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Table 2. Reaches Mapped for the South Platte Drainage. Separate polygons define the <br />phreatophyte vegetation that occurs within the alluvial boundary and divisions into the <br />following reaches for ET accounting and tabulation: <br />Main Stem South Platte Divisions <br />Reach 1. Mountain Front to to Denver Gauge <br />Reach 2. Denver Gauge to Henderson Gauge <br />Reach 3. Henderson Gauge to Ft. Lupton Gauge <br />Reach 4. Ft. Lupton Gauge to St. Vrain confluence <br />Reach 5. St. Vrain confluence to Big Thompson confluence <br />Reach 6. Big Thompson confluence to Kersey Gauge <br />Reach 7. Kersey Gauge to Ft. Morgan Canal <br />Reach 8. Ft. Morgan Canal to Beaver Ditch <br />Reach 9. Beaver Ditch to Cooper Bridge near Balzac <br />Reach 10. Cooper Bridge near Balzac to Sterling Irrigation Ditch #1 <br />Reach 11. Sterling Irrigation Ditch #1 to Harmony Ditch #1 <br />Reach 12. Harmony Ditch #1 to Liddle Ditch <br />Reach 13. Liddle Ditch to State Line <br />Cache la Poudre River Divisions <br />Reach A. Mountain front to Larimer-Weld Irrigation Canal <br />Reach B. Larimer-Weld Irrigation Canal -New Cache la Poudre River Canal <br />Reach C. New Cache la Poudre River Canal -South Platte Confluence <br />uth Platte Tributaries (from riaht to left on F <br />Red (T1): Sections of Beaver and Badger Creek drainages <br />Green (T2): Bijou Creek drainage <br />Blue (T3): Sand Creek drainage <br />Purple (T4): Box Elder Creek drainage <br />Yellow (T5): Portions of First and Second Creeks <br />Orange (T6): Portions of Cherry Creek <br />4.2 Correction of TM Data for Atmospheric Scatter <br />Each image was atmospherically corrected using the Dark Object Subtraction method <br />(DOS; methods described in Chavez, 1988) to minimize data scatter associated with local <br />atmospheric conditions at the time of the satellite overpass. DOS is a process that <br />minimizes the effects of atmospheric scattering by subtracting the reflectance value of the <br />darkest objects (usually bodies of water) in each channel from all pixels within the area <br />of interest. The theory for this process is that the darkest objects within each band should <br />have zero reflectance. Therefore, given that zero values should be present, whatever non- <br />zero reflectance value measured at the lowest limit of reflectance is most likely displaced <br />upward due to atmospheric scattering. DOS values were determined as the lowest values <br />for Bands 3, 4, and 5 of each image from the region within the alluvium-defined study <br />area (Table 3). Only these TM bands were of interest for the analysis of phreatophyte ET <br />in the South Platte system. <br />12 <br />