The Ceres water uptake model (Ritchie and Otter, 1985) uses the “law of limiting” approach, in which the soil resistance is compared to the root resistance and the greatest resistance determines the flow rate of water into roots. If the soil limited water uptake exceeds the maximum plant limited flow rate, then the water uptake rate is set equal to the maximum plant limited flow rate. If the total possible water uptake exceeds the potential transpiration, the possible water uptake rate calculated for each layer is reduced proportionally so that the total water uptake equals the potential transpiration.

Crop water uptake depends on root length in each soil layer (i).

Water uptake per unit root length for each soil layer is calculated by:

 

 

where Urli is the attainable water uptake for unit root length (m3 m-1 d-1), θi  is the volumetric water content (m3 m-3), θpwpi is the volumetric water limit of plant extraction (m3 m-3), ρrli is the root length density (m3 m-3) and all coefficients are derived empirically.

The attainable crop water uptake at each soil layer is:

 

 

where Urli is the attainable water uptake for unit root length (m3 m-1 d-1), Uai is the attainable water uptake at each soil layer (m3 m-1 d-1), ρrli is the root length density (m3 m-3) and Zi is the layer thickness. Attainable crop water uptake from soil profile Ua is the sum of Uai.

Actual crop water uptake from soil profile U is the minimum between potential transpiration (Tmax) and the attainable value from soil profile.

 

 

In order to know the final value of actual water uptake at each soil layer the ratio between actual and attainable water uptake from soil profile are needed.

 

Actual crop water uptake from each soil layer is given by:

 

 

If there is not root length density as an input the model calls a method (van den Berg and Driessen, 2002) to calculate it as a function of roots biomass and depth.

 

A composite strategy WaterUptakeCeresRootDensityVanBerg is available for this purpose.

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