Most of the northern half of Warren County is in the Northwestern Glaciated Plateau Section of the Appalachian Plateaus Physiographic Province. The remainder of the county is in the High Plateau Section. The glacial outwash sand and gravel hydrogeologic unit is the most extensively used unconsolidated unit for water supply in Warren County because it is capable of yielding large amounts of water to wells and it is situated in populated valleys. The median well yield for 47 specific-capacity tests was 25 gal/min (gallons per minute); well yields ranged from 2 to 1,600 gal/min. Acceptable well yields for domestic supply also are available from other unconsolidated hydrogeologic units including alluvium, colluvium, glacial drift, ice-contact stratified sand and gravel, and undifferentiated alluvium and glacial lacustrine. The median well yields during specific-capacity tests of wells in these five hydrogeologic units ranged from 8 to 18 gal/min.

A comparison of the median specific capacities for wells in the unconsolidated and bedrock hydrogeologic units indicates that wells completed in the outwash sand and gravel hydrogeologic unit had the highest median specific capacity of 6.0 (gal/min)/ft (gallons per minute per foot); specific capacities for wells completed in the outwash sand and gravel unit ranged from 0.14 to 300 (gal/min)/ft. For wells completed in the bedrock hydrogeologic units, their corresponding median specific capacities are Pottsville Group, 0.5 (gal/min)/ft; Shenango Formation, 0.44 (gal/min)/ft; Cuyahoga Formation, 0.24 (gal/min)/ft; Knapp Formation, 0.45 (gal/min)/ft; Corry Sandstone through Riceville Formation, 0.67 (gal/min)/ft; Riceville Formation, 1.5 (gal/min)/ft; Oswayo Formation, 0.07 (gal/min)/ft; Venango Formation, 1.0 (gal/min)/ft; and Chadakoin Formation, 0.71 (gal/min)/ft.

Annual precipitation at Warren for the years 1984-87 was above the long-term mean. The 4-year average of the annual hydrologic balance for 1984 indicated 40 percent of the precipitation was lost to evapotranspiration. Ground-water discharge, commonly defined as base flow, accounted for about 29 percent of precipitation, and surface runoff made up 31 percent. During 1984-87, ground-water discharge made up from 47 to 50 percent of total runoff or streamflow. In 1990, ground-water withdrawals made up only 1.3 percent [13.8 Mgal/d (million gallons per day)] of the total withdrawals for the county. However, ground water is the predominant source for domestic, municipal, and industrial water supplies in Warren County outside of the larger cities.