StateMod calculates reservoir evaporation every time step as a function of the reservoir’s surface area, and the combination of precipitation and evaporation stations assigned to each reservoir. The calculation is done at the end of a time step after all water rights have operated as follows:
- Net evaporation (evaporation less precipitation) is calculated in units of feet (
- The average surface area is calculated to be the average of the surface area at the beginning of the
time step and the area at the end of the time step in units of acres (
- Total net evaporation is calculated to be the product of net evaporation (
Evap0) and average surface area (
- Total net evaporation is distributed to the reservoir accounts that share in net evaporation.
- If the distribution to accounts equals the total net evaporation, total evaporation is applied.
- If the distribution to accounts does not equal total net evaporation, distribute any remaining net evaporation beginning with the last reservoir account first.
- The calculations performed in the last step are sometimes required because the average area is an approximation that can, under certain circumstances, result in more net evaporation than is available in the reservoir at the end of a time step (e.g. net evaporation is calculated to be 10 acre-feet but the ending storage is 2 acre-feet).