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(1991- as cited in 903_App_C NativeET, available on line at cdss.state.co.us). This <br />curve was finalized and presented by Cooper (2004) and is summarized in Table 9. No <br />field ET measurements have been made that would enable fitting such a curve <br />specifically for the South Platte system. Hence, the curve for riparian vegetation from the <br />RGDSS effort will be used for SPDSS after scaling for the affect of the different <br />reference ETo in the two systems. <br />The curve for riparian vegetation from the RGDSS is presented in Table 10. This curve <br />was fitted with afour-segment spline curve. With the water table at the surface, this curve <br />defines ETg as equal to the maximum possible rate, defined as ETo -precipitation. With <br />the water table at the extinction depth, 15 feet, ETg = 0. At intermediate depths, ETg is <br />defined by the position on the spline fit according to the values presented in Table 9. <br />Table 9: Points defining the segments used in the RGDSS to describe ETg by dtw. <br /> Point 1 Point 2 Point 3 Point 4 Point 5 <br />X value DTW (ft} 0 2 4 ~ 15 <br />Y Value Fraction of ETolyr (ft} 1.000 0.625 0.219 0.230 0.000 <br />To adapt these points for conditions present in the South Platte River system, ETo, the <br />reference ET for grass, may be multiplied by the decimal fraction representing the <br />fraction of water use (Y) in Table 9. Note that ETo, grass reference ET, should be used <br />for this purpose since it most accurately portrays ET processes for riparian vegetation. A <br />raster of the 11-year average ETo is included in the electronic data accompanying this <br />report to enable this calculation. <br />Representing the spline fits for the four segments as decimal fractions (ultimate row of <br />Table 9) and fitting curves for the segments yielded a set of four curves for estimation of <br />reference ETg for any given depth to water for any pixel (Table 10). <br />Table 10. Spline-fit segments for ETg in feet/year by depth to water for the ith pixel. <br />GurVe Segment Defining Points Depth Inter~+al Formula Far ETg <br />1 Points 1 #0 2 S~~i~face #0 2 ft. ETgi = (-0.187 * depth + 1.00} *EToi <br />2 Points 2 to 3 2 to 4 ft. ETgi = (-4.2{]3 * depth + 1.031 } *EToi <br />3 Poin#s 3 to 4 4 to 8 ft. ETgi = (-0.0367 * depth + 0.367} *EToi <br />4 Points 4 to 5 8 to 15 ft. ETgi = (-0.0103 * depth + 0.154} *EToi <br />9. Comments and Concerns <br />9.1 Repeating these Anal <br />The methods used for this report are readily reproducible with the description and the <br />data processing parameters listed. For this reason, these methods can be used to <br />periodically assess riparian ET, especially if some systematic change has occurred that <br />21 <br />