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<br />CHAPTER 3. RUNOFF FROM SNOWMELT <br /> <br />Section 3.01. Introduction <br /> <br />Snow is hydrologically important in many parts of the world. Generally, <br /> <br /> <br />snowmelt is an important factor in hydrograph analysis in mountainous and <br /> <br /> <br />high plains basins where the snowpack accumulates throughout the snowfall <br /> <br /> <br />season and causes flooding during the melt season. Also, the presence of <br /> <br /> <br />snow can greatly influence the short-term runoff characteristics of a basin <br /> <br /> <br />under certain conditions of air temperature and rainfall. The evaluation <br /> <br /> <br />of snowmelt quantities contributing to runoff can be important for (a) the <br /> <br /> <br />determination of floods for planning and design purposes, (b) operational <br /> <br /> <br />flood forecasting, and (c) hydrograph analysis for determining unit hydro- <br /> <br /> <br />graph and loss rate characteristics. <br /> <br /> <br />A snowpack can be considered a frozen reservoir for which the water <br /> <br /> <br />equivalent can be measured quantitatively if given enough time and effort. <br /> <br /> <br />The problem, however, is to be able to adequately predict energy supplies <br /> <br /> <br />so that an estimate can be made of the melt rate or the rate of delivery <br /> <br /> <br />of liquid water to the snow-soil interface. As opposed to rainfall, snow- <br /> <br /> <br />melt quantities cannot be measured directly, but must be estimated from <br /> <br />meteorological parameters. The rate of overall basin snowmelt is governed <br /> <br /> <br />by the transfer of energy involving radiation, convection, advection, and <br /> <br />conduction. The relative importance of each of these processes of heat <br /> <br /> <br />transfer is highly variable, depending upon conditions of weather and local <br /> <br />environment. The natural sources of heat energy in melting snow are absorbed <br /> <br /> <br />solar radiation, net longwave (terrestial) radiation, convection and advec- <br /> <br /> <br />tion heat transfer from the air, latent heat of vaporization by condensation <br /> <br /> <br />from the air, conduction of heat from the ground and from rain water. Since <br /> <br /> <br />heat from the ground is usually minor in relation to the other sources of <br /> <br /> <br />heat where the snowpack is significant, it is usually neglected in deter- <br /> <br /> <br />mining snowmelt quantities during floods. <br /> <br />3-01 <br />