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<br />The primary objective of the plan is to reduce salt load pickup by <br />improving irrigation efficiency in the cultivated area and by improving <br />watershed conditions on privately owned land in the diffuse area. <br />Seepage losses from unlined farm delivery and tailwater collection <br />ditches and deep percolation from field irrigation were considered to be <br />major sources of water that carries salt to the river. Therefore, <br />providing more effective and efficient irrigation is the purpose of the <br />on-farm improvement program. To achieve this purpose, improvements in <br />the irrigation system and management practices are needed. <br /> <br />Improvements in the irrigation systems consist of (1) ditch lining or <br />pipelines to reduce seepage losses, (2) water measuring and control <br />structures and land leveling for control and uniformity in applying <br />water during each irrigation, (3) subsurface drains for existing open <br />drains to combine fields into more efficient units, and (4) ditches or <br />ponds to control surface water leaving the fields. Although subsurface <br />drains also are needed to lower the existing high water table that <br />impedes cultivation and crop growth, this practice is not considered <br />applicable for sal inity control. <br /> <br />A basic assumption in irrigation water management is that soil moisture <br />to be replaced is a fixed amount for each irrigation depending on <br />allowable moisture depletion for the soil and the crop. The frequency <br />of irrigation, therefore, will vary during the irrigation season with <br />the variation in daily water requirements of each crop grown. Manage- <br />ment factors which the irrigator can control include the time of set, <br />the flow rate to each furrow in the field, and the number and timing of <br />irrigations during the season. These are the management variables used <br />in formulating each alternative. <br /> <br />Runoff from watersheds carries sediment and causes bank erosion along <br />channels that convey the water to the river. The salt-laden sediment <br />adds about 80,000 tons of salt to the river annually. Therefore, <br />identifying the sediment sources and applying treatment measures to <br />reduce runoff and erosion are needed in the grazed range sites. <br /> <br />On-going Program - Effects of present irrigation practices were deter- <br />mined to establish a base for recommending improvements and to measure <br />the effects of those improvements. It should be noted that results <br />from evaluating the present conditions are conservatively stated. The <br />procedure contained no basis for considering effects of unleveled fields <br />or of inaccuracy in water measurement. Therefore, present deep perco- <br />lation losses may be somewhat higher than indicated because of the lack <br />of uniform water application on unleveled fields. <br /> <br />\~~~ <br /> <br />37 <br />