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Selection of the RGDSS study period is constrained by the accessibility of diversion and climate <br />data, both of which are available electronically only from about 1950 onward. Electronic data for <br />streamflows, diversions, and climate extend through 1997. Therefore a study period that <br />encompasses the years 1950 through 1997 will make maximum use of available electronic data <br />sets. Extending this study period to earlier years would require data entry work from paper <br />records. <br />Because a significant drought occurs at the very beginning of this recommended study period, <br />special attention will need to be paid to the modeling assumptions that reflect initial conditions of <br />surface and ground water storage. The modeled system may encounter the stresses of the drought <br />(i.e., low inflows and high demands) before it has an opportunity to fill. Thus the starting <br />contents assumptions for surface water reservoirs, for example, may influence the simulated <br />performance of those reservoirs during the drought period. <br />An RGDSS study period extending from 1950 to 1997 exceeds periods used in any previous <br />modeling study in the Rio Grande Basin within Colorado. We believe that this period represents <br />a diverse range of hydrologic conditions over which to evaluate Rio Grande water supplies. We <br />also believe it is the most cost effective choice given the current availability of electronic data. <br />Alternative Study Periods <br />An alternative study period that should be considered would be one that includes the drought of <br />the 1930's, the most extensive drought on record. However, to extend the study period back to <br />1930 would require data entry and quality control for over 9,000 station-years of daily diversion <br />data. If there were significant amounts of long-term carryover storage in the basin, then the value <br />of having this more extensive drought in the study period would be increased. However, given <br />the relatively small amount of surface water storage in the basin, we believe that the costs of data <br />development to support simulation of the 1930's drought outweigh any benefits that would be <br />gained by including it. <br />Any alternative study period that began later than the 1950s would represent only wetter-than- <br />average conditions. Furthermore, little would be gained from a shorter period; there are several <br />streamflow-gaging stations that began after 1950 that could add important information to the <br />model, but few of these continued past 1980. Starting later than the 1950s would thus shorten the <br />period of available gage coverage. <br />Comments and Concerns <br />None <br />C:Acdss\Task2-3.doc Identify Study Period April 16, 1999 Page 15 of 18 <br />