Typically, reservoirs are operated based on policies that involve multiple pools that are defined to be used for different purposes. An example of a typical multi-pool reservoir is provided in the example GoldSim model, where the reservoir is divided into surcharge, flood control, conservation, and dead pool zones.
The flood control zone is to remain empty except during the times following a flood event upstream of the reservoir. Flood control zones often include a surcharge zone, which is the uncontrolled storage volume above a spillway elevation. Usually, it is not in the interest of reservoir operators to spill water over the spillway because it is uncontrolled and poses a risk to the channel downstream. The flood zone is typically drained in a controlled manner through use of an outlet works with an operated gate or valve.
The conservation pool is used to store water temporarily for downstream uses such as power generation, recreation, navigation, irrigation, municipal and industrial water supply, and instream flows for habitat. This pool is only drawn down if a request is made in behalf of one of these uses. Often times the top of the conservation pool varies seasonally as shown in the example plot image at the bottom of this article. Typically, the top of the conservation pool rises during the part of year that additional storage is needed to supply water for beneficial use later on. A rise in the conservation pool poses greater risk on the operations of the reservoir because it requires that the flood pool zone is decreased, which gives the reservoir less opportunity to evacuate flood flows before the reservoir level enters the surcharge stage.
Often times, the top of the conservation pool acts as a guide, target, or rule curve for operators. The reservoir is operated in order to try and keep the reservoir level as close as possible to the top of the conservation pool. Obviously, it is critical that the seasonally varying conservation pool limit is first optimized using appropriate methods prior to actual use in the field.