TOWARD UNDERSTANDING SOIL WATER UPTAKE BY PLANT ROOTS

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Abstract
Plant water uptake from a uniformly rooted soil volume was studied by numerically solving the nonlinear diffusion equation for a soil cylinder bounded externally by an insulating surface, located at the half-distance between adjacent roots and internally by the root surface. A sinusoidally varying uptake rate is employed to simulate the diurnal evaporative demand, while the actual water flux across the inner surface is controlled by the root surface matric potential through a stomatal adjustment function. The analysis is conducted for Chino clay, Pachappa sand, and Indio silt loam, using hydraulic parameters determined from piecewise continuous exponential functions of the dependent variable matching data for these soils. The analysis uses root radii of 0.003, 0.005, 0.010 and 0.020 cm and root densities of 0.080, 0.142, 0.318 and 0.650 cm root length/cm3 of soil. The transient matric potential distribution in the soil depends strongly on the precise nature of the hydraulic prameters for each soil: as the soil loses water by transpiration, the value of the bulk soil matric potential decreases monotonically at a rate dependent upon the specific water capacity of the soil. For initial bulk soil potentials of -0.3 bar in the Chino and Indio, and -0.2 bar in the Pachappa, the initial rate of potential decrease with time is greatest in the Chino, followed by the Pachappa and Indio, respectively. The corresponding radial potential distribution around individual roots depends upon the instantaneous value of the soil water diffusivity, which decreases as the soil dries, creating steep potential gradients adjacent to the root, with the largest initial gradients occurring in the Chino clay, followed by equivalent gradients in the Pachappa sand and Indio silt loam. The relative rate of decrease in the bulk soil potential and the drawdown around a root for the 3 soils, however, changes with time as the soil dries. The drawdown around a root is reduced by increasing root densities, so that at low densities the roots surface potential depends more closely upon the diffusivity, whereas at high densities the root surface potential depends more closely upon the specific water capacity. In addition, roots of smaller radii exhibit greater drawdown as the total root surface area decreases and the flux of water at the root surface increases. The simulation of root water uptake using piecewise continuous functional relationships for the diffusivity and specific capacity produces results that form a coherent illustration of plant survival in 3 diverse soil environments.