A Water Budget for Lake Tahoe James C. Trask and Graham E. Fogg Hydrologic Sciences Program, University of California, Davis The annual water budget for Lake Tahoe is an estimate of the water inflows and outflows to and from the Lake over a period of one year. The main sources of Lake Tahoe water are precipitation directly onto the Lake surface, the many streams flowing into Lake Tahoe, and groundwater inflow. These inputs are balanced by outflow via the Lower Truckee River, evaporation, and perhaps groundwater outflow. Knowledge of the Lake Tahoe water budget is important for assessing water quality and resource issues. For example, nitrogen, phosphorous, and sediment concentrations have been monitored in some streams flowing into Lake Tahoe. To convert these concentrations into actual amounts reaching the Lake, estimates of streamflow magnitudes are also needed. Furthermore, potential impacts of groundwater pumping and export of wastewater cannot be determined without a basin water budget. Previous water budget estimates. A number of published and unpublished estimates of the Lake Tahoe water budget are documented in reports dated from 1902 to 1999. Each updated estimate includes either (a) a water budget for a specific time period; or (b) a refinement of previous budget estimates based on new measurements and/or methods. New water budget estimate. The water budget reported here is a long-term estimate of average annual inflow and outflow rates, combining information from previous reports for the period between 1901 and the 1980's. Except for evaporation from the Lake surface and groundwater net inflow, all inflow and outflow estimates used in this budget are based on measurements. Accurate measurements of Lake evaporation have only recently been initiated , but high-quality evaporation measurements at several other Sierra Nevada lakes together with recent measurements at Lake Tahoe provide adequate information for estimating Lake Tahoe evaporation. Because local and regional measurements of groundwater flow in Tahoe Basin aquifers are insufficient to provide direct estimates of groundwater flow into the Lake, groundwater inflow was estimated as a residual term (that is, by subtracting non-groundwater outflow from non-groundwater inflow). Summary of Lake Tahoe Water Budget Total Annual Inflow (avg.) = Total Annual Outflow (avg.) = 650,000 acre-feet. Figures 1a and 1b are long-term estimates of the average annual magnitudes of the principle inflows to and outflows from Lake Tahoe. The total inflow or outflow is equivalent to 5.3 ft of water over the area of the Lake. Note that runoff (mainly from streams) supplies more water into Lake Tahoe than does precipitation (mainly snow and rain) onto the Lake surface. About 3.5 feet of water evaporates from Lake Tahoe each year. This is about double the average annual outflow via the Lower Truckee River. Sources of the estimates in Figures 1a and 1b are given below: Precipitation directly onto Lake Tahoe: The amount given for precipitation in Figure 1a is the average of the values reported by Crippen and Pavelka (1970) and Marjanovic (1989). In each of these reports, precipitation onto Lake Tahoe was estimated using data from rain gages that had been in place over various time periods at many locations in the Tahoe Basin. Watershed Runoff into Lake Tahoe: The runoff estimate in Figure 1a is based on the estimate reported by Marjanovic (1989) weighted by the average precipitation estimate. Lake Tahoe receives runoff from 63 tributaries and 44 intervening areas. Marjanovic used stream gage data from 12 streams; which drain about half the watershed surface area. For non-gaged streams, runoff coefficients were estimated based on a watershed slope index developed by Marjanovic using the gaged watersheds. For intervening areas without streams, standard handbook values for runoff coefficients were used. Groundwater flow into Lake Tahoe: The estimate of groundwater flow in Figure 1a is computed as the residual, such that total non-groundwater inputs (Fig. 1a) match total water outputs (Fig. 1b). Outflow from Lake Tahoe via Lower Truckee River: The estimate for outflow in figure 1b is the average of the values reported by Crippen and Pavelka (1970) and Marjanovic (1989). The Lower Truckee River is the only surface water outlet from Lake Tahoe. The dam at Tahoe City defines the starting point of the Lower Truckee River; and is used to control the flow rate from Lake Tahoe into the Lower Truckee River. This outflow magnitude has been monitored by stream-gages. Evaporation from Lake Tahoe: The estimate for evaporation in Figure 1b is the average of reported estimates for Shaver, Huntington, and Florence Lakes (3.17 feet) and of an estimate for Boca Reservoir (3.96 feet). Shaver, Huntington, and Florence lakes are located at elevations 5376, 6954, and 7345 feet, respectively, in the upper San Joaquin River Basin (Note: Lake Tahoe elevation is ~6225 feet). Evaporation from each of these lakes was estimated using the product of measured pan evaporation and a pan coefficient of 0.74 (Longacre and Blaney, 1962). The average value of estimated annual evaporation from these three lakes is 3.17 feet. Boca reservoir is located 15 miles north of Tahoe City at an elevation of 5575 feet. Annual average evaporation of 3.96 feet from Boca reservoir was estimated using National Weather Service pan evaporation data and a pan coefficient of 0.74.