West Mojave Plan FEIR/S - Desert Managers Group

eastern cities such as Twentynine Palms (4 inches) (see also precipitation tables in Appendix H).All of the weather stations in the planning area receive some of their precipitation assnow. The total average snowfall ranges from under one inch in Trona to over three inches atHaiwee reservoir and Lancaster.A cyclic weather phenomenon called the El Nino brings increased precipitation toportions of the eastern Pacific Rim. This is especially true in the western Mojave Desert.Weather Bureau records indicate that there have been 23 El Nino years since 1931. These 23years represent approximately 1/3 of the years, but on the western edge of the desert, those yearsaccount for 65% of the precipitation. This east to west variability is also reflected in thepronounced east to west difference in the influence of the El Nino years. In Twentynine Palms,for example, only 44% of the precipitation falls in El Nino years as opposed to 65% along thewestern edge of the desert.During the summer the western edge of the Mojave Desert is heavily influenced by thedry southwest airflows resulting in typically very dry weather. The influence of the southwestwinds diminishes toward the eastern Mojave Desert. This results in a more continental influenceand its resulting monsoonal weather patterns. This is illustrated by comparing Randsburg (alongthe western edge of the planning area) with Needles (in the eastern Mojave). In Randsburg, onlytwo percent of the Julys and six percent of Augusts have more than 1 inch of precipitation. Bycomparison, in Needles, more than 1 inch of monthly precipitation falls in sixteen percent of theJulys and twenty-seven percent of the Augusts. Even sites within the eastern portion of theplanning area (such as Twentynine Palms) average more precipitation in July and August thanthey do in January and February.The consistent occurrence of two wet seasons in the eastern portion of the planning areais reflected in the vegetation. There is a distinction between plants having most of theirphotosynthetic activity during the late spring and summer (warm season plants) and plantshaving most photosynthetic activity during the winter (cool season plants). The vegetation in theeastern Mojave Desert includes warm season plants such as Mojave yucca (Yucca schidigrea),galleta grass species (Pleuraphis spp.) and others in addition to the cool season plants. Thewarm season plants are absent from the western edge of the desert. The break between the warmseason area and the cool season area follows a north south line along the Mojave River and justwest of Harper Dry Lake. The cool season areas are the Indian Wells and Searles Valleys souththrough the Antelope Valley and east to near the Mojave River.Extreme variability is another characteristic of the precipitation. Some locations such asMojave have a mean precipitation of 6.06 inches and a standard deviation of 4.04 inches. Thismeans that the normal precipitation ranges from a low of 2.02 inches to 10.10 inches. This is an80 % variation in precipitation volumes.Drought: When precipitation is below average, it is considered a drought. The PalmerDrought Severity Index (PDSI) has become the semi-official drought index. The PDSI usesprecipitation and other moisture data to develop a dryness index. The index uses “0” as normalfor a site and negative numbers to indicate severity of a drought and positive numbers to indicateChapter 3 3-44