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3.2 ET Estimates From NDVI* Using Local Weather Data <br />For calibration of phreatophyte greenness (as NDVI*) to yield ETg, peak greenness, i.e., <br />NDVIs, was set equivalent to the peak ET rate. Appropriate for riaparian vegetation, this <br />peak ET rate can be estimated by a grass reference ET, ETo, determined by the Penman- <br />Monteith equation using methods in Allen et al. (1998). The average seasonal peak ET <br />rate was found to be proportional to annual total ET (Groeneveld et al.--in review). In <br />much the same way as in Equation 2, this calculation used a derivation of ET that was <br />normalized by regional conditions for annual precipitation and ETo according to: <br />ET*; = ET,P; = ETA (3) <br />ETo-; - P; ETo-; - P; <br />Where all values are annualized totals, and <br />"i" denotes the ith pixel (a convention continued throughout) <br />ETo is the annual total grass-reference ET <br />P is the total annual precipitation <br />Note that the numerator of Equation 3 is ETg, since it is actual ET minus the component <br />provided by precipitation. Given identical vegetation cover and vigor, ETg changes <br />spatially according to the distribution of precipitation and ETo. <br />For a large study area such as the South Platte drainage basin, annual totals for <br />precipitation and ETo change across the basin. Spatial variations in these parameters were <br />tracked by two files of computer-interpolated values from five weather stations with the <br />interpolations modeled by northing and Basting position. The base data for developing <br />these files used the calendar total ETo expressed as the annual average for an 11-year <br />period, 1993-2003. The precipitation average was 1951-1980 based on mapping <br />developed by the Colorado Climate Center. Average annual ETo was obtained by <br />Leonard Rice Engineers (LRE) for five meteorological stations: Crook, Fort Collins, Fort <br />Lupton, Greeley, and Holyoke. Using the inverse distance weighted interpolation tool in <br />ArcMap, a raster surface was created using point values from these five ET stations. This <br />raster surface enabled smoothly interpolated values as the best estimates of ETo between <br />and beyond stations. Rastor surface of annual precipitation was developed from the <br />Colorado Climate Center average annual precipitation contours. <br />Average ETo and precipitation are given in Table 1 for four stations. The raster surfaces <br />developed from these data are contained in the package of electronic files accompanying <br />this report. <br />Table 1. Ten-year average ETo values in inches used to create ETo raster dataset. <br /> <br />Station <br />Lat. <br />Long. <br />Elev. {ft} 11-Year <br />avg. ETo {in} 30-Year <br />avg. Precip. {in} <br />Ft. Collins 40.59 -105.137 5120 41.78 17.5 <br />Greeley 40.44 -104.647 4680 44.30 10.5 <br />Ft. Lupton 40.00 -104.859 5055 45.71 9.2 <br />Holyoke 40.49 -102.089 3735 48.75 13.5 <br />6 <br />