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SPDSS Memorandum <br />Final <br />To: Ray Alvarado and Ray Bennett <br />From: LRE -Erin Wilson and Beorn Courtney <br />Subject: Task 77 -Perform Analysis of Deficit Irrigation <br />Date: July 13, 2006 <br />Introduction <br />Consumptive use methods, such as the modified Blaney-Griddle formula, can be used to estimate <br />crop consumptive use. These methods estimate the crop consumptive water requirement under a <br />full water supply. In the case of lands irrigated solely by surface water supplies, historical <br />diversion records, less losses, are used to estimate the portion of the full demand that was <br />actually met. For lands supplied by ground water or on-ditch reservoir releases, for which <br />historical records are generally unavailable, consumptive use methods can be used to estimate <br />pumping and on-ditch reservoir releases. Pumping and reservoir releases can be estimated as the <br />irrigation water requirement (IWR) divided by an estimated application efficiency (IWR is <br />potential CU less the effective precipitation). <br />During the SPDSS feasibility study, some water users indicated the consumptive use approach <br />may overestimate the irrigation water requirement, and associated pumping and on-ditch <br />reservoir releases, in the water-short South Platte River basin. Deficit irrigation, defined here as <br />the method of applying less water than the amount that would be estimated using a consumptive <br />use method, was again reported to occur in the South Platte River basin by users interviewed in <br />Phase 1 and Phase 2. <br />This memorandum presents the general approach for Task 77: <br />Perform an analysis to determine whether estimating the use of supplemental supplies (i. e. <br />ground water and reservoir releases) to meet full or partial potential use is appropriate for <br />regions within the South Platte. <br />The findings summarized in this memorandum pertain to deficit irrigation on lands served by <br />ground water or on-ditch reservoir releases. To the extent that deficit irrigation occurs on lands <br />served solely by surface water diversions, the deficit can be characterized through the historical <br />diversion records. <br />