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<br />U'v <br /> <br />/-t/-9t.f <br /> <br />11 <br /> <br />N <br />~ GRAND VALLEY SALINITY CONTROL UNIT, COLORADO <br />N <br />~ The Grand Valley in Colorado illustrates the geo-hydro-chemica1 <br />process in the Colorado River system where the unit is underlain <br />by a massive Mancos Shale Formation with an unlimited salt <br />supply. The Mancos shale being weathered by deep percolating <br />irrigation water is about 4000 feet thick and provides an <br />infinite source of salts. <br /> <br />The Colorado River provides an unlimited water supply during the <br />growing season, from April to October, for irrigation of 65,000 <br />acres of cropland. Cropland lying in a block along the north side <br />of the river is relatively uniform, consisting of soils derived <br />from Mancos shale. The land slopes 1 to 5% to the river, and is <br />generally underlain by a confined aquifer. The crops are surface <br />irrigated predominantly by furrow irrigation. The major crop is <br />alfalfa. Some small grain, pasture, corn, and orchard crops are <br />grown. About 5,000 acres are idle each year. The USDA salinity <br />control plan proposes irrigation system and management <br />improvements on about 80 percent of the area or 53,000 acres. <br /> <br />USGS computed the salt loading from the Grand Valley irrigated <br />area by 5 methods, using good inflow and outflow stations in the <br />reach. A considerable number of daily flow and salinity records <br />were evaluated with the computer. The long term average is an <br />indication of the salt loading from irrigation in the Grand <br />Valley reach. <br /> <br />The salt pickup from the Grand Valley irrigated area based on <br />USGS inflow and outflow evaluations of gaging stations on the <br />river system is 580,000 tons salt pickup/year plus or minus <br />90,000 tons (USGS 1976 & 1986). <br /> <br />This means that the deviation from the long term average (over 40 <br />years) is plus or minus 90,000 tons. The projected salt load <br />reduction from the USDA onfarm irrigation improvement program is <br />93,000 tons per year. Gaged data is very useful and important in <br />looking at long term averages and trends. About 23 stations in <br />the Colorado River system provide the overall loading and <br />concentration of salt for basin-wide analysis. <br /> <br />In the Grand valley irrigation use and operations have changed <br />very little and very slowly over the years and diversions are <br />about the same every year. Therfore the salt loading from <br />irrigation should c') about the same every year. However, <br />computations of salt loading from the Colorado River reach based <br />on gaged data, varied from 100,000 to a million tons/year and <br />monthly and daily data is not always logical. Annual water use <br />based on measured inflow and outflow varied from minus to plus <br />several feet and specific annual values are not meaningful. It <br />is nearly impossible to use inflow and outflow gaging data for <br />