(These parameters apply to the Soil Moisture method. For the MABIA Method, see MABIA, Flooding.)

These variables are used to model flooding for rice cultivation, managed or unmanaged wetlands, and river flooding onto a floodplain.

Maximum Depth

The maximum depth of water above ground. For rice cultivation or managed wetlands, this is typically the height of the dike that contains the water. Once the water reaches this height, any additional water will run off (subject to the Flood Return Fraction, below). 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.  For floodplains, this is the maximum depth of water that will pool on the land surface, as defined by the topography (see Volume Area Elevation Curve below).

Minimum Depth, Target Depth

Minimum Depth is the minimum required depth for healthy plant growth. When the level of above-ground storage falls to 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. Will default to 0 If blank.  Set equal to Target Depth to 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 no irrigation will be applied.  For floodplains, these will typically both be 0.

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.

Fraction Flooding Received

Land use branch’s share of flood flow from river to this catchment.  Allows for multiple land use branches each receiving some of the flooding.  If catchment is connected to multiple river reaches, same fraction will partition the flood inflow from all.  Leave blank (zero) if land use is not a floodplain.  See River Reach Inflows and Outflows for information about linking the catchment to a river reach.

Flood Return Fraction

% of water above Maximum Depth that flows out of branch in one time step.

Volume Area Elevation Curve

Relationship among Volume, Surface Area and Elevation, as defined by the topography.  VSE curves determine extent (area) and depth (elevation) of flooded area, given the volume of floodwater in catchment.  Leave blank if ground is level (no slope), which will mean that any amount of flooding will flood the entire area of the branch to an equal depth.

Initial Surface Depth

Initial value for surface depth at beginning of simulation

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: Flooding, Tabs: Maximum Depth, Minimum Depth, Target Depth, Release Requirement, Initial Surface Depth.