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Show PART II PRESENT STATUS OF IRRIGATION Return flows Even -with the best controlled irrigation system, some water is lost during an irrigation to surface runoff, deep percolation, and evapora- tion. Most of the water from surface runoff eventually finds its way back to the stream. Other water which enters the soil percolates through the root zone, contributes to the ground "water resource, and eventually gets back to the stream through springs, seeps, or underflow. Diverted flow not stored in the root zone or consumptively used that finds its way back to a source of supply is termed return flow. Because of differences in topography, soils and geology, crops, methods of irrigation (amounts of water and number of irrigations), and other related factors, return flow varies widely. On many tributary streams the entire flow is often diverted at an upstream location but water is available from return flow for downstream use. Through return and reuse it is not uncommon for the total diver- sions from a stream to exceed the apparent streamflow. Return flows that occur as ground water underflows to the stream channel commingle with surface flows and are extremely difficult to mon- itor. Because of these conditions measurements and records of return flow are meager. Return flows in connection with large projects are commonly estimated as the difference between the irrigation diversion and the computed depletion (water consumptively used), including inci- dental uses. Data relating to actual project water efficiencies is meager. As a general estimate the overall project water efficiencies probably average about 30 percent. Even -though the return flows are used several times in different areas within the region, the quality of surface water (with the excep- tion of small isolated areas) has been acceptable for irrigation pur- poses. Research studies are needed to determine the mineral composition and concentration of water returning to the streams and ground water aquifers. Crop consumptive use In lieu of adequate diversion and return flow records and on-site consumptive use measurements, the empirical formula developed by H. F. Blaney and W. D. Criddle was used in estimating the amount of water con- sumed by irrigated crops (Appendix V-Water Resources). The methodology used is the same as the one developed by H. F. Blaney and adopted by the Upper Colorado River Basin Compact Commission. Slight modifications, based on more recent studies, were made in determining the effective pre- cipitation (net amount of precipitation consumed by crops). 26 |