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• Spatial and temporal variations are the biggest impediment to predicting runoff <br />(Michaud, 1992). <br />• Runoff predictions are worst for the small, frequent events such as those most <br />likely to be captured at the Rillito Project site. <br />For Rillito Creek, watershed streamflow and precipitation data were available from three <br />sources: USGS stream gauge records, National Weather Service climatic stations (rainfall), <br />and the district's ALERT sensor records. Precipitation sensor records that pre -date stream <br />sensors are of limited use since precipitation data cannot be correlated to flow hydro - <br />graphs or peak flow rates. <br />Review of this data base, as well as the research mentioned above, led CHZMHilI (1994) to <br />recommend that the dam operations plan be primarily based on data other than rainfall, <br />such as stream gauging. This was recommended because of problems with short lengths <br />of records, poor calibration, and sparse sensor density, as well as poorly understood <br />theoretical relationships and natural variability. The ALERT system, modified to include <br />more stream gauges, would provide the data for future refinements in understanding <br />rainfall- runoff relationships. <br />The district's ALERT and USGS stream, gauge data were evaluated to determine if the <br />hydrograph for the "typical" (1 -year recurrence interval) flow event could be refined. <br />Although the ALERT data were easy to access and review —and sufficiently precise to <br />record hydrographs —there were few actual recorded flow events. Many of the gauges <br />were not installed until 1989 or later, and most stations did not have rating curves until <br />1992. Smaller flows may completely elude detection because of the wide, sandy stream <br />bottoms which characterize some stations. <br />The data indicate that: <br />• There may be fewer than 36 flow events per year on the average at the Rillito <br />Recharge Project site. <br />• Average annual flow volumes probably exceed 10,100 acre -feet. <br />• Typical winter flows have durations longer than 24 hours and have lower peaks <br />than summer events. <br />• Little information is available on snowmelt- runoff relationships. <br />• Shorter duration, smaller flows may be completely absorbed by channel <br />transmission losses. <br />Travel Time Estimate <br />The stream gauge data were also evaluated in conjunction with HEC -2 step backwater <br />models of the Rillito Creek and its tributaries to estimate flood water travel times <br />Program Summary Report Part /// — Demonstration Project Summary Reports 9 <br />