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<br />" <br /> <br />7 <br /> <br />N <br />a4 <br />f\:. <br />..... <br /> <br />precipitation is added to groundwater. Irrigation in the semi <br />arid and arid lower portions of a basin results in a volume of <br />subsurface return flow that is added back to natural streamflow. <br />The depth to the water table and the base flow is seasonally <br />responsive to irrigation. The volume of salt loading is <br />directly proportional to the volume of subsurface return flow and <br />reductions are dependent on irrigation water management. <br /> <br />The salinity concentration of subsurface return flow from <br />irrigation is constant regardless of the amount of canal seepage <br />or onfarm deep percolation in those areas underlain by ancient <br />lake and seabeds. There is an unlimited (infinite) supply of <br />salt. Salinity concentration is dependent on the geo- hydro- <br />chemical characteristics of the underlying aquifer in each unit. <br />The unique nature of the geology formations, the hydraulic and <br />subsurface flow characteristics, and the chemical composition and <br />solubility of the salts in the aquifer underlying the irrigated <br />area have to be understood to develop water and salt budgets. <br /> <br />This concept has been discussed and accepted by state, federal <br />and other agencies making salinity studies in the Colorado River <br />since 1973. Participants to this agreement include the seven <br />basin states, a consortium of seven universities in the region, <br />and the Bureau of Reclamation, Soil Conservation Service, <br />Agricultural Research Service, u.S. Geological Survey, <br />Environmental Protection Agency, private consultants, and the <br />Salinity Forum. <br /> <br />Monitoring has substantiated this concept. USBR with assistance <br />from USGS has carried out an intensive monitoring program in Reed <br />Wash, Grand Valley, Colorado. Water and salt budgets for before <br />and after conditions following installation of 7 miles of canal <br />lining and 30 miles of lateral pipelines showed no change in <br />salinity concentration of subsurface returnflow over time. The <br />results are discussed later in this paper. <br /> <br />Salt is added to the return flow by mixing and displacement of <br />ground water and the dissolution of salts from the underlying <br />saline aquifer. In most areas the deep underlying aquifers are <br />under some hydrologic pressure from the surrounding watershed <br />runoff. They act as a pump supplying very saline water which <br />mixes with seepage and percolating irrigation water and is <br />displaced, forcing it back to the stream system. Seepage from <br />canals, laterals, and ditches and deep percolation onfarm also <br />develops a hydrostatic head on the ground water aquifer back to <br />the stream system. Monitoring the salinity of the inflow <br />irrigation water and the drain water as near to the receiving <br />~tream as possible, represents the increase in salinity <br />concentration. The other parameters needed are the volume of <br />irrigation water "in" and the volume of water "out". A reduction <br />in deep percolation and seepage from irrigation will make a <br />proportional reduction in salt pick up. <br />