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