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<br />nDn331. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />PART v. mSTORIC IRRIGATION DEPLETION <br /> <br />The depletion to the Mancos River resulting from historic irrigation <br /> <br /> <br />practices was estimated by analyzing the historic ditch diversions, irrigated <br /> <br /> <br />acreage, and the estimated irrigation requirement for the crops grown in the <br /> <br /> <br />Mancos Valley. For the purposes of this analysis, depletion is defined as the <br /> <br /> <br />reduction in streamflow that occurs as a result of irrigation practices, taking <br /> <br /> <br />into account stream diversions, consumptive use of water, and return flows <br /> <br /> <br />including the lagged effects of groundwater return flow to the stream. Consump- <br /> <br /> <br />tive use is that portion of the irrigation water that is consumed through evapora- <br /> <br /> <br />tion or crop evapotranspiration. Irrigation efficiency is defined, for the purposes <br /> <br /> <br />of this analysis, as the percentage of the water diverted from the stream that <br /> <br /> <br />is consumed. Overall irrigation efficiency in the Mancos Valley is increased <br /> <br /> <br />as a result of the reuse of return flows from one ditch to another. <br /> <br />Crop Consumptive Use <br /> <br /> <br />Based on field inspections of the area and on interviews with the Division <br /> <br /> <br />Engineer and Water Commissioner, it was determined that the principal crops <br /> <br /> <br />grown in the Mancos River Valley are grass hay mixtures and alfalfa. Cultivated <br /> <br /> <br />row crops and orchards make up a very small percentage of the total irrigated <br /> <br />area. Considerable timbered lands north of the town of Mancos have been cleared <br /> <br /> <br />and irrigated to improve the range pasture. Much of these areas do not appear <br /> <br /> <br />to be intensely irrigated. <br /> <br />The consumptive use of inigation water was estimated using the modified <br /> <br /> <br />Blaney-Criddle procedure, calibrated to the area through use of data from Iysime- <br /> <br /> <br />ter studies conducted at the Navajo Indian Irrigation Project (NIIP). Temperature <br /> <br /> <br />data for the Mancos valley were obtained by averaging the recorded monthly <br /> <br /> <br />temperatures at the Durango and Fort Lewis weather stations. The mean eleva- <br /> <br /> <br />tion of the irrigated area in the Mancos valley is about 6800, which is approxi- <br /> <br />mately midway between the elevation at the Durango (Elev. 6550) and Fort <br /> <br /> <br />Lewis (Elev. 7610) weather stations. Although the Mesa Verde weather station <br /> <br /> <br />is actually closer to the Mancos valley, the average monthly temperatures at <br /> <br />-15- <br /> <br />" <br />