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- Approximately 2.2 million acres in the South Platte River basin are forested, based on a <br />GIS analysis of late-1980s/early-1990s satellite imagery in a 2001 USGS National Land <br />Cover map. The CU of mainly coniferous forests was researched and an estimated CU <br />value for forests was determined (16.4 inches per acre) based on an average of three <br />published estimates that ranged between 14 inches per acre to 17 inches per acre.' g' 10 The <br />CU of forested areas, which cover approximately 16 percent of the South Platte River <br />basin, is estimated to total about 2.9 million acre-feet. <br />- The area covered by phreatophytic species in the basin is unknown. The Colorado Office <br />of the State Weed Coordinator tabulates exotic species in the state, by county. The exotic <br />species include primarily Russian olive and tamarisk (salt cedar). It is reported that the <br />county tabulations are not wholly complete or up-to-date.9 The State Weed Coordinator's <br />values also do not include native phreatophytes, including cottonwoods and poplars, <br />which are likely present in the South Platte River basin in greater numbers than the exotic <br />phreatophytic species. The CU of phreatophytes is species-dependent but the Tamarisk <br />Coalition provided an estimated CU rate of about 3.0 acre-feet per acre for exotic species,6 <br />which is on the order of the CU rates used in the studies reviewed (2.67 acre-feet per <br />acre). Although the amount of phreatophytic species in the basin is quite possibly in the <br />hundreds of thousands of acres, if the species within the tributary basins are included, it <br />would take over 300,000 acres of phreatophytes to consume one million acre-feet at the <br />CU rate listed above. <br />- The remaining land in the South Platte River basin totals over 8.5 million acres, assuming <br />the above estimates of land use are correct. The remaining native vegetation CU totals <br />approximately 11.8 million acre-feet (= 15.7 - 2.9 - 1), or about 16.5 inches per acre. <br />Note that the native vegetation CU rate in the studies reviewed for the RGDSS water <br />budget effort ranged from about 5.8 inches per acre to 10.2 inches per acre. These native <br />vegetation CU estimates are about half the unit rate calculated for the South Platte River <br />basin. This comparison should be re-visited as the water budget inputs are revised based <br />on further analyses within the various SPDSS sub-tasks. <br />- Inclusion of CU from wetlands, non-irrigated lands, and other non-managed areas may <br />account for a portion of the native vegetation CU not addressed above. Further analyses <br />within the various SPDSS sub-tasks may provide further information on these uses. <br />North Platte River and Laramie River basins <br />The unit rates of native vegetation CU calculated for the northern river basins are about 30 <br />percent to 35 percent higher (NPR-approximately 22.4 inches per non-irrigated acre; LAR- <br />approximately 21.1 inches per non-irrigated acre) than the rates calculated for the South Platte <br />River basin. Further analyses within the various SPDSS sub-tasks may provide additional <br />information on the input data for the other water budget components which will affect the native <br />vegetation value calculated in the water budget runs. <br />• Exports <br />Transmountain exports in the North Platte River water budget (1.9 kaf/yr) and the Laramie River <br />water budget (19 kaf/yr) are similar to those from the studies reviewed (NPR-1.3 kaf/yr; LAR- <br />19.7 kaf/yr) with the differences likely due to variable study periods. <br />Task83 final.doc 9 of 15 Apri17, 2005 <br />