Measuring and modeling root water uptake based on³⁶chloride discrimination in a silt loam soil affected by groundwater

Water uptake by plant roots was successfully simulated by use of a volumetric sink term S(z) added to the continuity equation for soil moisture flow, which generally requires detailed information about the root system as functions of root density, root distribution, and root length. Unfortunately, these factors are difficult to evaluate. This paper describes a simple method for the estimation of soil water extraction by roots based on root discrimination of selected solute species such as chloride. A silt loam soil planted with carrots and affected by groundwater at different depths was used for the investigation. The soil was characterized by soil matric potentials close to hydrostatic equilibrium conditions. Aeration in the root zone was impeded by high moisture content. Because chloride was strongly discriminated by the roots, root water uptake was found to be related to the increase in soil³⁶chloride solution concentration. Consequently, the chloride in the soil water was found to be an ideal indicator of water uptake in plants. Based on this proposed approach, patterns of water extraction by carrot roots could be described using a quasi steady-state model. We also found that in the groundwater-affected silt loam soil with impeded aeration, about 80% of the water transpired was extracted from the top 5 cm of the root zone.