Watershed functions that dominate the hydrologic environment are identified and discussed. Hydrological and ecological functions are considered in relation to the storm and annual hydrographs, and to water quality. Two integrative watershed responses to these functions are also articulated. Since most of the Earth's water is in storage, consideration of the hydrologic cycle as movement between water storage sites enhances this functional and response characterization of the watershed which, in turn, suggests guidance and direction for the restoration of watershed functions.

Consideration of a drainage basin as the basic unit of supply and management goes beyond merely calling attention to the need for watershed management based on what we typically visualize of the natural environment. This article seeks to establish a broad base for anyone who wishes to base policy decisions on sound science, in this case, on some readily identified and easily comprehended fundamentals about natural and disturbed hydrologic environments. This article organizes, defines, and explains the importance of seven basic functions of watersheds, augmented by some essential topics aimed at a more thorough understanding of the functions.

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Long ago, Fernow (1893)-wrote concerning "the desirability of utilizing the Weather Bureau, the various agricultural experiment stations, and other forces, in forming a systematic service of water statistics, and in making a careful survey of the conditions of water supplies, which may serve as a basis for the application of rational principles of water management. " Over the intervening years, many of these statistics have been amassed for other States, but many still are unavailable for Alaska. This report is a step for Alaska along the path pointed out by Fernow. Although Alaska has about 200 currently active climatological stations, it has only 0. 03 rain gage per 100 square miles, or about one-tenth the gage density for conterminous United States. This density is inadequate for realistic maps of precipitation, temperature, or runoff. Nevertheless, the U.S. Weather Bureau is accumulating a large and growing record of precipitation and temperature over the State, and the U.S. Geological Survey heads an expanding streamgaging program. There has been less attention to evaporative losses which determine differences between precipitation income and water available for human needs. As Thornthwaite (1948) pointed out, wet and dry climates are determined neither by total nor seasonal precipitation but by the relation of precipitation to the evaporative demand. For example, precipitation amounts are nearly equal in California's Mojave Desert and in Alaska's forested and frequently boggy interior. The important and often overlooked difference between climates of these regions is the amount and timing of the evaporative demand-over 100 inches yearlong on the Mojave; only about onesixth as much during summer in Alaska's interior. The evaporative demand usually is established at climatic stations containing evaporation pans and associated meteorological instruments requiring regular observation and service by trained personnel. Before 1963, there were only two evaporation pans in Alaska; since then, four more have been installed and others are planned. Even this severalfold expansion of evaporation measurement constitutes a minute sample in a State one-fifth the area of the 48 conterminous States. Lacking direct measurement of evaporation, one can estimate evaporative losses from weather data which have been routinely obtained at hundreds of climatic stations. The importance of the evaporative loss is attested by the development of many formulas for estimating it from these more easily obtained climatic data. The large number of formulas also attests that none is wholly suited to its purpose. Penman's (1948) equation is accepted as best founded theoretically, but requiring sunshine, humidity, and wind data which are reported at only four climatic stations in Alaska. Papadakis (1961) fitted climate for a few Alaska stations into his worldwide classification. Thornthwaite ' s (1948) equation has Names and dates in parentheses refer to Literature Cited, p. 27. probably been tested more widely than any other. Penman (1956) re...