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