Historical, future, and potential stream capture from groundwater pumping in the middle Humboldt River Basin (MHRB), Nevada, is estimated using a calibrated numerical groundwater flow model. The model was developed to estimate (1) stream capture, which is the change in flux between the groundwater system and the Humboldt River and tributaries, and (2) change in streamflow, which is the change in streamflow estimated for the Imlay gage on the Humboldt River (U.S. Geological Survey streamgage 10333000). Historical stream capture for water years (WYs) 1961–2015 is estimated using recorded and estimated groundwater pumping during that period. Future (predictive) stream capture was based on historical stresses (WYs 1961–2015) using a scenario that simulated non-mine pumping from WY 2015 at a uniform rate for 100 years into the future. Potential stream capture throughout the middle Humboldt River Basin from groundwater pumping during varying durations of time are presented in a series of capture maps. Maps also are presented that show the potential to capture from groundwater evapotranspiration, as well as the storage changes for pumping duration of 100 years.

Estimates of historical stream capture from the mainstem Humboldt River during the early 1960s are less than 400 acre-feet per year (acre-ft/yr) when groundwater withdrawals and pumping rates were relatively small compared to more recent times. In the late 1980s and early 1990s, groundwater withdrawals increased and estimated historical stream capture also increased from about 4,000 acre-ft/yr in the late 1980s and early 1990s to as much as 18,800 acre-feet (acre-ft) in WY 1998. In WY 2015, estimated historical stream capture declined to about 13,000 acre-ft because of decreasing groundwater withdrawals and lower streamflow during the drought of WYs 2012–15, resulting in less stream water available for capture. Stream capture was estimated for 100 years into the future based on WY 2015 non-mine pumping rates and mine-dewatering activity through WY 2015. Stream capture is forecast to increase to about 23,000 acre-ft/yr, and streamflow in the Humboldt River could decrease by as much as 19,000 acre-ft/yr.

Pumping for mine-dewatering and the associated discharge of that water affects streamflow in the Humboldt River at Imlay, Nevada (U.S. Geological Survey streamgage 10333000). Historically, from WYs 1991 to 2015, streamflow was greater at Imlay gage during active mine-dewatering from mine-water discharge operations and increased by as much as 105,000 acre-ft in WY 1998. The increase was attributed mostly to the discharge of groundwater from mine-related dewatering operations directly into the mainstem Humboldt River or its tributaries, with some of this increase associated with return flows from discharge to rapid infiltration basins. Results indicate that streamflow at Imlay gage is expected to decrease by as much as 1,600 acre-ft/yr 30 years after mine-related pumping and discharge are discontinued. The streamflow reductions at the Imlay gage are expected to then decrease to around 500 acre-ft/yr, 100 years after mine-related pumping and discharge are discontinued.

Potential capture maps were produced for pumping durations of 10, 25, 50, and 100 years. Capture map results indicate that areas of greater potential stream capture occur adjacent to the Humboldt River and for upstream tributaries areas north of the Humboldt River.

The Humboldt River in the middle Humboldt River Basin (MHRB) is a water source that supports substantial agricultural development in northern Nevada. Additionally, groundwater in the MRHB is pumped to support agriculture, energy, municipal, and mining operations. This study evaluates the effects of groundwater pumping on streamflow and estimates stream capture for the Humboldt River and MHRB. A calibrated numerical groundwater-flow model was used in this study to estimate historical and future stream capture from groundwater pumping in the MHRB. Historical stream capture for the Humboldt River and its tributaries, specifically from water year 1961 to water year 2015, was determined based on recorded and estimated groundwater pumping during that period and was about 400 acre-feet per year during the early 1960s, 4,000 acre-feet per year in the late 1980s and early 1990s, and 13,000 acre-feet per year in water year 2015. Stream capture from the Humboldt River is forecasted to increase to as much as 23,000 acre-feet per year 100 years into the future, an increase from the estimated historical stream capture. Forecasted streamflow in the Humboldt River could decrease by as much as 19,000 acre-feet per year after 100 years of pumping for agricultural, municipal, and energy-related uses. Historical pumping for mine-dewatering and the associated mine-water discharge are forecasted to reduce streamflow at the Imlay streamgage in the Humboldt River by as much as 1,600 acre-feet per year 30 years after mining operations are discontinued. Streamflow reductions from historical mining operations are forecasted to be 500 acre-feet per year 100 years after mining operations are discontinued.