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. i'1tl""Q~1 u ,..10,.1,)'1 . PART vm. HISTORIC DEPLETION . The use of water in the San Juan River basin results in depletion to the river system. In this analysis, depletion is defined as the reduction in streamflow that occurs as a result of water useage, taking into account stream diversions, consumptive use of water by crops, evaporation, and return flows including the lagged effects of groundwater return flows to the river system. Consumptive use is that portion of the diverted water which is lost to the river system, primar- ily through evaporation and crop evapotranspiration. Depletions to the San Juan River occur primarily through trans-basin diversions by the San Juan-Chama Project and irrigation and crop evapotranspiration, with minor amounts of deple- tion resulting from reservoir evaporation and municipal uses. The depletions to the San Juan River resulting from historic, irrigation practices were estimated by analyzing irrigated acreage and estimated irrigation requirements for crops grown in the basin. Municipal depletions are generally negligible, except for those associated with lawn and park irrigation which are relatively small com- pared to the overall basin irrigation. Although there are many small irrigation and stock water ponds within the basin, there are only three reservoirs of signifi- cant size in the basin (excluding Navajo Reservoir). The reservoirs do not appre- ciably affect the river flows with storage and releases, and contribute only minor river depletions with evaporation losses. The larger reservoirs are Echo Canyon Reservoir on Echo Creek south of Fagosa Springs, Fagosa Springs waste water disposal ponds, and Dulce Lake on Dulce Creek in New Mexico. . . . . . Crop Consumptive Use Based on interviews with the Water Commissioner, it was determined that the principal crops grown in the San Juan River basin are grass hay mixtures and minor amounts of alfalfa. Some timbered lands have been cleared and irri- gated to improve the range pasture. Most of these areas are not intensely irri- gated. . . The consumptive use of irrigation water was estimated using the modified Blaney-Criddle procedure, calibrated to the area through use of data from lysi- -18- . ;it..:..