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<br />Ie <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2-1406 <br />5 Jan 60 <br /> <br />CHAPTER 5 <br />TEMPERATURE INDEXES OF SNOWMELT <br /> <br />5-01. GENERAL. Snowmelt evaluation for basins may be accomplished either through use of <br />simplified generalized equations as presented in the preceding chapters, or indirectly through <br />use of snowmelt indexes. In basin applications for design floods, the former arc more appro- <br />priately used, because of the requirement for direct rational evaluation of all factors affecting <br />snowmelt and extending them to the given design condition, This involves detailed computations <br />of major scope, but they are justified for the design of major water control projects. For daily <br />streanlflow forecast.ing uses, however, a simple snowmelt index is usually adequate, when con- <br />sidering the overall accuracy of forecasts and time limitations in their preparation. <br />Because of the availability of air temperature data and simplicity in application, air tem- <br />perature measurements arc often used as indexes of daily snowmelt quantities. From chapter 2, <br />it vms seen t.hat temperature of the air is only OIle of several factors affecting snowmelt, unu that <br />some of the causative factors, such as the snow surface albedo, wind, and moisture content of the <br />air, arc not directly related to air temperature, It was also pointed out that air temperature is a <br />better index of snowmelt for forested areas than for open areas. Air temperature, thl'n, is rdated <br />only in it gencral way to sllmvmclt, and its use in estimating snowmelt varies with Lnsin forest <br />cover, meteorological conditions, snow surface albedo, time of year, and other miscellaneous <br />factor3. Accordingly, there is no one temperature index that is uniwrsally applicable for <br />estimating snowmelt.. <br /> <br />5-02. POINT MEtT RATES. Investigations of the Snow Laboratory data haw shown the <br />magnitude and range of melt rates using temperature indexes for conditions at a point. During <br />the spring snowmelt season, the mean value of snmvpack ablation for several years of observatiun <br />at each snmv course was related to the air temperature at un adjacent station. The following <br />tabulation summarizes these point melt rates: <br /> <br />Point Jh::ll Rates: Degree-Day Factors* <br /> <br />Lahur.,tory <br /> <br />I \la\lmllUl melt I ~llTllmllm mdt j \fean all <br />_ _ ----'.t-"""'- _I ",'wn _I~~'''w,,,_ <br /> <br /> <br />o 128 I 0 Of)b 0 106 <br />. 131 . OS.! . 090 <br />.1081 .02G .OGO <br />. ] 22 I . 04!l . 085 <br /> <br />. 054 I ' 029 ' 045 <br />.041 i .020 .035 <br />. aGO I' . 015 .034 <br />. 032 . 021 . 038 <br /> <br />A. Based on mean daily temperature:::: <br />CS~k ___~_~___ _________ <br />1;CoL_ <br />WBSL__ <br />MearL_____ <br />H. Based on maximurn daily temperatures: <br />CSSL______~ <br />l:CSL______~_ <br />WBSL___n <br />l\lean__________________________ ___________ <br /> <br />-------------------1 <br />i <br /> <br />'inches or melt per degree-day ahuve 320 F. <br /> <br />17 <br />