Numerical Layers

Specific Yield of the upper SZ numerical layer

The specified value for specific yield is not used for the specific yield of the upper most SZ numerical layer if UZ is included in the simulation.

By definition, the specific yield is the amount of water release from storage when the water table falls. The field capacity of a soil is the remaining water content after a period of free drainage. Thus, specific yield is equal to the sat­urated water content minus the field capacity.

To avoid water balance errors at the interface between the SZ and UZ mod­els, the specific yield of the top SZ layer is set equal to the he saturated water content minus the field capacity. The value is determined once at the begin­ning of the simulation. The water content parameters are taken from the UZ layer in which the initial SZ water table is located.

In principle, having different values between the SZ and UZ models does not directly cause a water balance error, but it may cause numerical problems that could lead to water balance errors. By definition, the steady-state water table location will be identical in both the SZ and UZ models. Pumping from the SZ will lower the SZ water table by an amount equal to the specific yield divided by the cell area times the pumping rate. However, if the field capacity is not correlated to the specific yield, then the amount of water released from storage in the UZ will be more or less than the amount extracted from the SZ cell. This will result in different water tables in the SZ and UZ models. If pumping stops, the system will again reach an equilibrium with the same water table in both the SZ and UZ simply because of the pressure head redis­tribution.

As mentioned, the upper Sy value is calculated only at the beginning of the simulation based on the UZ layer in which the initial SZ water table is located. If the soil profile has multiple soil types with different field capacities and satu­rated water contents, then the specific yield in the SZ and UZ model may diverge during the simulation. With slowly moving water tables, the differ­ences may not be that large and the errors generated will likely be tolerable. If the water table drops into a lower SZ layer, then the specified Specific Yield will be used.

The actual value used in the model is displayed in the pre-processed tab under Specific Yield.