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It may be noted that some of these small water bodies are often ephemeral and therefore, <br />may not consume water during the entire year. Balancing this potential, however, is their <br />presence during mid-summer that suggests that they are strong contributors to ET <br />consumption through the mid summer period since they were mapped during this time. <br />Thus, small water bodies are likely to remain present, or even expanded, during the <br />remainder of the year. This could be determined using a time series of imagery that was <br />beyond the scope of this investigation. <br />6. ET Estimates from South Platte Phreatophyte Lands <br />Tables 5 and 6 contain total annual ETg and ET, estimated for each reach of Table 2 and <br />Figure 7 for vegetative conditions represented on the 2001 imagery. These data have been <br />further broken down into acres of vegetation, vegetation ETg, acres of smallwater bodies, <br />small water body Eg and combined ETg totals, expressed in units of acre feet. Also, the <br />average depth equivalent water use (feet of consumptive use/year from acre-feet/ acre) <br />have been calculated for each reach and for 2001. These summaries are contained within <br />the ultimate rows of Tables 5 and 6 as total acres, total acre feet and average depth of <br />water consumed over the area of the entire riparian polygon. Table 5 provides ETg, while <br />Table 6 provides the calculation of ET, by adding in the raster-based precipitation. <br />The data in Tables 5 and 6 are based upon multi-year weather records (n=11 for ETo: <br />1993-2003; n=30 for precipitation:1951-1980) because these give the most accurate basis <br />for evaluation of phreatophyte water use. Averages for these parameters were chosen for <br />two reasons: (i) basing the calculations on only one year may represent conditions that <br />are not typical for the system, and (ii) long-term average data enable two ways to adjust <br />ET outputs to better reflect conditions in a particular year. The adjustment methods are <br />described in Section 7. <br />The analyses in this report are based upon the conditions during the 2001 mid-summer <br />snapshot by the satellite. As a generality, phreatophyte vegetation tends to remain <br />relatively stable over time, unless directly impacted by fire or scoured during flood flows. <br />This is because there is constant annual replenishment of water in the root zone from <br />shallow groundwater, seepage return flow from irrigation and irrigation structures, <br />overland flow, springs and seeps etc., and head control by topography, distribution <br />structures and stream gradients that remain relatively constant over time. Consistent <br />inflow and head control buffers the mainstem riparian environments against swings in <br />water availability that potentially effect vegetation cover and vigor among years. Closer <br />connection with the water means higher vegetation growth and productivity and higher <br />disposition of water. This is the basis for the `ET predicted by depth-to-water' approach <br />employed in many groundwater models and that will be examined in Section 8. <br />17 <br />