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surrogate is lake evaporation. Annual average values range from 50 centimeters (20 inches) in northern Maine to 215 centimeters (86 inches) in Southwest California (Figure 3.6). The amount of mean annual lake evaporation reflects the major physical controls of latitude, altitude, and relative humidity. Like precipitation, PET has an annual temporal pattern (systematic variance through the course of the year) as well as year-to-year variation. One important consideration in watershed management is what proportion of the year's moisture loss occurs during the growing season (May to October). Values range from a high of more than 80 percent in the northern U.S. to below 60 percent in south Florida (Kohler et al., 1959).
Soil Water Budgets
Soil water—that is, water contained in soil—is necessary for most plant and animal life. The availability of soil water is a function of both precipitation (counted as income) and evapotransporation (counted as expenditures). These budgets or balances can be expressed in diagrammatic form to show soil water surpluses, deficits, recharge, and utilization of stored soil water (Figure 3.7). Such water budgets not only give insight into a major control on natural processes, they also provide information relevant to irrigation and other water requirements. Amounts of water that exceed a soil's holding capacity move down through the soil into groundwater for aquifer recharge. Some of the groundwater provides baseflow for streams and leaves the region as runoff. Representative soil water budgets for the United States show that the magnitude of deficits and surpluses varies greatly by region and season. The greatest deficits occur in the Southwest, while the strongest surpluses occur in the Pacific Northwest and the East. Short but significant soil water deficits may occur; even in humid areas, near the end of the growing season. Given variations in precipitation and evaporation, water budgets can vary significantly from year to year. ''Drought" occurs when precipitation is far enough below the long-term average to create a soil water deficiency great enough to adversely affect economic and social systems. There are great regional differences in the United States regarding the severity of drought (USGS, 1970).
Streamflow plays an important role in water supply, flooding, navigation, pollution, and recreation. It is composed of two major components: baseflow and stormflow. Baseflow is the more or less continuous flow that results from groundwater and a surplus of soil water. Stormflow results from rainfall or snowmelt events. In soils with high infiltration capacities and hydraulic conductivities, most stormflow may be subsurface except where soil is absolutely saturated. Where infiltration and/or conductivity is low, as in areas affected by compaction