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and simulation mode over the period 1950 to 1997. During this period, historical flow <br />observations at numerous gages, recorded water deliveries to diversion structures, and historical <br />reservoir end-of--month contents were used to develop baseflow hydrologic conditions. The <br />model was then run in simulation mode using historical diversions as a surrogate for water <br />demand. This "historical simulation" utilized numerous operating rules to reflect water <br />management practices over the study period. <br />Model calibration can be defined as simulating a system such that results agree reasonably well <br />with historical observations. The Rio Grande Surface Water Model was calibrated through <br />several adjustments to hydrologic conditions (baseflows) and operating rules. Efforts towards <br />calibration focused on three areas: comparison of historical to modeled diversion amounts, <br />comparison of historical to modeled stream flows, and comparison of historical to modeled <br />reservoir contents. The calibration period extended over the entire study period (1950 - 1997). <br />Due to improvements in data collection and changes in river administration, the most recent 20 <br />years of the study period were examined more closely. Calibration targets were chosen to <br />generally simulate conditions within 5 to 10 percent of observed conditions. <br />The steps used in the calibration process were as follows <br />Diversions: <br />1 Compare simulated diversions to historical diversions to identify shortages <br />2 Determine reason for any shortage <br />3 If required, adjust proration factors, return flow pattern/location, operational rights <br />4 Regenerate baseflow and rerun simulation <br />Reservoirs: <br />1 Compare simulate EOM contents to historical EOM contents to identify differences <br />2 Determine reason for any EOM content differences <br />3 If required, adjust proration factors, operational rights <br />4 Regenerate baseflow and rerun simulation <br />Streamflows: <br />1 Compare simulated stream gage flows to simulated flows <br />2 Repeat diversion and reservoir steps until simulated gage flows approximate historical <br />Results - Baseflows <br />Final average annual baseflows for the Phase la surface water model are summarized in Table 2. <br />Average monthly baseflows at seven key points along the Rio Grande and four key points along <br />the Conejos River are shown graphically in the Figures 1 and 2, respectively. It is interesting to <br />note substantial losses to the river system between the Rio Grande gages at Del Norte (USGS <br />08220000) and Monte Vista (USGS 08221500). Model results suggesting losses in this reach of <br />the Rio Grande averaging approximately 27,900 of per year are generally consistent with <br />observations of the District 20 Water Commissioners. The baseflow calculations also suggest <br />system losses in lower reaches during the non-irrigation season During peak runoff months and <br />throughout much of the irrigation season, baseflows, as expected, generally increase in the <br />downstream direction as the River gains from additional tributary inflow and return flow. <br />C:Acdss\Task6-14.doc Phase la Model (Flistoric Monthly) February 22, 2000 -Page 3 of 10 <br />