EPIC approach is a simple algorithm that estimates crop water uptake from any soil layer deploying a function that takes into account the potential transpiration, the root depth, the soil water content and an empirical water extraction distribution parameter. The water uptake is calculated, for available water content greater than 25% of the plant available soil water, as:

where Tmax is the crop transpiration (mm d-1), Di is the depth from the soil surface to the bottom of the layer i (m), Drz is the root zone depth (m), Uk is the water uptake for all layers above layer i (mm d-1), Λ is the a water extraction distribution parameter (unitless).

If the soil water content is less than 25% of the plant available soil water:

and

where θfci is the volumetric field capacity (m3 m-3), θpwpi is the volumetric wilting point (m3 m-3) and θi is the volumetric initial water content (m3 m-3).

The maximum water uptake Smax distribution estimated with EPIC model varies considerably with different Λ value. As Λ increases, Smax increases in the upper layers and decreases in the lower layers. Selection of proper Λ value depend on crop characteristics and soil properties. For example, crops which have nearly uniform root distribution throughout the whole rooting depth, like wheat, should be assigned relative low Λ value (=2). Currently, default value in the model is 2.

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