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7. Method for Evaluating ET at Monthly Intervals <br />Yearly estimated ETo can be divided into monthly components based upon the percentage <br />contribution towards total annual ETo. Monthly averages were calculated using 11 years <br />of ETo data (1993-2003) from Ft. Lupton (Table 8), chosen due to its central location <br />relative to the study area. Annual values of ETg_; can then be multiplied by these monthly <br />percentages to estimate monthly ET contribution from each 0.2 acre pixel. <br />Table 8. Monthly average ETg percentages for <br />South Platte system. These data are <br />calculated using the total yearly ETg from <br />Table 5, above. Each month's contribution is <br />scaled to average monthly ET° data from Ft. <br />Lupton site, years 1993-2003. <br /> Average <br />ETo (in.) % Annual <br />ETo <br />Janua 1.03 3.79% <br />Februa 1.22 4.49% <br />March 1.96 7.21 <br />April 2.42 8.90% <br />Ma 2.92 10.74% <br />June 3.62 13.31 <br />July 3.94 14.49% <br />Au ust 3.31 12.17% <br />September 2.59 9.52% <br />October 1.92 7.06% <br />November 1.18 4.34% <br />December 1.09 4.01 <br />Total 27.20 100% <br />The simple approach used in Table 8 is only a rough approximation of the distribution of <br />ETo through the year, with the summer ETo percentage probably underrepresented. This <br />is because the monthly representation of ETo has simply been parsed by the relative <br />driving force, regardless of the vegetation cover present. Vegetation is absent during the <br />winter, hence transpiration is zero, while transpiration dominates ET processes during the <br />growing season. By contrast, transpiration is non-existent during the winter months and <br />ET is dominated by physical process of evaporation from water or sublimation from ice <br />or snow. Thus, as represented in Table 8, ET rates during the summer are probably <br />smaller than actual, while those during the winter are larger than actual. The distribution <br />in Table 8 can be improved but only through considerable additional analysis beyond the <br />scope of this investigation. <br />8. ET by Depth to Water for South Platte River Riparian Phreatophytes <br />A curve for estimating ET as a function of depth to the water table (dtw) is necessary to <br />enable the groundwater model to provide dynamic simulation of ET from the riparian <br />corridor. This work borrows from the Rio Grande Decision Support System (RGDSS) <br />that significantly enhanced (with field measurements) a curve first produced by Emery <br />20 <br />