West Mojave Plan FEIR/S - Desert Managers Group

pulled back by gravity. This causes them to impact other particles and set them into motions andcan account to 50 to 80 percent of total soil movement. (NRCS, 29Palms)3.2.3 WaterThe planning area is one of the most arid areas in the nation; the potential annual waterloss through evapotranspiration exceeds the annual water gain from precipitation even at thehigher elevations. On the valley floor the evaporation exceeds the precipitation by at least 25:1.Prominent mountain ranges have an important influence on moisture distribution withinthe plan area. As moist, unstable air masses from the Pacific Ocean rise up the windward slopesof the Southern Sierra, San Gabriel and San Bernardino Mountains, the air is cooled and watervapor condenses and falls as rain, snow, or ice. When these air masses descend the leewardslopes, they become warmer and more stable and thus retain most of the remaining moisture.Consequently, precipitation amounts are much greater on the windward slopes of the mountainranges, whereas arid conditions prevail leeward of the mountains. All of the study area, exceptthe Kelso Creek area is on the leeward side of these major mountain areas.Because of the arid nature of the study area, water supply is the single most importantresource. The presence or absence of a reliable supply of good quality water has determined thepattern of agricultural, urban, and industrial development and will continue to do so.Groundwater withdrawn by wells furnishes nearly all of the developed water. Many of the Stateor federally listed or BLM sensitive species, discussed elsewhere in this document, aredependent upon the presence of groundwater either directly or for their habitat.Surface water is very scarce. Streams that originate high in surrounding mountains on thewest and south may have perennial flow in the higher altitudes; at the lower altitudes andthroughout the area virtually no water exists in streambeds or riverbeds, except locally afterinfrequent, heavy cloudbursts. The playas may be covered by water from the runoff for as longas two months a year. There are many locally important springs and seeps most of which areassociated with the mountain areas.3.2.3.1 Groundwater BasinsThe water yielding materials in this area are in valleys and basins, and consist primarilyof unconsolidated alluvial-fan deposits, although locally flood plain and lacustrine (lake) beachdeposits may yield water to wells. The valleys and basins are internally drained; that is, waterfrom precipitation that falls within the basin recharges the aquifer and ultimately discharges tothe land surface and evaporates within the basin. Ground water is generally under unconfined, orwater table, conditions at the margins of the basins, but as the unconsolidated deposits becomefiner grained toward the centers of the basins, the water becomes confined. Rarely, basins mightbe hydraulically connected in the subsurface by fractures or solution openings in the underlyingbedrock. These multiple-basin systems end in a terminal discharge area, or sink, from whichwater leaves the flow system by evaporation. Along the Mojave River several basins or valleysare hydraulically connected, and ground water flows between the basins, mostly through theunconsolidated alluvial stream/flood plain sediments of the present and ancient river.Chapter 3 3-60