Abstract
The Walker River is the main source of inflow to Walker Lake, a closed-basin
lake in west-central Nevada. The only outflow from Walker Lake is evaporation
from the lake surface. Between 1882 and 2008, upstream agricultural
diversions resulted in a lake-level decline of more than 150 feet and
storage loss of 7,400,000 acre-feet. Evaporative concentration increased
dissolved solids from 2,500 to 17,000 milligrams per liter. The increase
in salinity threatens the survival of the Lahontan cutthroat trout,
a native species listed as threatened under the Endangered Species Act.
This report describes streamflow in the Walker River basin and an updated
water budget of Walker Lake with emphasis on the lower Walker River
basin downstream from Wabuska, Nevada. Water budgets are based on average
annual flows for a 30-year period (1971–2000).
Total surface-water inflow to the upper Walker River basin upstream
from Wabuska was estimated to be 387,000 acre-feet per year (acre-ft/yr).
About 223,000 acre-ft/yr (58 percent) is from the West Fork of the
Walker River; 145,000 acre-ft/yr (37 percent) is from the East Fork
of the Walker River; 17,000 acre-ft/yr (4 percent) is from the Sweetwater
Range; and 2,000 acre-ft/yr (less than 1 percent) is from the Bodie
Mountains, Pine Grove Hills, and western Wassuk Range. Outflow from
the upper Walker River basin is 138,000 acre-ft/yr at Wabuska. About
249,000 acre-ft/yr (64 percent) of inflow is diverted for irrigation,
transpired by riparian vegetation, evaporates from lakes and reservoirs,
and recharges alluvial aquifers.
Stream losses in Antelope, Smith, and Bridgeport Valleys are due to
evaporation from reservoirs and agricultural diversions with negligible
stream infiltration or riparian evapotranspiration. Diversion rates
in Antelope and Smith Valleys were estimated to be 3.0 feet per year
(ft/yr) in each valley. Irrigated fields receive an additional 0.8 ft
of precipitation, groundwater pumpage, or both for a total applied-water
rate of 3.8 ft/yr. The average corrected total evapotranspiration rate
for alfalfa is 3.2 ft/yr so about 0.6 ft/yr (15 percent) flushes salts
from the soil. The diversion rate in Bridgeport Valley was estimated
to be 1.1 ft/yr and precipitation is 1.3 ft/yr. The total applied-water
rate of 2.4 ft/yr is used to irrigate pasture grass.
The total applied water rate in the East Fork of the Walker River and Mason Valley was estimated to be 4.8 ft/yr in each valley. The higher rate likely is due to appreciable infiltration, riparian evapotranspiration, or both. Assuming a diversion rate of 3.0 ft/yr, stream loss due to infiltration and riparian evapotranspiration is about 3,000 acre-ft/yr along the East Fork of the Walker River and 14,000 acre-ft/yr in Mason Valley.
In the lower Walker River basin, overall and groundwater budgets were calculated for Wabuska to Schurz, Nev., and Schurz to Walker Lake. An overall water budget was calculated for the combined reaches. Imbalances in the water budgets range from 1 to 7 percent, which are insignificant statistically, so the water budgets balance. Total inflow to the Wabuska–Walker Lake reach from the river and others sources is 140,000 acre-ft/yr. Stream and subsurface discharge into the northern end of Walker Lake totals 110,000 acre-ft/yr. About 30,000 acre-ft/yr is lost on the Walker River Indian Reservation from agricultural evapotranspiration, evapotranspiration by native and invasive vegetation, domestic pumpage, and subsurface outflow from the basin through Double Spring and the Wabuska lineament.
Alfalfa fields in the upper Walker River basin are lush and have an average corrected total evapotranspiration rate of 3.2 ft/yr. Alfalfa fields on the Walker River Indian Reservation are not as lush and have a total corrected evapotranspiration rate of 1.6–2.1 ft/yr, which partly could be due to alkaline soils that were submerged by Pleistocene Lake Lahontan. The total applied-water rate is 7.0 ft/yr, almost twice the rate for alfalfa in the upper Walker River basin. Most of this excess water becomes induced recharge, which is irrigation water that infiltrates to the water table.
Surface and subsurface inflow to Walker Lake total 130,000 acre-ft/yr. Virtually all outflow is evaporation from the lake and totals 162,000 acre-ft/yr at the 1971–2000 average lake altitude of 3,959.3 feet. The difference between inflow and outflow is ‑32,000 acre-ft/yr. Storage change estimated directly is ‑29,000 acre-ft/yr for an imbalance of 3,000 acre-ft/yr (2 percent). This imbalance is insignificant statistically so the water budget balances.
Water budgets were calculated to provide managers a range in supplemental inflows needed to maintain dissolved-solids concentrations at 8,000, 10,000, and 12,000 milligrams per liter (mg/L). From about 700,000 to 2,000,000 acre-ft are needed to dilute the lake to these concentrations from the current (2008) concentration of 17,000 mg/L. From 26,000 to 53,000 acre-ft/yr of supplemental inflow is needed to maintain concentrations of 8,000 to 12,000 mg/L. Years of supplemental inflow, above average inflow, or both, will be needed to raise the lake-surface altitude and dilute salts.