Ponding

(These parameters apply to the Soil Moisture method. For the FAO Method, see FAO Method, Climate.)

These variables are used to model flooding for rice cultivation, or for wetlands (managed or unmanaged).

Maximum Depth

The maximum depth of water above ground. This is typically the height of the dike that contains the water. Once the water reaches this height, any additional water will run off. This height may vary from one timestep to the next. For example, if the rice fields are flooded in May and drained in September, using a dike 150 mm high, the following expression would be used: MonthlyValues( Apr, 0, May, 150, Aug, 150, Sep, 0 ). In this case, any remaining above-ground storage from the end of August would become surface runoff in September.

Minimum Depth, Target Depth

Minimum Depth is the minimum required depth for healthy plant growth. When the level of above-ground storage falls below this height, WEAP will irrigate with enough water to bring the level up to the Target Depth. If the level is between the Minimum Depth and the Target Depth, no irrigation will be applied. Leave Minimum Depth blank to have it default to the Target Depth--this will cause irrigation to be applied each month to maintain the level at exactly the target level. The Minimum Depth is analogous to the Lower Threshold variable for irrigation; Target Depth is analogous to the Upper Threshold variable.  For an unmanaged wetland, enter 0 for minimum and target depth so that irrigation will never be applied.

Release Requirement

When modeling surface water storage on a land class, either for rice cultivation or a managed wetland, the Release Requirement is the amount of water to be released in the timestep, to be replaced with new supply. This is typically used to maintain proper water temperature for rice. The release requirement for the timestep will never exceed the amount of surface storage at the beginning of the timestep. For example, if the release requirement for August is 100 mm, but the amount of water at the beginning of August is only 75 mm, then the release requirement will be reduced to 75 mm. Therefore, all of the water will be released, and replaced by new supply.

Note: Ponding can exist only when the root zone is saturated. This corresponds to z1 = 1 in the Soil Moisture Method calculations, meaning that the soil water balance for the top bucket is 100%.  The Soil Moisture Method calculates fluxes out of the root zone due to evapotranspiration, interflow and deep percolation, which will occur at their maximum rates when z1 = 1.  As water leaves the top bucket, ponded water will enter the soil to take its place, causing the depth of water above ground to decrease.  In addition, any water released because of the release requirement will also decrease the depth above ground.  

Entered on: Data View, Branch: Catchments, Category: Ponding, Tabs: Maximum Depth, Minimum Depth, Target Depth, Release Requirement.