decreases in macropore space. Liddle and Moore (1974) found walkers and vehicles to reduce diffusivity in wet soils, but not in dry soils. Studies attempting to examine reversibility of compaction generally indicate that short term, even intense, compaction is reversible. Long term compaction tends to cause more serious changes in soil structure, especially in deeper soil layers (Liddle and Greig-Smith 1975), often resulting in a site which does not readily recover to its pre-disturbance condition. In the desert, vehicles tend to primarily disturb the surface, which still takes as much as a decade to repair (Vollmer et at. 1976, Webb 1976). Studies of off-road vehicle effects on structure of deeper soils indicate possible serious changes, effects of which may not become expressed ecologically until years after the original impact (Vollmer et at. 1976, Wilshire and Nakata 1976). Studies of a wide variety of soils indicates that compaction has more serious structural changes in wet soils than in dry soils (Bates 1935, Parker and Jenny 1945, Martinion and Olmstead 1949, Jamison et at. 1950). Scheduling periods of heavy maintenance pressure during predictable dry periods could possibly reduce the potential for soil compaction. In order to limit the amount of long-term compaction occurring after construction, vehicles should be excluded from as many areas as possible, being confined to specific roads in the collector fields. Vehicles have been found to significantly increase penetration resistance more than bulk density in sand (Liddle and Greig-Smith 1975). Increases in penetration resistance would decrease infiltration rate, and could inhibit penetration of the root systems of ephemeral plants. Deeper levels of soil would also be compacted by vehicles as opposed to limited compaction due to occasional trampling. Introduction of grazing is not recommended after a period of intensive construction-type surface disturbance, as soil recovery may be retarded (Lowe 1955). Runoff and Erosion In the southwestern deserts, a list of Candidate Sites for photothermal solar conversion units (Black and Veatch 1977) indicates topography to be an
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