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composition and density. The model determines the form of precipitation <br />~, <br />' <br />` <br />' (rain or snow), the melting process, and sno~.apack condition in terns of <br />~ energy levels an d free water requirements. <br />~ <br /> The energy balance is indexed by shorto-rave and longo-rave radiation <br /> which is co-~puted from base-station temperature measurec~ents . Where <br /> forest cover is present, shortti:ave radiation reaching the sn ovrpacl; surface <br /> is estimated by means of a transmissivity coefficient ~rhich varies as <br /> the species and density of the forest cover change. <br />The sno~,apack is assumed to behave as a dynamic heat reservoir; thus, <br />all elements in the sno~,Telt portion of the model, including amount and <br />type of precipitation, are expressed in units of heat. Te„~peratures ~•rithin <br />the sno~.apack are computed using unsteady heat-flow theory. The pac4: will <br />yield melt rater only when it has been priced (temperature = 0°C, and its <br />free-grater-holding capacity is satisfied). <br />Evapotranspiration is computed by a modified version of the Hamon <br />equation (Ha.^.,on, 1961), which is adjusted in proportion to the radiation <br />actually received each day. The adjusted evapotranspiration is then re- <br />defined according to its source, which can include: (1) evaporation from <br />• , snow interce?ted by the forest canopy, (2) evaporation from the snot•rpack <br />~: <br />surface, and (3) evapotranspiration durinc the groaning season. <br />If the source is evapotranspiration, further adjustments are made for <br />(1) available soil water in open or forested areas by a method similar to <br />that proposed by Denmead and Shaw (1952), and (2) the reflectivity of open <br />• or forested areas. "evaporation can take place from either the sno~•rpack <br />'I sur`;c2, or front snoar beneath the trees and from tree canopies, depending <br />en :he presence or absence of forest cover. <br />;.?ut to the watershed system is derived from snoa.~nelt and rainfall. <br />Once evapotranspiration requirements have been satisfied, subsequent input <br />is used to replenish any soil-water deficit. The residual input becomes <br />water available for streamr"loan (generated runoff). Figure 10 is a flow <br />diagram of the system. <br />r1 <br />f`J <br />G-24 <br />