A Mechanistic Root Water Uptake Model Including Root Resistance.

Modeling of root water uptake and its partitioning over depth is relevant for hydrological, meteorological and crop growth modeling. It is commonly accepted that under dry conditions soil hydraulic conductivity is the limiting factor to water transport in the soil-plant-atmosphere pathway, therefore determining transpiration; under wetter circumstances soil hydraulic conductivity increases and the conductance within the plant determines root water uptake rates. Assuming a leaf water pressure head, pressure head in the root xylem and at the root surface can be calculated if radial and axial hydraulic resistances and water flux density are known. Problem to perform this calculation is that the flux density is directly dependent on transpiration, one of the unknowns whose value is one of the supposed modeling outcomes. We dealt with this problem considering (1) that pressure head is a continuous function along the pathway whose derivative is discontinuous at the interface soil-root and (2) that the first derivative of matric flux potential M is continuous along the pathway, whereas the function itself is discontinuous at the transition from soil to root. The resulting differential equation could be analytically solved for a special case of Brooks and Corey soils; a more general application required an iterative procedure which was optimized, included in the SWAP hydrological model and compared to experimental field data with Common Bean and Soy Bean. Results show an significant improve in model performance when compared to a previous model not including internal plant resistance, only soil resistance to water flow.