Physical Disturbance Future construction of solar collection "farms'’ on large tracts of desert land will have an impact on the environment of the construction site. These impacts will range from minor trampling by maintenance personnel to complete surface clearing of areas in the vicinity of structural supports of individual collectors. The total amount of disturbance in both construction and maintenance phases will have important consequences with respect to the ecological potential of the desert beneath the collector fields, and is suggested to be of significantly greater importance than the shading and wind deflection effects imposed by the collector arrays, at least in proposed photovoltaic systems (Sears st at. 1977). The desert ecosystem has reached its relatively stable but delicate balance over a long period of time. The desert ecological literature presents ample evidence that where man has concentrated his activities in the desert, the desert ecosystem is drastically altered and, even though the activities may be discontinued, recovery may take many decades or centuries before the desert ecosystem again reaches a state of dynamic equilibrium. Some of these studies can be applied to the current problem of what changes may occur in soil structure, moisture regime, surface characteristics, and biota when large solar conversion units are placed and operated in the desert. Soil Structure Several parameters have been utilized experimentally by researchers to assess the effects of compaction on soil structure. Bulk density, macropore space, diffusivity, and penetration resistance are parameters which indicate changes in soil structure. Knowledge of changes in soil structure is important, due to its effects on water availability, root growth, and aeration of the soil. There are numerous studies indicating that walkers (Bates 1935, Chappel et at. 1971, Liddle and Greig-Smith 1975) and off-road vehicles (Davidson and Fox 1974, Webb 1976, Wilshire and Nakata 1976) have caused substantial increases in bulk density and penetration resistance, and
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