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<br />~ <br />.1 <br /> <br />tJOl.j56 <br /> <br />. <br /> <br />Derivation af Hili.. Coefficients <br />Based on review of the report and discussions with Dr. Hill, Hill's SCS-BC coefficients were <br />derived as shown in equation 1. <br /> <br />Kc. <br /> <br />(Kcm · Etrll (Kt · It · p 110011 <br /> <br />Equstlon 1 <br /> <br />, <br /> <br />where: <br /> <br />Kc. <br /> <br />SCS-BC coefficients for Irrlgated meadow.s shown In Table 12 of the Bear <br />River report and rellsted In Table 1 above. <br /> <br />Kcm. <br /> <br />Mean crop coefficients for use with Penman Etr velues. The coefficients <br />are shown In Figures 13 snd 14 of the Bear River report and described in <br />equetlonl 8 and 9 Of the report. <br /> <br />Etr . <br /> <br />Reference crop evapotranspiration ai cal4;ulated using the 1972 Kimberly <br />Penman equation which uses an .Ifalf. reference crop. <br /> <br />Kt - <br /> <br />Temperature coefficient al described In SCS Technical Release 21 i1nd II <br />equal to 0.173 . .314 t. <br /> <br />t . <br /> <br />Mean monthly air temperature In degrees Fahrenheit taken from the nearest <br />National Weather Station (NWS). <br /> <br />p - <br /> <br />Monthly percentage of annual daylight hours. <br /> <br />It is evident In the report. and confirmed by Or. Hill, that the Bear River Iyslmeters were <br />irrigated throughout the summer; the Kcm coefficients utilized to derive Hili's scs-ec coefficients <br />represent adequate irrigation before July 15 and no Irrigation after cutting. The effects of no <br />Irrigation after cutting was determined empirically by .ssumlng that on July 15 the soli moisture <br />was full and no further Irrigations occurred. Then using estimated Et, the soli moisture was <br />consumed at potential Et until 50% of the soil moisture was depleted. From that point, crop Et <br />was linearly reduced such that when the available soli moisture wes fully depleted. crop Et would <br />be equal to zero. For exsmple, If the soli moisture was 55% depleted, the crop evapotranspiration <br />was estimated to be 90% of potential. ' <br /> <br />It is important to note that the derivation of the coefficients Involved the use of <br />temperature measurements at two different locations. In determination of Etr, the temperature <br />was measured using the electronic weather station .t the Iyslmeter sites. The value .t. wu <br />messured at the nearest NWS. Since derivation Of Hill's scs-ec coefficient use the NWS <br />temperature, sny aridity effect or local conditions which may influence the NWS temperature .re <br />accounted for in the coefficients. In other words. the coefficients are locally calibrated for use <br />with the NWS. It ia Important to note that Hlllet al. reported the maximum temperature increased <br />and minimum temperature decreased with Increasing elevation at the NWS; wherea. at the <br />electronic station both temperature parameters decreased with Increasing elevation. <br /> <br />... - _J;).J>. <br />b-' <br /> <br />Evaluation at HIli'. SCS.BC C~el8nb <br />Evaluadon of the coefficients for use in the Yampa and White Rivet basins w.. <br />accomplished by using data from the NWS at Steamboat Springs and Maybell. Evaluation of the <br />coefficients for use in other part of the Colorado Rivet basin and at lower elevation. wa. <br /> <br />U38.Il\MEMO _ . '.2S.IS <br /> <br />2 <br /> <br />A -Ill WALTER <br /> <br />~O'd ~OO'ON 6v:~1 <br /> <br />56 5~ 6nl;j <br /> <br />~08~-19L-~O~-1:131 <br /> <br />JOSSl;j ~ Jalaa4~ .~.~ <br />