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<br />From: Ro.n 811esner To: ShIrley Mondy <br /> <br />Date: 01/11/02 Time: 2:56:42 PM <br /> <br />P2ge4017 <br /> <br />,...., <br />r'1 <br />..... <br />,'., <br /> <br />~ <br /> <br />It is proposed that for natural flows the time series of diversions and return flows be used. <br />For modeled cases, the forecast losses and return flows should be utilized, considering <br />expected time lag in return flow. Data will be provided to the connnittee for review and <br />approval. <br /> <br />Proponents of any new proj ect would be asked to submit their estimate of incidental <br />losses along with other project information for review by the Hydrology Connnittee prior <br />to modeling. The best available information and scientific methods should be used to <br />estimate incidental losses for these new proj ects. <br /> <br />Off-Stream Depletion <br /> <br />In the present natural flow analysis and model nms, off-stream depletions (depletions that <br />occur within a stream reach on an ungaged water supply) have been handled differently <br />for different states and for different time periods. Quantification of these demands in <br />tenns of impact upon streamflow is always problematic. Timing is not welllUlderstood. <br />Demands must be limited to natural gains (adjusted for pbreatophyte loss) in the model to <br />prevent a call on the reservoir, yet must numerically match the values in the natural flows <br />to preserve gage balance. It is proposed that this process be significantly simplified. <br /> <br />Since assumptions in the model will also be used in computation of natura1 flow and both <br />the model and natura1 flow will be calibrated to gage flow, we propose that off-stream <br />depletions not be considered in either the natural flow computation or model runs. nus <br />will eliminate the need for going through complicated computation to limit depletions <br />and will substantially improve the accuracy of the gage estimate. The computed gain- <br />loss, which is the balance term in each reach, will account for existing off-stream <br />depletion. This approach will be accurate as long as new off-stre8m depletions are not <br />added. Even then, they would have to be quite large to have a measurable impact due to <br />the extremely small and unreliable water supply that typically exists in these ungaged <br />drainages. <br /> <br />For purposes of state depletion accounting, the anticipated depletion associated with these <br />projects could be computed and footnoted in the depletion tables as not being included in <br />the model water balance. <br /> <br />Tributary Disaggregation <br /> <br />Monthly model tributary flows of the La Plata and Mancos rivers and McElmo Creek <br />must be disaggregated into daily flows at their confluence with the San Juan River for <br />operation of the daily time-step main stem model. It is proposed that the existing gage <br />record on each tributary be used to calculate a time series of daily flow percentages <br />(percent of the monthly flow that occurred on a given date). These flow percentages do <br />not change with time, so they can be computed once and the time-series data set included <br />as a model input file. This facilita\es the disaggregation of monthly to daily data within <br />the model. The monthly model tributary flows would be multiplied by the time series of <br />daily flow percentages to produce a daily record. <br /> <br />Keller-Bliesner Engineering <br />Draft San Juan Model Input Data Generation <br /> <br />January 11, 2002 <br />Page 3 <br />