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Ground-water flow in Paradise Valley, a basin tributary to the Humboldt River in north-central Nevada, was studied as part of a regional aquifer-system analysis in the Great Basin region of Nevada, Utah, and adjacent States. The valley was chosen because it is typical of the many basins that drain into the river. The principal technique used to analyze ground-water flow in Paradise Valley was with a computer program that simulates ground-water flow in three dimensions. Results from computer simulations are used to illustrate the possible effects that increased pumpage could have on ground-water flow in Paradise Valley and, by analogy, in other similar basins tributary to the Humboldt River. Basin fill, which consists mostly of discontinuous lenses of gravel, sand, silt, and clay, and lesser quantities of volcanic deposits, forms the primary aquifer in Paradise Valley. The deposits may exceed 8,000 feet in thickness near the center of the valley but pinch out around the valley edges where consolidated rocks are exposed. The elongate valley (at most 13 miles wide and about 40 miles long) trends northeasterly and is bounded by mountains on the north, west, and east sides; it merges with Humboldt River Valley to the south. Recharge to the basin-fill aquifer is primarily from streams that enter the valley from the surrounding mountains and from two streams that enter the northeastern part of the valley from adjacent areas by way of narrow canyons. Estimated average annual streamflow into the valley is about 72,000 acre-feet. During periods of high streamflow, surface water flows to sand dunes that cross the southern end of the valley, forming a lake that remains until the water seeps into the ground, is evaporated, or is drained to the Humboldt River if a temporary channel is dredged through the dunes. Discharge from the basin-fill aquifer is primarily from evapotranspiration in the valley lowlands adjacent to streams. Results of a simulation of conditions prior to ground-water pumpage indicates that southward ground-water flow from Paradise Valley into Humboldt River Valley was only 1,800 acre-feet per year, whereas as much as 1,300 acre-feet per year may have been flowing northwestward from Humboldt River Valley into Paradise Valley near Golconda Butte. Ground-water pumpage in Paradise Valley increased slowly from about 200 acre-feet in 1948 to 6,800 acre-feet in 1970. Pumpage increased dramatically in the 1970's when the southern end of the valley was determined to be an ideal location for growing potatoes; in 1982 pumpage totaled about 44,000 acrefeet. The increase in pumpage altered the direction of groundwater flow, which prior to development was from the margins of the valley toward its axis and then southward toward the Humboldt River. As of 1982, ground-water flow in Paradise Valley was toward a water-table depression near the heavily pumped area in the southern end of the valley. Five different development scenarios were simulated in which pumpage was concentrated in different areas of Paradise Valley. The purpose of these simulations was to illustrate the effects of different ground-water development patterns on ground-water flow in Paradise Valley. The first scenario assumed pumpage equal to net pumpage (total pumpage less what was estimated to return to the aquifer) of about 39,000 acre-feet per year estimated for 1982: 36,000 acre-feet per year in Paradise Valley and about 2,800 acre-feet per year in adjacent Humboldt River Valley. After 300 years of pumpage, results showed water-level declines in the southern part of Paradise Valley of more than 200 feet. These declines were near the area where pumpage was most concentrated in 1982. Ground-water flow from Paradise Valley to Humboldt River Valley had decreased 1,700 acre-feet per year, whereas groundwater flow from Humboldt River Valley into Paradise Valley increased 7,400 acre-feet per year. Net pumpage for the other four scenarios was set equal to 72,000 acre-feet per year, but the location of pumpage in Paradise Valley changed. The distribution of pumpage in the valley for each simulation included, respectively, wells concentrated at the southern end; wells concentrated at the northern end; wells concentrated along the central part; and wells distributed throughout the area of evapotranspiration prior to pumpage. Concentrating pumpage at either end of the valley resulted in simulated water-level declines that locally exceeded 400 feet after only 75 years of pumping in the northern end and after 100 years of pumping in the southern end. In addition, pumpage concentrated at the southern end of the valley produced a reversal of net flow between Paradise Valley and Humboldt River Valley. Prior to pumping, the estimated net flow was 500 acre-feet per year from Paradise Valley into Humboldt River Valley, whereas after a simulated period of 50 years, the net flow was 15,000 acre-feet per year from Humboldt River Valley into Paradise Valley. Simulations in which pumpage was concentrated in the central part of the valley and distributed throughout the area of evapotranspiration resulted in water-level declines of generally less than 200 feet after 300 years; both simulations nearly reached a new equilibrium and captured much of the predevelopment evapotranspiration.

Ground-water flow and simulated effects of development in Paradise Valley, a basin tributary to the Humboldt River in Humboldt County, Nevada