The soil moisture redistribution function forms part of the catchment drying process. It determines the quantity of water (mm) transferred between soil zones within a time interval. The expression used is:

 

where:

RD = Soil moisture redistribution between lower and upper soil zone

US = current volume in upper soil zone store (mm)

LS = current volume in lower soil zone store (mm)

A1 = soil moisture redistribution constant

A2 = soil moisture redistribution constant

RQ = redistribution factor

The four soil moisture constants must be supplied as data. A value for A0 may be obtained from continuity considerations when LS = 0, as follows:RQ is evaluated within the program by the equation:

RQ = TIxA0(1+A1)eA3xLS  RQ>20.                 (18)

RQ = 20.0 RQx20.0                                        (19)

where:

TI = time interval (min)

A0 = soil moisture redistribution constant

A1 = soil moisture redistribution constant

A3 = soil moisture redistribution constant

 

where:

A = hydraulic conductivity parameter typically equal to Ko/2.8

A1 may be evaluated as the ratio of the capacities of the upper and lower soil zones, that is:

 

where:

LSC = maximum moisture content of lower soil zone (mm).

A2 and A3 are usually set to zero.

If the result obtained by evaluating Equation (17) is negative then the flow is from the lower soil zone to the upper soil zone. If the result is positive then the flow direction is from the upper soil zone to the lower zone.