A feedback mechanism to the unsaturated zone has been included in the module to model a redistribution of water in favour of evapotranspiration from the low wetland areas located adjacent to most rivers.
The UZ feedback is not relevant if the Richards Equation method is used and the feedback fraction should be set to zero.
Water is redistributed from the base flow reservoirs to the unsaturated zone model in the lowest interflow reservoir zone of the subcatchment, if there is a water deficit in the root zone. The base flow discharge is distributed to all UZ columns in the lowest interflow reservoir.
The UZ deficit in the lowest interflow reservoir is called the Field Moisture Deficit (FMD) in units of metres, and is calculated as the total amount of water required to bring the entire root zone in all the UZ columns back up to field capacity. Thus,
(30.53)
where qFC is the moisture content at field capacity, q is the moisture content in the root zone, n is the number of UZ cells in the root zone, and Dz is the height of the UZ cell. This is then summed over all of the UZ columns in the lowest interflow reservoir.
Now, the amount of water that is available from the linear reservoirs to be redistributed to the unsaturated zone is calculated as a fraction of the baseflow:
(30.54) 
where UZfrac is a specified fraction of the baseflow that is allowed to recharge the unsaturated zone.
If Savailable is larger than or equal to the Field Moisture Deficit, then the water content of each of the root zone cells is increased to field capacity.
If Savailable is smaller than the Field Moisture Deficit, then the water content of each of the root zone cells is proportionally increased by
(30.55) 
After the feedback calculation, the amount of baseflow to the river is reduced by the amount of water used to satisfy the Field Moisture Deficit in the unsaturated root zone, which is QUZ, in Figure 30.10.
