Macroscopic approaches to root water uptake as a function of water and salinity stress

Computer simulation models can be important tools for analyzing and managing site-specific irrigation, soil salinization, or crop production problems. For many of these problems, computer analyses require simulations of root water uptake as a function of water and salinity stress. We give an overview of macroscopic modeling approaches that are based on the Richards equation with a sink term specifying water uptake. Various parameterizations of the sink term as functions of water and salinity stress are reviewed. An example application demonstrates the simulation of drainage for a wide range of salinity and water stress conditions. The example shows that in practice it is very difficult to discriminate among the various functional forms proposed for the sink term. Future improvements to models may involve the incorporation of more dynamic root functions, such as the ability of roots to respond actively to growing conditions. As an example, we consider modeling compensated uptake, in which plants react to stress in one section of the root zone by increasing uptake in sections with more favorable conditions. Lastly, the challenge of estimating uptake reduction parameters from crop salt tolerance databases is discussed.