Evaluating cumulative effects of ground-water withdrawals on streamflow

Cumulative effects of ground-water withdrawals on streamflow of many large river basins are difficult to evaluate because of climate variability and ever-changing interactions between ground water and streamflow. The Humboldt River basin in Great Basin region of north-central Nevada was used to evaluate the cumulative effects of ground-water withdrawals from a basin with considerable variation in topography, climate, and geology.; Statistical analyses and a numerical ground-water flow model were used to determine historical effects and to make predictions once ground-water withdrawals cease. Statistical analysis of ground-water withdrawals in relation to climate variability in the basin upstream of Palisade, Nevada had no effect on annual runoff of the Humboldt River from 1950-99. During the 1990's, ground-water withdrawals resulted in more annual runoff downstream of Palisade because water pumped from three gold mines was discharged to the Humboldt River or one of its tributaries.; The numerical model provided a means for separating annual climate variability with effects of ground-water withdrawals and allowed for an analysis once withdrawals cease. Model simulations of historical withdrawals were consistent with the statistical analysis. Annual runoff in the Humboldt River was simulated to decrease downstream of areas of ground-water withdrawals once ground-water withdrawals cease and water no longer is discharged to the river from the mines. Greatest decease in annual runoff was simulated at the downstream most location (Comus gage) immediately following the cessation of withdrawals although the additional losses were a small fraction of the annual runoff. Annual volumes of additional loss were less during drought years and more during years of high runoff. However, the percentage of the additional loss was more during drought years than during years of high runoff because there simply was less flow in the river. Annual volumes of runoff at the Comus gage continued to be affected by the groundwater withdrawals 63 years after all withdrawals stopped, although the loss was less than 1 percent of the annual runoff. This suggests the decrease in annual runoff from groundwater withdrawals could continue long after withdrawals cease, but the additional losses likely will not be discernable from natural losses.