Laserfiche WebLink
<br />.' <br /> <br />Climate <br /> <br />Virgin Valley Sub evaluation Unit is in an arid climatic regime. <br />The frost-free season (in the irrigated area) averages about 245 days. <br />Based on observations at Lake Mead since 1936, the average annual evapora- <br />tion from a free water surface is 80 inches. The maximum evaporative <br />rate occurs during the summer with as much as 20 percent of the total, <br />or 16 inches monthly. Temperatures in the bottom lands vary in July <br />0' 0 <br />from an average mi~ of 70 F. to an average maximum of 105 F., <br />and in January from an average minimum of 300 F. to an average maximum <br />o <br />of 60 F. <br /> <br />Precipitation in the irrigated area varies from four inches during <br />an average year to about six inches in a wet year. About 50 percent <br />occurs primarily in the form of rain during the winter months (November <br />through February). A few summer convective storms are likely to occur <br />and can cause serious problems such as flooding, crop damage, and erosion. <br /> <br />Water Resource <br /> <br />Virgin River water has been used for irrigation in Mesquite and <br />Bunkervi11e for about 100 years. The mineral content of the water <br />makes it undesirable for domestic use. Groundwater is used for domestic <br />and municipal needs, stock watering and irrigation. <br /> <br />Surface Water - The source of Virgin River water is the high plateaua <br />and mountains of southwestern Utah and northwestern Arizona. The lower <br />regions of the central and southern part of the basin contribute a <br />small proportion of runoff. About one-half of the runoff comes during <br />February through May. Only four to eight percent of the annual yield <br />occurs during anyone of the remaining months. A series of highly <br />saline springs are located along the stream channel from the "narrows" <br />downstream of St. George, Utah. to Littlefield, Arizona. These springs <br />provide a significant part of the irrigation supply during periods <br />of low flow. Virgin River joins Colorado River at Lake Mead in Nevada. <br />During the irrigation season, the river flow is low and uaually only <br />large storm water flows reach Lake Mead during this period. <br /> <br />The mean annual discharge of Virgin River at Littlefield, Arizona <br />(USGS Gaging Station) is 164,400 acre-feet based on 50 years of record. <br />However, annual stream flow has varied from 78,000 to 400,000 acre-feet <br />during this period. The average water yield for the five year period <br />(Water Years 1972-74 and 1978-79), when there was a corresponding flow <br />record at Riverside, Nevada, was 220,800 acre-feet. The USGS Gaging <br />Station near Riverside measures outflow from the program area. Average <br />outflow was 192,500 acre-feet for the five year record cited. Beaver <br />Dam Wash, the only tributary in the subevaluation unit that contributes <br />a significant volume of water, enters Virgin River above Littlefield <br />stream gage. <br /> <br />U,- , I ': r l~ C <br />~l...vv <br /> <br />9 <br />