Navajo Reservoir Operations Vol. II

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Hydrologic simulation models, such as RiverWare, are essentially mass balance models operating within a rule-based framework to simulate hydrologic interactions between water ~~.___ __ _ _..J ..1_ _:_ ___ _ _ .. _:_.._ :__:_ _ _ ____..__ L _1__ ~ _ _ ___.__ _ ..L _.. ..L _ _..~ _~ ;_.C1_n.~ 1~~~ ..L_ ~.._ ;:)UW"'~~ auu Ll1'-'l1 U~t:~. IVld.Ull4ll1l11e a Wc1ll;;l Ualalll"t: a~~uu;::, lllaL Lilt:: ':'UIU VI lIJ..lIVW;:) 1"';:);:) Ul\" ;:)U.L1.l of outflows equals the change of storage within the basin. Water inflows consist of natural stream flows, trans-basin inflows (e.g., Dolores Project return flows), and precipitation. Outflows consist of water flowing across the downstream basin boundary (San Juan River at Bluff), consumptive use (crops, M&l, natural vegetation, free water surface evaporation, etc.), and trans-basin diversions (San Juan-Chama). Water storage consists of the water within basin lakes and reservoirs, soils, and groundwater aquifers. . In the San Juan River model, only unnatural (man-induced) hydrologic effects are explicitly modeled. The model begins with the natural inflows and natural, ungaged, gains and losses to river reaches. Starting from this basis eliminates the need to model natural hydrologic processes such as rainfall/runoff. Thus, precipitation falling upon natural vegetation, consumptive use by natural vegetation, runoff of excess precipitation, evaporation from the free water surfaces of rivers, etc. are assumed to be reflected in the natural inflows and reach gains and losses and are therefore not modeled. Likewise, it is assumed that precipitation runoff from man-affected areas (agricultural lands, cities, etc.) is not significantly different from natural conditions to warrant explicit modeling treatment. . Thus, the inflows for the simulated water balance of the San Juan River Basin consist of the estimated natural inflows, stream reach gains, and the Dolores Project return flow to the San Juan River Basin. The outflows consist of the man-affected (gaged) flow of the San Juan River at Bluff, consumptive irrigation (irrigated crop evapotranspiration less effective precipitation), M&! depletions, net (in excess of natural) evaporation from manmade reservoirs and stock ponds, and the San Juan-Chama trans-basin diversion. The change in storage is reflected in the difference between beginning and ending reservoir content and groundwater volume. Groundwater storage in the current model includes the underlying NIIP and the irrigation in McElmo and Montezuma creeks. The effects of soil water storage for irrigated lands are assumed to be reflected in the effective rainfall and consumptive irrigation calculations and are not explicitly modeled. The 1970 to 1993 monthly natural flows expected at 23 gaging stations along the San Juan River and its tributaries above Mexican Hat, Utah, were calculated by Reclamation. The monthly natural flows were estimated by adjusting gaged flows (Hydrosphere, 1998) to account for upstream irrigated crop depletions, reservoir influences (operational and evaporative), trans- basin diversions, M&! uses, and flows directly bypassing the gage. Natural reach gains and losses were calculated as the difference in the natural flow estimates between gauging stations. No lagging of return flows (diversions less depletions) was incorporated except for the three areas underlain by the simulated groundwater storage. Irrigated crop depletions were calculated using the SCS TR21 modified Blaney-Criddle consumptive use less effective precipitation (Soil Conservation Service, ! 970). When water supplies are insufficient to meet diversion requirements for full crop demand, shortages are simulated following Reclamation Type! study approach (Reclamation, 1971). Reclamation's XCON program was used to compute both non shorted and shorted irrigation depletions. A-3 'f1{1r356