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'e e e EM 1110-2-1406 5 Jan 60 CHAPTER 9 SYNTHESIS OF SNOWMELT HYDROGRAPHS 9-01. GENERAL CONSIDERATIONS. Synthesizing streamflow from snowmelt involves many of the same basic prohlems as for rainfall, but witb several additional major considerations peculiar to snowmelt alone. Thcse include; (1) tbe maximum rates of snowmelt arc moderate and are generally far less than rates of maximum rainfall; (2) the duration of snowmelt may extend over a period of a month or two, while storm rainfall is generally restricted to much shorter times; (3) the area of snow cover continually changes during the melt period, so tbat tbe basin area con- trillUting to runoff varies througbout the melt period; (4) unlike rainfall, snowmelt is not a measured quantity in hydrologic practice, and tbe variation in snowmelt must be estimated indirectly by snowmelt indexes or general cquations; (5) initial requirements of water to meet soil moisture deficiencies arc generally satisfied early in tbe snowmelt period, so tbat during tbe major runoff period losses are at a minimum; (6) a larger proportion of water input travels tbrougb tbe base flow component of streamflow than for storm rain runoff; (7) in some cases (particularly during the winter period), the snowpaek must be "primed" by liquid water to produce water excesses at the snow-ground interface. In general, the problem of synthesizing streamflow from springtime snowmelt would be analogous to developing streamflow from more or less continuous rainfall of moderate intensities, extending over a period of a montb or two, with a continually decreasing area of contribution. Hydrograpb synthesis may be required either for tbe springtime period when runoff is produced predominantly by snowmelt witb occasional rainfall, or for the winter rain-on- snow situations. The principal consideration in establishing methods for reconstituting snowmelt hydrograpbs is that the system provides for realistic evaluation of eaeb of tbe several major variables affecting runoff. These variables are (1) snowmelt, (2) snow-covered area, (3) initial losses, and (4) time delay to runoff through basin storage. A rational method should be based on explicit evaluation of each of these elements. Tbe streamflow hydrograpb represents tbe summation of the com- bined effects of the variables, and an unrealistic evaluation of anyone will necessarily result in an erroneous compensation to be forced into anotber element, in order to acbieve a balance for the hydrograph as a whole. This is undesirable, particularly wbere tbe elements are extrapolated to design conditions. 9-02. ELEVATION EFFECTS. Because tbe snowpaek exhibits its principal variation witb eleva- tion, there arc two general approaches to the problem of computing the runoff from snow-covered areas. The first is to divide the drainage basin into bands of equal elevation and to compute the snowmelt, rainfall, and losses separately for each band. The second general approacb is to treat the basin as a unit, making corrections for non-snow-eovered area or other noncontributing areas. In general, the first method is more adaptable for winter rain-on-snow situations, wbile the second method is usually preferable for springtime clear-weather snowmelt. 9-03. MELT PERIOD. For small basins (generally less than 1,000 sq mil witb sbort storage times, diurnal fluctuations to streamflow occur as the result of daily fluctuation of snowmelt. If necessary to reproduce such fluctuations, melt periods of not more than 6 hours are required. For larger basins (in excess of 3 or 4 thousand sq mil or those with relatively long storage times, snowmelt may be routed in time increments of one day. 9-04. SNOWMELT. Daily quantities of basin snowmelt may be determined by general snowmelt equations, as set forth in chapters 3 and 4, or by derived basin snowmelt indexes as described in chapter 5. ",Vhen 6-hour values of clear-weather snowmelt are required, a fixed distribution pattern 31