Reservoirs are top level objects each of which contain a fixed collection of slots which represents its important hydrologic processes. The slots are (click for a legend for the standard slot display):

• Inflow. This slot collects all of the sources of water entering the reservoir. This includes virgin or unregulated inflows and releases from other structures upstream.

• Outflow. The outflow slot handles all releases from the reservoir. It is a controllable slot. The outflow slot is connected to the inflow slot of any water handling object that is downstream. The outflow slot may be configured by adding facets through the user interface. Facets represent typical outlet works such as valves, gates, weirs and generators. Facets may be configured as desired.

• Storage. This slot represents the volume of water impounded. It is a controllable slot. The storage slot has two internal variables, one for the volume at the start of the time step and one for that at the end. The ending value is what is reported and is used by contraints. The beginning value is available to stack calculators.

• Surface Elevation. The elevation of the water surface at the end of each time step. For this variable to be computed, the reservoir must have an area capacity table.

• Tailwater Elevation. The elevation of the water surface immediately downstream of the outlet works of the reservoir at the end of each time step. For this variable to be computed, the reservoir must have a tailwater discharge table.

• Surface Area. The surface areal extent of the free water surface of the reservoir at the end of each time step. For this variable to be computed, the reservoir must have an area capacity table.

• Evaporation. The evaporation loss for the reservoir at the end of the time step. This is a function of the surface area. For this variable to be computed, the reservoir must have an area capacity table.

• Evaporation Coefficient. Coefficients which when multiplied by the surface area of the reservoir will give the evaporation for the time step. This is typically a time series.

• Seepage. The seepage loss for the reservoir. Note that this is not leakage through the dam or outlet works which shows up downstream as inflow to the next structure. Rather, this is storage that seeps into the groundwater. It is a function of storage for the time step involved.

• Seepage Coefficient. A coefficient (or time series of coefficients) which, when multiplied by the storage in the reservoir will give the seepage loss.

• Area Capacity Table. This is an optional table which contains the relationships among the surface elevation of the reservoir and its storage volume and surface area. This table has three columns of numbers which, when entered with a value from any column, will return the values in the other two by linear interpolation. There is no particular limit on the number of rows in this table. Note that, while this table is optional, a number of variables such as evaporation depend upon it.

• Tailwater Discharge Table. This is an optional table which contains the relationship between discharge rate and tailwater elevation. This table has two columns of numbers, one for discharge rate and the other for the corresponding tailwater elevation. The table works by linear interpolation. Note that while this table is optional, it is used in the calculation of head for hydropower.

The primary purpose of a reservoir is to impound water and release it for downstream use in accordance with some policy established by the modeler.

The reservoir responds to all of the messages in the meta algorithm and to no others. During all steps of the MA except task and finish time step, it simply passes the message down to each of its slots in turn. For example, the begin time step response of a reservoir is simply to send a BTS message to each of its slots.

During the task step of the MA, the reservoir performs all calculations needed to insure that end of period storage is equal to beginning of period storage plus inflow and less release and losses. In doing this, the reservoir may treat either inflow, ending storage or outflow as the dependent variable depending on the policy being implemented. Thus, if the reservoir is operating to satisfy a downstream demand, the reservoir may take the water from storage, pass the request upstream or some combination. In making that decision, it checks the storage policy and is free to communicate with upstream water sources if necessary.

During the finish time step portion of the MA, the reservoir calculates and saves values for derived variables such as surface elevation and surface area. It then passes the FTS message to each of its slots in turn. Finally, the reservoir sets its beginning storage for the next step to the ending storage for the current step.

While the slots and tables available to a reservoir may not be changed by the user, all of the values of those objects may be changed freely. The initial and target values in the slots may be changed by using any of the management policy controls. The one area wherein you may change the reservoir's structure is in the addition of facets. Facets are added to the outflow slot and represent outlet works such as generators, valves, weirs, etc.