The major source of water to the ground-water flow system is precipitation in the form of rain and snow that infiltrates the land surface, moves downward through the unsaturated zone, and enters the water table as recharge. Water that is evaporated or transpired by plant roots as it percolates to the water-table surface is not ordinarily counted as recharge.

The amount of recharge can vary considerably across the landscape depending on the soil type, precipitation (rates, types, timing, and amount), vegetation, the slope of the land surface, and other factors. The cross section below shows an area of relatively high recharge below a small depression in the land surface which collects runoff and focuses flow downward:

Schematic section of ground-water recharge (source: D.T. Feinstein and J .T. Krohelski, U.S. Geological Survey)

Urbanization changes the natural rate of recharge by adding impervious surfaces (rooftops and pavements) and storm sewers. Irrigation water diverted from surface water can be an important way of locally increasing recharge to the ground-water system in agricultural areas (irrigation is only a minor source of water in southeastern Wisconsin ). Wells by themselves have little effect on the natural rate of recharge, but under some circumstances they can reverse natural gradients and result in transfer of water from streams to the ground-water system.