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Operational failures of the ALERT system components were addressed through <br />redundant sensors, operator overrides, alarms, and reports. Due to the system design, <br />departures from the normal operating condition of the project would probably result in <br />losing recharge opportunities. <br />Sensor Allocation Plan <br />The district operates the ALERT system of weather sensors for flood warning. Federal <br />funds helped develop and expand this system to detect and evaluate surface flows for <br />recharge. The Rillito Project Development Plan proposed a dense network of <br />precipitation, streamflow, weather, evaporation, and water quality sensors in the <br />watershed above the project site. The proposal for the network was based on the belief <br />that a rainfall- runoff model would be the primary method used to determine when flows <br />would occur at the project site and when to raise or lower the dam. <br />Hydrologic analysis performed by CH2MHi11 (1995) indicated that the hydraulic methods <br />based on measured streamflows would be more reliable than precipitation -based models <br />for the Tanque Verde and Pantano watersheds, so the sensor allocation plan was revised. <br />Operational Algorithm <br />A dam operations algorithm to operate the inflatable dam used input from the ALERT <br />system and the system operator (CH2MHill, 1995). The algorithm was designed to <br />operate the recharge facility without human input but could accept operator overrides. <br />The algorithm used real -time measurement of streamflow at sensors located upstream, <br />and rainfall- runoff predictions for the few small tributary watersheds a short distance <br />from the recharge site. <br />The algorithm consists of three basic components: systems check, RTU override, and dam <br />operations routines. If certain key components are not responding, the RTU override <br />routine is initiated, and the dam may be lowered. Other less important process problems <br />would result in alarms or reports. <br />Rainfall /Runoff Data <br />Hydrologic data were first reviewed with respect to the operational needs of the <br />inflatable dam system. The district sponsored an attempt to use eight ALERT <br />precipitation sensors in the upper Tanque Verde watershed to predict runoff. This study <br />indicated that observed data do not provide a clear picture of rainfall- runoff relationships <br />(Hendrickson, 1990). This is due to varying antecedent moisture conditions, uncertain <br />transmission losses, and spatial variation of rainfall, among other factors. <br />Later, independent research at the more highly instrumented Walnut Gulch watershed <br />indicated that: <br />• Precipitation gauge networks seldom are reliable or dense enough to accurately <br />predict runoff events. <br />High Plains States Groundwater Demonstration Program <br />