In order to calculate crop water uptake, the soil profile is divided into layers. Water uptake of each layer is calculated from the difference between the water potential in the soil and the plant xylem, multiplied by plant conductance (mainly determined by root conductance).

 

This model requires the fraction of the roots present in each soil layer as input.

Soil conductance is assumed to be large compared to root conductance, and so water uptake can be assumed to be unlimited by water movement towards the roots.

Currently, the algorithm for water uptake is already available in the software and can be used, but its use is not recommended as it is still undergoing testing and debugging.

 

The CropSyst approach (Stöckle et al., 1997, 2003) calculates water uptake as the product of the difference between the water potential in the soil and the crop canopy and the crop hydraulic conductance.

The maximum crop hydraulic conductance is calculated as

 

 

where Cmax  is the maximum crop hydrologic conductance (kg s m-4), Umax is the maximum uptake rate (kg m-2 d-1) and Ψl,sc is the canopy average leaf water potential at the onset of stomata closure due to water deficit (J kg-1)

 

 

where C is the crop hydraulic conductance (kg s m-4) and fir is the fraction of incident radiation intercepted by the crop canopy.

 

 

where Ci is the hydraulic conductance for each soil layer (kg s m-4) and fri is the fraction of  root length present in the layer (0-1)

If there is not root length as input, it can be calculate by two ways:

1- if there root length density for layer and roots depth are available as inputs:

 

 

where RLD is the root length density per layer and TotRLD is the total root length density in the soil profile (m m-3).

 

2- if only roots depth is known the model calls the method Berg-Driessen. Then water uptake from each soil layer can be calculated by:

 

Ui=water uptake from each soil layer (kg m-2 d-1)

Ψsi=soil layer water potential (J kg-1)

Ψl= canopy average leaf water potential (J kg-1)

 

 

U=sum of the uptake from all soil layers in which roots are present

 

 

Ψs= weighted average soil water potential in the root zone (J kg-1)

Tmax=crop transpiration (kg m-2 d-1)

 

If Ψl is lower than Ψl,sc, the atmospheric evaporative demand cannot be met by attainable water uptake. The Ψl is then recalculated as:

 

 

Each water potential can be calculated by Campbell’s or van Genuchten’s function from soil water content.

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