A Matric-Flux Potential Expression to Predict Limiting Hydraulic Conditions for Root Water Uptake.

Soil hydraulic properties together with limiting crop water potentials allow to characterize crop water stress by predicting the threshold value of water content or pressure head that delimits the constant and falling water uptake rate phase. Therefore, the accurate assessment of soil water availability depends on an adequate theoretical model together with thoroughly determined soil hydraulic properties. Such a model was developed by De Jong van Lier et al. (2006), who proposed an equation to calculate the limiting matric flux potential (threshold value between constant and falling rate phase) for a given transpiration rate and root length density in a single-layer soil. We further developed this equation to a version allowing multiple soil layers. In order to illustrate the new approach, a scenario with soils from two Brazilian climatic zones (semi-arid and sub-humid) was tested. The applied scenario was composed by soil hydraulic properties, determined for several layers throughout soil profile for both climatic zones; vertical root length distribution corresponding to a tropical semi-deciduous and deciduous forest; and potential transpiration rates ranging from 0.1 to 6.0 mm/d. The novel equation allows verifying aspects of soil-water supplying capacity related to the vulnerability of ecosystems and agricultural soils to drought.