Specific Yield of upper SZ layer
MIKE SHE forces the specific yield of the top SZ layer to be equal to the “specific yield” of the UZ zone as defined by the difference between the specified moisture contents at saturation, qs, and field capacity, qfc.This correction is calculated from the UZ values in the UZ cell in which the initial SZ water table is located.
Ensuring the correct UZ thickness
The following procedure could be used to ensure that the unsaturated zone does not drop below the bottom of the first calculation layer of the saturated zone:
1. After a simulation, create a map of grid statistics of the potential head in the first calculation layer of the saturated zone
2. Subtract the map of the minimum potential head from the map of the bottom level of the first calculation layer of the saturated zone.
3. View the difference map. If the difference is very small in some areas of the map (e.g. <0.5 m), it is strongly advised to move the bottom level of the first calculation layer of the saturated zone downwards.
4. Repeat this procedure until there are no small differences.
Evaluation of the UZ-SZ Coupling
The WM_Print log file generated by MIKE SHE should be reviewed after each simulation to evaluate the performance of the UZ module. If the user specified maximum UZ iterations is exceeded an excessive number of times and there are no problems with the soil data used in the UZ module, the UZ and SZ time step should be evaluated. Sometimes it is possible to reduce the number of times the maximum UZ iterations is exceeded by making the UZ and SZ time steps more similar. Typically the SZ to UZ time step ratio should be no larger than four.
It is also useful to save the value of Ecum as a grid series output, or as a detailed time series output at critical locations. These plots can be used to determine if there are locations or periods of time during the simulation where the Ecum term exceeds Emax. This can occur if
· the water table drops below the first SZ calculation layer (positive value),
· the water table rises above the top of the first SZ calculation layer (negative value),
· the vertical hydraulic conductivity in the upper SZ calculation layer is much greater than the saturated hydraulic conductivity used in the UZ, or if
· the drainage time constant is too high.
In the first two cases above, the epsilon term can exceed Emax because the UZ module cannot get rid of epsilon because there is no available storage for the error term. In the third case, the UZ and SZ hydraulic properties should be consistent or it will be difficult for MIKE SHE to simulate consistent vertical flow rates. In the last case, the drainage time constant should be reduced to prevent excessive and unrealistic drainage outflows from the SZ module.
Limitations of the UZ - SZ coupling
The coupling between UZ and SZ is limited to the top calculation layer of the saturated zone. This implies that:
· As a rule of thumb, the UZ soil profiles should extend to just below the bottom of the top SZ layer.
· However, if you have a very thick top SZ layer, then the UZ profiles must extend at least to below the deepest depth of the water table.
· If the top layer of the SZ model dries out, then the UZ model usually assumes a lower pressure head boundary equal to the bottom of the uppermost SZ layer.
· However, if the top layer of the SZ model dries out, and you are using the Richards Equation method, then you should ensure that there is one UZ node below the bottom of the top SZ layer. Otherwise, an error may be generated if there is an upwards capillary pressure gradient.
· All outflow from the UZ column is always added to the top node of the SZ model.
· UZ nodes below the water table and the bottom of the top SZ layer are ignored.
More detail on interaction between the lower UZ boundary and the SZ Layer 1 is given in the Section: Lower Boundary (V1 p. 557).