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climate C) '0) ~ ~ ~ ~ The Moapa Valley Unit is in an arid climatic regime. The frost- free s~ason (in the irrigated area) averages about 240 days. Based on observations at Lake Mead since 1936, the average annual evaporation from a free water surface is 110 inches. Evaporation rates ~re highest during July, August and september when 10 to 12 percent of the total evaporation occurs each month. At Overton, the December mean temperature is 4So F. while the July mean is 8So F. The precipitation in the irrigated area varies from four inches during an average year to about six inches in a wet year and co~es in the form of rain primarily in the winter months. A few summer convective storms are likely to occur and can cause serious problems such as flOOding, crop damage, and erosion. Surface Water The hYdrologic watershed area for Muddy River starts about lS0 miles ~orth, near Ely, Nevada as the White River. Water from White River does not pass the Pahranagat Lakes area, even during storm ,events and is therefore a non-contributing area to Muddy River surface flows. Upper Muddy River is a low elevation drainage that only flows during storm events. The water for MUddy;River comes primarily from the Muddy River springs and Warm sprin~s in the Upper Valley Which flow year round. Meadow Valley Wash, another large tributary, joins the Muddy River at Glendale. It starts about 100 miles to the north but gener~lly only contributes surface water during storm events. Waterifor irrigation in the lower Meadow Valley Wash area comes from wells. California Wash also enters near Glendale from the southwest. Weiser Wash enters from the northeast. other washes enter the Lower Moapa valley from the southwest. They are all basically dry except during storm events. White River, Upper Meadow Valley Wash and Upper Muddy River are not included in the study area. Their contributions of surface and ground water and salt were considered in the water and salt analy~es. The study area consists of Hydrologic Units lS010012 and iS010013 as defined by the U.S. Geological Survey in their Hvdroloqic Unit Map - 1974. State of Nevada. Muddy River water has been used for irrigation in Moapa Valley since the latter 18S0's. The surface water supply from springs in the Upper Moapa Valley flows about 2S miles to the southeast where it empties into Overton Arm of Lake Mead, just south of Overton, Nevada. USGS ,Moapa stream gage (#9-4160), located in Upper Moapa Valley below the springs, has been in continuous operation since October 1944; The average annual flow for the 19S1 through 1989 period of record is 30,672 acre-feet. 17