ZIMshed properties
ZIMsched Soil model computations
ZIMsched for Rice field
ZIMsched for Rice field computations
The parameters for the ZIMsched soil water model are the same as those of the FAO 56 model. Additionally, two extra parameters need to be specified:
· Sat. hydraulic conductivity. The saturated hydraulic conductivity is also called the saturated drainage coefficient [mm/h]. Since it is known that the drainage of water from an unsaturated zone diminishes with diminishing soil moisture, water may not drain instantaneously from the time when the water is above field capacity. In such cases water will be available for transpiration for a short period depending on the magnitude of the saturated drainage coefficient.
· Depth to groundwater. The unsaturated soil zone extends from the ground surface to the groundwater table [m]. Based on the depth of the top soil and the root zone layers, the lower soil layer can be calculated. From this zone deep percolation to the groundwater takes place using the same drainage formula as for the root zone layer.
ZIMsched Soil model computations
The ZIMsched soil water model is one step more complex than the FAO 56 soil model and simulates the water balance in the unsaturated soil column in three compartments. The first two are identical to the ones presented in the FAO 56 soil model. The third layer is denoted as:
· The lower zone layer. It covers the soil column from the root zone to the groundwater table. If the depth to the groundwater table below ground surface is called Zgwt, the thickness of this layer is time varying equal to: Zlower(t) = Zgwt - Zroot(t) - Ze.
Fig A.3.2 shows the schematic presentation on the soil column and the flows involved.
In the ZIMsched model, the root zone percolation also only occurs when the water content is above field capacity, and but the percolation is not instantaneous and is soil-dependent, described by the saturated drainage coefficient of the soil:
(A.3.3)
Where Ks is the saturated drainage coefficient [m/s].
The updating of the soil moisture content in the root zone is calculated by:
(A.3.4) 
The deep percolation PercD from the lower soil layer is calculated by a similar equation as for the root zone percolation, and the new soil moisture content in the lower zone is calculated by:
(A.3.5) 
The ZIMsched for Rice, a soil water model for rice crops, is an extension of the ZIMsched model with an additional storage on the ground surface. This allows water to be stored on the ground surface during growth of rice. The model requires an extra parameter describing the maximum water depth that can be stored on the ground (often called detention storage):
· Max water depth. The maximum water depth [m] specifies the maximum water depth that can be held back on the ground surface before surface runoff takes place. From this storage water can evaporate and infiltrate into the top soil storage.
ZIMsched for Rice field computations
In the computations of the soil model for rice crops the actual water depth is added to rain (not to the irrigation because the wetting factor would be applied) and at the end of the calculation any excess of water on the top soil is considered as ponded water (up to the maximum water depth). The exchange of flow between different soil layers is identical to the ZIMsched model.
As in ZIMsched model, first Top soil layer is filled, then the Root zone layer and finally the lower zone. If there is an excess of water in the lower compartment (above field capacity), deep percolation occurs. At the end of the calculations, the excess of ponded water may generate runoff.
At the beginning of nursery stage (until start of land preparation) only a fraction of the field is used. The fraction, called Nursery area is specified in the Crops module.
