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<br />(::) <br />W <br />IV <br />00 <br /> <br />PART VI. BASIC STUDIES <br /> <br />A. Ob.iectives <br /> <br />Studies of past and future effects of storage and irrigation proJ- <br />ects on the quality of water downstream depend primarily on records of <br />streamflow and quality of water collected before the project was con- <br />structed as well as afterward. Many projects were built before the need <br />for antecedent data was recognized} and as a result data adequately de- <br />fining the effects of existing projects are rare. <br /> <br />The primary requisites for a comprehensive quality of water study <br />of an irrigated area are inflow and outflow measurements of both quan- <br />tity and ~uality of water. Each gaging station and quality sampling site <br />is expensive to install and maintain, so with limited funds available <br />care mUst be exercised in the selection of special study areas. If gag- <br />ing stations are already in operation, these are used with the addition <br />of quality sampling. If gaging stations do not exist, funds are advanced <br />to the Geological Survey through a cooperative program to install and <br />maintain stations and to collect and analyze the water samples. <br /> <br />A meaningful study should be based on a period of at least four to <br />ten years on the smaller or well-defined basin areas with the length of <br />period partly dependent on how stable the irrigation practices are. It <br />must be recognized that each area will have a different effect on water <br />quality. To reflect the effect of continuing development in larger ba- <br />sins, studies will need to be continued for a long period. <br /> <br />So far the studies in the basin have been limited to a comparison <br />of the total dissolved solids in the inflowing water and the return flow <br />water. No attempt has been made to determine losses of water or total <br />dissolved solids by deep percolation, to detect underground aquifers.that <br />might be augmented with return flow, or to evaluate changes in chemical <br />characteristics (other than dissolved solids) resulting from development. <br /> <br />Studies prior to irrigation would be helpful, but they have not been <br />made in most areas, so comparisons must be made when new land is added or <br />new storage is made available. A study is planned for the Seedskadee <br />project area at a later date. This will give a clear-cut comparison be- <br />tween "before" and "after" irrigation conditions since only the land in <br />an experimental farm is presently irrigated on the Seedskadee project. <br /> <br />Favorable salt balance conditions exist when the amount of dissolved <br />solids carried off the land by irrigation return flows.is e~ual to or <br />greater than the amount c.arried to the land by irrigation water. The <br />term "pickup of dissolved solids from irrigated lands" as used in this <br />report applies to dissolved solids picked up in addition to favorable <br />salt balance conditions. <br /> <br />19 <br /> <br />