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<br />I. INTRODUCTION <br /> <br />SNOW IN THE MOUNTAINS, WATER IN THE RIVERS? NOT NECESSARILY... <br /> <br />Water in the West almost never flows unobstructed from where it fell and <br />melted (since it is often snow) to where it will rest in an ocean or lake. <br />Rather it is rerouted through tunnels and ditches, diverted onto croplands <br />and into treatment plants, and temporarily impounded in reservoirs many <br />times along the way. To keep the science of hydrologic forecasting and the <br />engineering of water management separate, the River Forecast Center (RFC) <br />and its associates forecast natural or unimpaired runoff, not observed or <br />regulated runoff. <br /> <br />WHY FORECAST NATURAL FLOW?.. <br /> <br />There are at least two major reasons for forecasting natural runoff. First, <br />it is not the mission or mandate of the RFC to manage water. Rather water <br />management falls to a large number of other agencies and interests that deal <br />with the physical, legal, and economic constraints of water supply. The <br />starting point for such attempts, however, is an accurate forecast of how <br />much water the watershed will yield, which leads to the second major reason <br />for forecasting natural runoff: it can be done well. The relationship <br />between observable hydrologic parameters (e.g., precipitation, snowpack, <br />terrain, etc.) and natural runoff is predictable and well-defined; the same <br />cannot be said for regulated (or managed) runoff. Further, due to the often <br />inverse nature between regulation and flow, inclusion of assumed water <br />management activities in forecasts tends to conceal and dilute the <br />hydrologic information. <br /> <br />SYNTHESIZING THE PAST... <br /> <br />To create a historical record of so-called natural flow, hydrologists take <br />records of observed (regulated) flows and adjust them by adding and <br />subtracting all known upstream diversions that have records. The result is, <br />at best, a close approximation of what the flow would have been if all of <br />the upstream adj ustments had not taken place. This record of adjusted <br />historical flows is then used to calibrate the model(s). <br /> <br />UNRAVELLING THE PRESENT... <br /> <br />Users of water supply forecasts need to reverse this adjustment process to <br />correlate that which is in the river to that which was forecast; eg., if <br />point A is forecast to receive 50 thousand acre-feet (kaf) , but an upstream <br />reservoir is scheduled .to store 15 kaf, then only 35 kaf is expected to flow <br />by point A. <br /> <br />3 <br />