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it-5 <br />• ~ 3. Adjust the average temperatures to the average watershed <br />elevation, using the method given below in Adiustment of <br />`~~.^peratures for altitude. This step ie omitted uhan elevation <br />data are crude or otheraiae unreliable. <br />4. Compute the watershed average daily temperatvree by averaging <br />the station averagee (adjusted for altitude, if desirable). <br />5, Subtract j2° F, from each watershed average daily temperature <br />- to get the deeree~lsys par day, <br />6. Use ecuet'_o:."~-1 to get an estimate of the potential anovmelt <br />for each day. See H factors below for selection of K, <br />7. Where the daily potential is not greater than the water <br />equiveleat remaining on the vaterahed, it ie ahcvn ae an <br />estate of snowmelt. <br />Once :.he estimates of snowmelt are obtained, thep.are urea to obtain <br />hy~'rpgraphs ea described in Chapter 16. <br /> Some hydrologists suggest that the effects of infiltration be subtracted <br /> frog the estimaLea snowmeit. rioxever, the 6 i'aotors as eeaerally <br /> developed already include the eifecte of infiltration, fhe effects of <br /> measures such as contour furrows are ootainea as aescribea it Chapter 12. <br /> The effects o: reservoirs, levees, etc. are obLainea ae usual. <br />• ?.efinaments in Lne aegree~ay maLnoa are best mace oy first i3proving <br /> .he accuracy of ceterminattons of snow wench and areal distribution on <br /> -,'he weLersdea. ''»;~en these are ;,no•.m wi.th.in small 1.=.;.a o: =_rror, then <br /> rater eetuvaienLS sbouia :;a ret~nea, wince Lhe ~/10 ratio is a rough <br /> _at-o:.:=scion. .:afirener.ts ~r..: Factors dil°aiQ coma inst. <br /> `i9~"oB-Qa~• eLhOa. a'9K ea 'w 8'vaT9-lea <br />~;~n.. The degree-day method has a ver limitod use, .f srt.Y at ~_, .or _iood <br />.?'"`~x:. - evaluations on gaged watersheds. :;pea eagi:.g station care are available, <br />•'"''" those data should be used to earl =ate ilcoa y.Ca,C9 z.d -:oft:=es cu :.;,:.ar <br />_~~,I, portions of the watershed, <br />}~~`^'' <br />.;.rs'.:•,,. Adiustment o_` '~~eret-,res 'r_ ~1~:~'=e <br />~'_,... In general, air temperatures decrease about 3° to i° `_r every 1,::_0 <br />r,, •~~ feet of ;,se in altitude. Cther factors :.nfluence this "lapse rate <br />" <br /> <br />_ '.:. d <br />^ <br />so that refinements ere rot justified, =nd `~ average decrease of .. <br />-• <br />_ <br />,, <br />:: <br />~ per 1,000 feet rise should be used, <br />'~ <br />:.: <br />'~ <br /> <br />~• ~ <br />:~ <br />y~r, ~~ I <br />L,. <br />