StateMod has two methods to spill water from a reservoir; Type 9 and Type 29. The Type 9 rule spills water from a
reservoir to meet a reservoir target. The Type 29 spills water from a reservoir for an administrative reason and is
typically used as part of a plan operation. When a spill occurs, two key variables associated with StateMod’s ability
to report streamflow at a river node and allocate water to a demand are affected; River Outflow and Available Flow.
River Outflow is reported in the Stream Balance (*.xdd) and represents water leaving a river node.
Available Flow is adjusted each time a diversion occurs during a time step. It is used to determine if water is available for diversion by a node located at or upstream of the reservoir. The Available Flow reported in the Stream Balance (*.xdd) represents the minimum value available at and downstream of the reservoir at the end of the time step after every water right has had an opportunity to divert water. The following are noted with regard to the two methods available to simulate reservoir spills by StateMod:
- Type 9 Operation: When a reservoir spills using a type 9 operating rule (spill to target) the River Outflow at the node containing the reservoir is adjusted to reflect the spill. However the Available flow (term used to determine if water is available to be diverted) does not get adjusted. The net result is that the River Outflow reflects wet water at the reservoir node while the Available Flow limits future diversions at and upstream of the reservoir.
- Type 29 Operation: When a reservoir spills using a type 29 operating rule (spill from a plan or reservoir) the user
has the ability to spill from a plan, from a reservoir or from a reservoir and a plan.
- If a plan is specified, with or without a reservoir, the user has the ability to control if the available flow at the node where the reservoir is located does or does not get adjusted. This capability is often required for a Changed Water Right Plan where the water may be diverted, temporarily stored in a plan that subsequently gets spilt for temporary storage in other plans associated with multiple users and ultimately released.
- If a plan is not specified, e.g. water is being spilled from a reservoir for an administrative purpose, the spill will occur at the reservoir node and the River Outflow and Available Flow are adjusted using the same approach as a type 9 operating rule.