely complex problem, and is further compounded by the various heliostat configurations proposed for use in solar thermal plants (ERDA 1976). Heliostats proposed by Honeywell and Martin Marietta consist of rectangular flat plates with gaps between the plates, while Boeing and McDonnel Douglas propose circular concave heliostats, with Boeing's enclosed in a plastic bubble. Small gaps in heliostats may reduce wind velocity under the heliostats, as porous windbreaks are generally more effective than nonporous due to the fact that air which passes through the gaps can prevent the turbulent return of air which has overtopped the windbreak (Miller et at. 1975). However, if the gaps are too large (e.g., Honeywell heliostat configuration, ERDA 1976), jetting effects may occur and increase turbulence (Rosenberg 1974). Rows of photovoltaic collectors should reduce velocity and turbulence, although our discussions with ERDA technicians and observations at the
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