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<br />,:") <br />-.1 <br />:1" <br /> <br />".,""'-\ <br /> <br />IV. STRATEGY <br /> <br />The USDA approach to monitoring salinity changes by improvement of irrigation <br /> <br />systems and management involves monitoring and evaluation of irrigation <br /> <br />mechanisms and translation of this information into salt load reduction. It <br /> <br />is nearly impossible to isolate and monitor complex hydrologic subsystems <br /> <br />for surface and subsurface inflow and outflow accurately enough over the <br /> <br />long-term to measure the salinity impacts of specific measures being <br /> <br />installed on scattered fields and farms throughout the salinity control <br /> <br />units. USDA recognizes the monitoring activities and analyses being made of <br /> <br />the system as a whole and for a few selected sites by USGS, USBR, BLM, <br /> <br />universities, and other state and federal agencies. I,e actively support the <br /> <br />basin-wide activity. <br /> <br />The USDA monitoring and evaluation plan described in this report, using <br /> <br />estimates of deep percolation and seepage reductions due to irrigation <br /> <br />improvements and translating these into salt load reductions, will provide <br /> <br />acceptable evidence of basin-wide salinity impacts. The irrigation return <br /> <br />flows from Grand Valley and the Uinta Basin add about one million tons of <br /> <br />salt (10 percent) to the Colorado River. Surface return flows pickup an <br /> <br />insignificant amount of salt. It is the deep percolation and seepage of <br /> <br />water into the underlying saline aquifers which results in salt loading. <br /> <br />The underlying saline aquifers have an essentially unlimited salt supply and <br /> <br />subsurface return flows will continue to return to the rive,r at about the <br /> <br />same concentration. Each acre-foot of deep percolating water picks up one to <br /> <br />ten tons of salt while it is in transit back to the river system, the amount <br /> <br />depending on which subarea is being evaluated. If irrigation systems or <br /> <br />management is improved and irrigation water does not seep from ditches or <br /> <br />16 <br />