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SPDSS Memorandum: <br />TO: Ray Bennett and Ray Alvarado, Colorado State Engineer's Office <br />Erin Wilson, Leonard Rice Engineers, Inc. <br />FROM: David Groeneveld and Michael Prescott, HydroBio, Inc. <br />RE: Estimating South Platte Phreatophyte Groundwater Evapotranspiration <br />Date: January 30, 2007 <br />Estimates were made of phreatophyte evapotranspiration from groundwater (ETg) from <br />the South Platte Basin between the mountain front and state line for the baseline year <br />2001. The methods used Landsat TM data (0.2 acre pixels) and a derivation of the most <br />commonly used vegetation index, Normalized Difference Vegetation Index (NDVI) as a <br />"crop reference". The derivation, NDVI*, stretches the result between the minimum and <br />peak vegetation responses in the system to yield a range between 0 and 1. The zero value <br />for this vegetation index was then equated to zero ETg, and peak vegetation response was <br />equated to peak ETg. Peak ETg was calculated by subtracting total annual precipitation <br />from FA056 grass reference ET (ETo -Precipitation). This method enabled mapping of <br />ETg represented by the vegetation index, modified by spatially dynamic ETo and <br />precipitation across the approximately 200-mile length of the South Platte study area. <br />Total ETg estimated for the study area was about 255,000 AF/yr, confined entirely to <br />riparian zones along the South Platte and its various tributaries. A curve was fitted to <br />enable spatial representation for the change of ETg that would occur if the depth to <br />groundwater changes within this system. <br />1. Introduction --------------- <br />2. Background -------------- <br />---- 2 <br />---- 2 <br />3. Method Used to Map Estimated Groundwater ET -------------------------- 3 <br />4. Calculations to Yield Spatially-Correct Phreatophyte ETg from NDVI* ---- 7 <br />5. Evaporation from Small Water Bodies ----------------------------------------------- 15 <br />6. ET Estimates from South Platte Phreatophyte Lands --------------------------- 17 <br />7. Method for Evaluating ET at Monthly Intervals ------------------------------------ 20 <br />8. ET by Depth to Water for South Platte River Riparian Phreatophytes ----- 20 <br />9. Comments and Concerns ----------------------------------------------------------------- 21 <br />10. Citations --------------------------------------------------------------------------------------- 23 <br />11. List of Electronic Products -------------------------------------------------------------- 24 <br />