Estimated effects of pumping on groundwater storage and Walker River stream efficiencies in Smith and Mason Valleys, west-central Nevada

First posted December 27, 2022 For additional information, contact: Director, Nevada Water Science CenterU.S. Geological Survey2730 N. Deer Run Road, Suite 3Carson City, 89701 The Walker River originates in the Sierra Mountains and flows nearly 160 miles to its terminus at Lake west-central Nevada. river provides a source of irrigation water for tens thousands acres agricultural lands California is principal inflow Lake. Extraction groundwater use became prevalent late 1950s early 1960s supplement demands not met by surface-water diversions during times drought. There growing concern that continued withdrawals within Basin are likely contributing depleted streamflow long-term depletion storage basin. This report documents changes storage-volume trends stream efficiency, measure change flow due gaining or losing conditions, two largest valleys Basin, Smith Mason Valleys, multi-decade period. Groundwater-level maps from previous studies were used beginning (1970) middle (2006) points this study. Groundwater levels measured 1991–95 2016–20 construct median groundwater-level represented conditions 1995 2020. Valley wide was calculated comparing periods 1970–95, 1996–2006, 2007–20 observing overall 1970 using previously defined specific yield values. Between 2020, declined 287,600 acre-feet 269,000 Valley. Using decline annual pumpage rates, maximum rate can be computed support management resources. In Valley, pumping excess 22,300 per year would result decline. 75,200 Stream efficiency continuous data monthly diversion volumes on reaches: (1) West 1948 2020 (2) 1958 non-irrigation season Valleys statistically significant 1.1 0.6 percent year, respectively. Trends corresponded occurrence prolonged drought, deviation average streamflows, total pumpage. Long-term declines demonstrate alluvial aquifer system becoming increasingly depleted, such no longer replenish while simultaneously balancing withdrawals. introduction supplemental intended offset deficits dry years; however, occurs even years when above streamflows meet demands. Reliance has resulted widespread decreased efficiency. With each successive drought cycle, ability sustain convey downstream diminished. Above wet have marginal short-lived effect rebounding outside corridor. Moreover, if trend continues, future cycle may further reduce supplies decrease reliability.