The Evaporation and Precipitation category methods are used to calculate the volume of Evaporation from and Precipitation to the surface of a reservoir over the timestep. Precipitation and Evaporation are used in the mass balance equations which are solved in the dispatch methods.

Some of the methods in this category only calculate evaporation.

The None method is the default method for the Evaporation and Precipitation category. It should be chosen if the user does not want to include Evaporation in the mass balance equation of the Reservoir. No calculations are performed by this method.

There are no slots specific to this method.

The Daily Evaporation method is used to calculate the daily evaporation volume and the flow rate of the precipitation. The daily evaporation volume is a function of the Evaporation Rate, average Surface Area, and Pan Coefficient.

The Input Evaporation method should be used when the user wants to input the Evaporation Rate directly. This Evaporation Rate is used to compute the volume of water that evaporated over the timestep. Also, the user can input the evaporation volume directly on the Evaporation slot. In that case, the Evaporation Rate is not used to calculated Evaporation.

In the Monthly Evaporation method, evaporation is calculated linearly from the Evaporation Coefficients entered for each month. This method will not work with a timestep longer than 1 Month. The total evaporated volume is a function of the average Reservoir Surface Area over the timestep, the Evaporation Coefficient, and the length of the timestep. The following slots are specifically associated with this method.

The Pan and Ice Evaporation method is used to calculate the volume of evaporation with one of two methods based on the value of the Pan Ice Switch slot for each timestep. The Pan Ice Switch slot is used as an indicator of whether ice is present on the surface of the reservoir. A value of 1.0 in the Pan Ice Switch slot indicates that there is ice cover on the Reservoir that must be taken into account when Evaporation is calculated. A value of 0.0 or any number other than 1.0 in the Pan Ice Switch slot indicates that there is no ice on the surface of Reservoir. The following slots are those specifically associated with this method.

This method is exactly the same as the Pan and Ice Evaporation method. It uses the same slots, has the same required inputs and performs the same calculations. The only difference is that this method uses the instantaneous, end of timestep surface area instead of the average surface area over the timestep.

Periodic Net Evaporation computes the gross evaporation from the reservoir and then subtracts out components of evaporation that would have occurred if the reservoir had not been built. This is the net evaporation and is set in the Evaporation slot. Each area of the submerged reservoir is separate including:

Each can have a separate evaporation coefficient and possibly additional components in its computation like temperature. The area of each region is specified in a separate table relating reservoir pool elevation to each region’s area. It is assumed that any precipitation that falls on the Remaining Area would have completely evaporated. The figure shows a sample of the different reservoir areas used in this method.

In the Single Evaporation method, evaporation is calculated linearly from the Single Evaporation Coefficient entered by the user. The total evaporated volume is a function of the average Reservoir Surface Area over the timestep, the Single Evaporation Coefficient, and the length of the timestep. The following slots are specifically associated with this method.