The water uptake approaches implemented in SoilW component is derived by Ceres (Ritchie and Otter, 1985) and EPIC (Williams et al.,1989) models. Other models under development are the approach implemented by CropSyst (Stöckle et al., 2003) and SWAP (van Dam et al., 1997).

Two simple approaches allow for external estimation of water uptake, checking, respectively if soil water content does not go below zero or below wilting point. CropSyst and Ceres models use a two-step approach for computing the actual water uptake from the soil:

1. the soil water content for each soil layer is compared with the root water request for each layer (which in turn depends on potential transpiration);

2. if the root water request cannot be completely satisfied for one or more soil layers, it is possible to use a compensating function to increase the root water request for soil layers which have more available water (if this compensation is not possible, or it is only partially possible, then the total root water request is reduced accordingly).

The above steps allow calculating the root water uptake and thus "calling" the chosen water redistribution method (Richards' equation, cascading) only when a reasonable quantification of root water uptake has been made.

Crop water uptake is simulated on the basis of roots fraction (CropSyst) or roots length density (Ceres) in each soil layer and potential water uptake.

In EPIC and SWAP is not implemented the root system ability to compensate water deficit in a soil layer, when more water is available in others.

In all the models above, the potential transpiration is an input that is given by the Crop component.

Potential crop transpiration can by given at each soil layer or as a cumulative value of soil profile.

The crop model can also compute the water uptake for each soil layer with their own algorithms. In this case, the soil component uses these values and offer the option to prevent that in some layer the requested water uptake brings the soil water content below the wilting point. If option to limit the water uptake to wilting point is active, and the request to a specific layer is greater than available water, actual transpiration in output is reduced accordingly. In any case, it is not allowed having water uptake that brings the soil water content below 0, and reduction of actual transpiration respect to the request is done.

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