9581

• UCREFRP Catalog Number: 9581
• Author: U.S. Bureau of Reclamation.
• Title: Final Environmental Impact Statement - Navajo Reservoir Operations Volume II Technical Appendices.
• USFW Year: 2006.
• USFW - Doc Type: Grand Junction - Durango, CO.
• Copyright Material: NO

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to simulate hydrologic interactions between water sources and their uses. Maintaining a
water balance assures that the sum of inflows less the sum 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&I, 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 modeP 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&I 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
NllP and the irrigation in McElrno 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&I uses, and flows directly bypassing the gage.
'Natural reach gains and losses were calculated as the difference in the natural flow
estimates between gaging stations. No lagging of return flows (diversions less depletions)
was incorporated except for the three areas underlain by the simulated groundwater
storage.

I San Juan Basin Model Disclaimer: Use of the model in the work of the SJRBRIP does not necessarily
constitute agreement or approval by individual program participants with the model data, methodologies, or
assumptions. Use of the model does not change the responsibilities of the respective States to maintain records
of water rights and water use. Official records of water rights and water use are maintained by the State agencies
statutorily charged with that responsibility.

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