Plant Water Uptake Terms Evaluated for Soil Water and Solute Movement Models

Two root water uptake terms, representative of the two major types commonly employed in soil-based models of water and solute movement, were evaluated. Simulations were run to test the sensitivity of the two terms to salinity and water content and to investigate the details of their respective form and function. The two root water uptake terms tested were: (i) a mechanistic equation based on Darcy's law (Type I), and (ii) an empirical equation relating soil water potential to relative water uptake (Type II). The Type I term was insensitive to salinity where no reduction in transpiration was shown for increasing irrigation water salinity from 0.0 to 6.0 dS/m and applying water equal to potential transpiration. The Type II term was sensitive to salinity and showed a 35% reduction in water uptake by increasing water salinity from 0.0 to 6.0 dS m⁻¹ and applying water equal to potential transpiration. Predicted reduction in water uptake due to matric potential was of the same magnitude as that due to salinity. The Type I term resulted in abrupt shifts in water uptake between full and zero transpiration, occasionally resulting in long periods of computed zero transpiration, uncharacteristic of conditions in the field. It was concluded that the Type I water uptake term may not be appropriate for models incorporating root water uptake, particularly under saline conditions. Contribution of the Univ. of California, Riverside. Sponsored by the Univ. of California's Salinity/Drainage Task Force.