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significant are events that might occur during the selected study period, thereby complicating <br />efforts to define system baseline conditions. <br />The development event with the greatest potential to affect study period selection is the change in <br />the way the Rio Grande Compact is administered in Colorado. Since the late 1960s, the Compact <br />has been strictly administered with the aim of meeting Compact obligations each year. This has <br />been accomplished by curtailing water use (both senior and junior rights) along the Rio Grande <br />and Conejos River. Prior to strict administration, no such curtailment occurred. This change will <br />require careful consideration in the way the Compact is represented in modeling efforts and in <br />any comparisons between model results and historical data. <br />Previous Modeling Efforts <br />There have been several previous surface water modeling efforts conducted for the Rio Grande <br />and Conejos River. The most significant of these efforts were the Conejos River Planning Model, <br />developed by Hydrosphere Resource Consultants, Inc. in 1993, and a model of the Rio Grande <br />River developed by Leonard Rice Consulting Water Engineers, Inc. in 1990 as part of the Rio <br />Grande Water Supply Study. A review of these modeling efforts is documented in memorandum <br />developed for Task 3 of the Surface Water RGDSS effort ("Review Past Modeling Efforts"). <br />This study periods chosen for both the Rio Grande Model and the Conejos Model were based on <br />the specific needs and uses of the modeling tools and reflected data availability at the time of <br />their development. The Conejos Model, completed in 1993, functions on hydrologic period <br />spanning 1970 through 1990. This study period was chosen in large part because of the <br />importance of accurately reflecting present-day administration of the Rio Grande Compact, and a <br />desire to avoid complications in handling historical data under different administrative practices. <br />The Conejos Model input data sets can readily be updated to include more recent hydrologic <br />conditions if desired. <br />The Rio Grande Model, completed in 1990, operates over the period 1948 through 1985. This <br />period was chosen based primarily on statistical review of unadjusted gage records at the Rio <br />Grande Del Norte gage. Other factors considered in selecting a study period were the availability <br />of diversion and water use records, inclusion of drought years, consistency with previous basin <br />studies, public perception of the study period, and the desirability of including recent years to <br />make the best use of the most reliable data. <br />Recommendations <br />Recommended Study Period <br />Based on climate considerations alone, the best RGDSS study period would be one that included <br />the drought of the 1930s, because it was the deepest and longest drought on record. It is also <br />followed by a brief period of recovery before another long drought begins. By comparison, the <br />1950's drought is equally as deep, although the duration is shorter. Consideration of a shorter <br />drought period is likely, however, inconsequential given the relatively small amount of carryover <br />storage capacity in the basin. We believe the reliability of the Rio Grande water supply system <br />can be thoroughly evaluated under the sequence of climate conditions from 1950 to the present. <br />C:Acdss\Task2-3.doc Identify Study Period April 16, 1999 Page 14 of 18 <br />