Parameterizing the soil-water-plant root system

Root water uptake is described from the local scale, to the field scale and to the regional and global scales. Locally, water uptake can be considered at two different Darcian scales, referred to as the mesoscopic and the macroscopic scales. At the local mesoscopic scale, root water uptake is represented by a flux across the soil–root interface, resulting from the mesoscopic Richards equation describing the flow of water from soil to plant root, supplemented by appropriate initial and boundary conditions. The mesoscopic model involves two characteristic lengths describing the root-soil geometry, and two characteristic times describing, respectively, the capillary flow of water from soil to plant roots and the ratio of supply of water in the soil and demand by plant roots. Generally, at a certain critical time, uptake will switch from plant-atmosphere demand-driven to soil supplydependent. The resulting expressions for the evolution of the average water content can be used as a basis for upscaling from the mesoscopic to the macroscopic scale. At the local macroscopic scale, the root water uptake is represented by a sink term in the macroscopic Richards equation. Reduction of water uptake due to water and salinity stresses is incorporated by either linear or non-linear response functions. The root water uptake is strongly related to root length density, but it is easier to obtain root mass density, and therefore a conversion relationship has been established between the bulk root mass and root length densities, both in space and in time. The local macroscopic model can be incorporated in Soil–Plant–Atmosphere Continuum (SPAC) numerical models, like the SWAP, HYSWASOR, HYDRUS, ENVIRO-GRO and FUSSIM models. These SPAC models in turn can be used for upscaling, first to the field scale and from there to the regional and global scales. As Global Climate Models (GCMs) show a strong sensitivity to continental evaporation, closer root water-uptake modeling might improve soil vegetation control instead of uncontrolled continental evaporation. It is concluded that the relationships between mesoscopic and macroscopic descriptions of water uptake provide a useful framework for interpreting laboratory and field data and formulating macroscopic sink terms. One-dimensional analysis of the root-zone water balance is now well developed and has strong computational capabilities. Joint stresses can be computed easily by multiplication of water stress with salinity stress. The implementation at the macroscopic scale of the mesoscopic description of simultaneous uptake of water and solutes is promising. Present Soil– Vegetation–Atmosphere Transfer (SVAT) schemes should be improved with respect to root water-uptake descriptions, using existing global data sets of root and soil properties. A first priority is to establish firmly the relationships of root biomass, rooting depth, root distribution and root functions with land-use type, soil type, soil texture, topography and climat