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<br />. <br /> <br />('I ';~)o')l <br />~j~) '..' Ci~, * <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />PART V. HISTORIC IRRIGATION DEPLETION <br /> <br />The depletion to the La Plata River resulting from historic irrigation <br /> <br /> <br />practices was estimated by analyzing the historic ditch diversions and the esti- <br /> <br /> <br />mated irrigation requirement for the crops grown in the La Plata River basin. <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 irrigated <br /> <br /> <br />crops grown in the La Plata River basin are grass hay mixtures and alfalfa. <br /> <br /> <br />Cultivated row crops and orchards make up a very small percentage of the total <br /> <br /> <br />irrigated area. <br /> <br />The consumptive use of irrigation water was estimated through use of <br /> <br /> <br />the modified Blaney-Criddle procedure, calibrated to the area through use of <br /> <br /> <br />data from Iysimeter studies conducted at the Navajo Indian Irrigation Project <br /> <br /> <br />(NIIP). Monthly precipitation data were obtained from the Fort Lewis weather <br /> <br /> <br />station at the San Juan Basin Experimental Station near Hesperus, where the <br /> <br /> <br />mean annual precipitation is 17.6 inches. Mean annual precipitation estimates <br /> <br /> <br />for areas throughout the remainder of the La Plata River basin were obtained <br /> <br /> <br />from the map entitled "Colorado Average Annual Precipitation 1951-80," pub- <br /> <br />lished by the U. S. Geological Survey (Scale 1:500,000). Precipitation in the <br /> <br /> <br />irrigated areas within the La Plata basin varies from about ten inches per year <br /> <br /> <br />near the Colorado - New Mexico state line to about 25 inches per year above <br /> <br /> <br />Hesperus. IsohyetaIs of mean annual precipitation are shown on Figure 3. The <br /> <br /> <br />average annual precipitation for the major areas of irrigation throughout the <br /> <br /> <br />basin is about 12 inches. For the irrigation season, average monthly precipitation <br /> <br /> <br />values for the irrigated areas were estimated by prorating Fort Lewis precipita- <br /> <br /> <br />tion records as follows: <br /> <br />-10- <br /> <br />.ii___.;:.;.; <br />