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<br />17 <br /> <br />released to the atmosphere from the tiny openings called stomata located on <br />the leaves and other plant surfaces. The term evapotI'anspiration has been <br />coined, and generally applies to the combined return olf moisture by both <br />evaporation and transpiration. Often, the terms evapo'ration, <br />transpiration, and evapotranspiration are ulled interchangeably. <br /> <br />Infiltration <br /> <br />Once precipitation reaches the land surface it either infllu'ates or <br />moves by overland flow. In undisturbed forests, infiltration ratellOI <br />soils generally far exceed the maxilllum rates of rainfall so that 1111 wster <br />enters the soil. Within the soil, vater is subject to gravitational and <br />capillary forces that cause it to move and frictional forces that tend to <br />restrict movement. Because of the slope of .ost watersheds and be,cause <br />soil conductivity generally decreases with depth, water entering the 8011 <br />begins to move downslope as it moves deeper into the soil. The di,rection <br />and rate at which the water moves depend on precipitation rates and the <br />texture of the land surface. Both rate and direction vary considerably <br />during the course of a storm (Harr 1977). <br /> <br />The litter layer within forests and riparian zOlles directly .~fects <br />rates of infiltration. A 5.08 cm (two-inch) layer of litter amounts to <br />about two tons per acre and is sufficient to minimize forest land erosion. <br />Functionally, litter absorbs the erosion energy of raindrops. As litter <br />decomposes and is incorporated into mineral soil, soil density is reduced, <br />porosity is increased, and infiltration rates of about 50.8 cm (20 inches) <br />to over 254 cm (100 inches) per hour are common (Dougl'ISS and Seehorn <br />1974). Since rain nearly alway. falls at rlLtea of leall than eight inches <br />per hour, virtually all rainfall infUtratel1 into fore,;t and riparian <br />soils, and surface runoff from upland forest soila is I~are. <br /> <br />The maximum velocity of soil water flo>1 is low and frequently about <br />equal to the average rate of rainfall durin~: a storm. This slow-mOVing <br />soil water is subject to evaporation and deplel:ion by plant transpiration. <br />The rate of evapotranspiration is largely re,lated to the solar energy <br />available for vaporization frOll leaves and the availabJllity and ease with <br />which water may be withdrawn from the soil. <br /> <br />Stream flow, on an annual or longer bas.:Ls, is the difference :between <br />precipitation and evapotranspiration losses (Harr 1976). Although the <br />amount of soil moisture IIl8Y change as a result of climlLte and <br />evapotranspiration, water not removed by plants ultimately moves downslope <br />as saturated or unsaturated flow to supply streams. TILe quantity ,md rate <br />at which water reaches the channel and passes through II watersbed llystem <br />during a storm is influenced by the amount snd duratiorL of precipitation, <br />watershed size, etc. Streams will rise and fall rapidly in relaticm to <br />precipitation, storm duration, snowmelt and I:he amount of water in the <br />soil. As the watershed responds to precipitlltion, stre,amflow incrnases to <br />a IIl8XilllUll known .. the 'peak flow.' <br />