After precipitation falls on the land, a portion of it evaporates, part of it infiltrates into the soil, and the remainder flows over the surface as runoff. The importance of each process varies as the cycles of temperature and rainfall change with the seasons. In general, however, if less water is able to infiltrate, then more of this water will become runoff. Water which infiltrates into the soil profile may be absorbed by plants and transpired, or it may flow downward through soil pores until it reaches a saturated zone, becoming temporary interflow or more permanent baseflow to streams (Fig. 13). Care must be taken to avoid adding pollutants, excess fertilization, or pesticides on soils that are aquifer recharge areas (Fig. 10) or have severe risk of leaching. Because of its slow rate of movement, if groundwater becomes polluted, it may remain so for many years, and environmental cleanup costs can be high.

Figure 13. Changes in watersheds and groundwater as a result of urban development and human modification (Schueler, 1987).