This is a special form of the second-type, Neumann, specified or constant flux boundary condition which is used in conjunction with evapotranspiration properties which can be defined as described in Section 5.8.5. It is an areal property and so you should first choose the subset of faces for which you want to apply the condition. These faces should be part of the top boundary of the grid, and in this case the top boundary must be coincident with the 2-D slice which was used to define the 3-D mesh. This means that you cannot use the boundary condition if the Y-vertical instruction has been used. This restriction arises because of grid numbering assumptions which are made when applying evapotranspiration as a function of depth.
The following instructions can be used as input to the Type instruction inside the Boundary condition...End instruction group to assign various specified flux boundary conditions:
Sets the input type to be a special specified flux boundary condition and converts the evaporative fluxes to nodal volumetric fluxes [L3 T−1] by multiplying by the contributing area of the chosen face.
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boundary condition type potential evapotranspiration face set top time value table 0.0 0.001 end end ! new specified head
would define a potential evapotranspiration of 0.001 from time zero for the duration of the simulation for all of the nodes contained in the face set top.
Although this instruction is currently restricted to be applied to porous media only, the potential evapotranspiration is applied to the surface water domain and subsurface domain in a stepwise manner. Nodes in the chosen faces and as a function of depth may be assigned a time-variable flux value, depending on factors such as the current nodal saturation, which may inhibit for example, transpiration from occurring.