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Figure 8. <br />NDVI* mosaic with the border between <br />scenes 32/32 and 33/32 shown as a <br />dotted line that crosses at an angle <br />toward the northeast. The center pivot <br />plot crossed by this line just north of the <br />phreatophyte boundary reveals a <br />temporal difference between the two <br />datasets due to crop development, yet <br />inside the phreatophyte boundary, <br />values are comparable for the <br />overlapped zone. This is disclosed by <br />inspection of both sides of the image <br />boundary (across dotted line) within the <br />blue phreatophyte boundary: these are <br />equivalent in magnitude as indicated by <br />brightness. Fields crossed to the south <br />of the phreatophyte boundary show <br />differences that may be related to alfalfa <br />harvest. <br />4.4 Estimation of ETg from NDVI* <br />The raster files of interpolated ETo and precipitation on a calendar year basis were used <br />to remove the effects of precipitation from the NDVI* that had been combined to form <br />one NDVI* raster for calculation of ETg_,, the individual pixel values of ETg. (Note that <br />the subscript "i" is present for all parameters because these parameters are estimated for <br />each individual pixel.) Per-pixel values of ETg_; were calculated using the raster values <br />for average annual ETo and precipitation, and the 2001 NDVI*; (equivalent to ET*,) <br />values according to rearranged Equations 3 and 4: <br />ETg ; =NDVI*, (ETo -p,) (s) <br />Calculations from Equation 5 resulted in an ETg_; raster for the conj oined phreatophyte <br />areas from the three TM scenes. The ETg_; raster was then clipped by polygons into the <br />reaches in Table 2 to define individual ET, raster datasets. <br />ETg can be thought of as a "generalized" measure of spatial ET since the presence of <br />groundwater supply for the phreatophyte vegetation is relatively constant from year to <br />year in comparison to the widely fluctuating supply that occurs for vegetation that <br />receives only precipitation. Except for changes in water table levels induced by <br />groundwater pumping, surface water diversion or return flows, ETg, expressed spatially <br />as it is for the South Platte study area, should remain relatively constant, as well. In the <br />14 <br />