StateMod can operate on a monthly or daily time step. See Section 7.17 for additional discussion on how to add daily capability to a monthly model. For simplicity StateMod estimates every February has 28 days, therefore any daily data provided for February 29 in a leap year is ignored. The daily capability can be implemented directly or by building upon a monthly model. Constructing a monthly model first is recommended for the following reasons:
- The most difficult part of developing a basin model is understanding the system. By first developing a monthly model, the system operation can be investigated without burdening the user with the volume of information ultimately required for a daily model.
- A daily model is typically developed to be able to simulate large and small flow events that occur within a monthly time step.
Therefore, although daily streamflow data will be required, the user may want to estimate some of the other terms required for a
daily analysis, such as diversion demands or reservoir targets, using a simplified approach. The ability to supply a simple distribution
method to estimate daily data includes the following options:
- Divide a monthly estimate by the number of days in a month or
- Set daily data to a monthly average or
- Use another gages daily distribution or
- Use a pattern developed by connecting the midpoints of monthly data (common for demand data) or
- Use a pattern developed by connecting the endpoints of monthly data (common for reservoir data).
- Daily baseflows may be developed directly as daily data or estimated from monthly baseflow estimates.
- For the case where a structure has both daily and monthly data which do not equal, the distribution method described above specifies which controls. This approach provides maximum flexibility to assign daily data.
- The routing of daily streamflows is accounted for by the gain and loss term that results from the base (natural) stream flows estimated by or provided to the model.
- Routing of reservoir releases are estimated to occur instantaneously in StateMod. The routing of reservoir releases is not included
- StateMod is a primarily a planning model,
- The additional detail required to properly implement reservoir releases with a travel time component is not justified since the system would have to include some kind of forecasting to know a reservoir release is required before a reservoir demand occurs and
- The volume of water potentially delivered early by ignoring a reservoir's travel time is offset by the potential over release that occurs after the demand is satisfied.