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<br />15 <br /> <br />evaporation-transpiration. A major amount of continental water arises as a <br />result of ocean evaporation that is evenly distributed among the land <br />masses. Evaporation from the oceans and land results in an average annual"" <br />precipitation within the conterminous U.S. of approxi:utely 76 cm (30 <br />in.). Approximately 53 cm (21 in.) of the 76 cm precipitated is .!vaporated <br />to the oceanic atmosphere. Therefore, other than a slllall aIDount (0.0001 <br />inch) that seeps into the groundwater, 23 em (9 in.) of the total <br />precipitation enters streams that return the ..ater to the oceans (Figure, <br />8). ' <br /> <br /> <br />-~ <br />-"" <br /> <br />FIRure t. Water lud.at for the continental United Stat... (U.S.C.St) <br />( lIy perwlufon from: W.K. Fre...n and Coap.n,~ San Franc.bco. L. I. <br />Leopold. 1974. Water A Primer) <br /> <br />The uneven distribution of precipitation within the conterminous <br />United States can be dramatically illustrated by drawing a line from north <br />to south along the borders of the states of Kansas and Missouri and <br />comparing average annual precipitation records on a re,gional basis (Figure <br />9). ApprolCillately 73 percent of the total I'rec1pitati,on falls on n states <br />east of the Kansas-Missouri state lines. The northwest states of <br />Washington and Oregon receive 12.7 percent .of the total and only 11,.2 <br />percent of the total falls on 14 add and s,!lIl1arid western states. Thus <br />unequal distribution and availability of water require many differE!nt types <br />of management strategies on a natiotlal basil!. <br /> <br />Interception <br /> <br />Any type of lIOisture falling Otl a watel:shed may rE,ach the groUlnd and <br />stream surfaces directly or be intercepted by vegetaticlD (Figure 7). Part <br />of this intercepted water may evaporate and reenter thE! atmosphere. The <br />remainder tends to reach the ground through stem-flow c,r drips through the <br />leaf cover and canopy. The relative amount of intercepted llOisture <br />reentering the atmosphere or entering the watershed depends on the extent <br />of vegetative cover, the intensity and duration of prec:ipitation, <br />temperature and other hydrologic factors. In humid, heavily vegetated, <br />old-growth forest areas, interception losses to the atulosphere may be <br />significant depending on local and seasonal climatic fa,ctOrs. For example, <br />evapotranspiration losses may be low during the winter period in a <br />decidious forest. <br />