SOIL MOISTURE MODELING USING HYDRAULIC REDISTRIBUTION AND ROOT COMPENSATION MECHANISMS OF PLANTS IN STRATIFIED VADOSE ZONE

Hydraulic redistribution (HR) refers to passive movement of soil water from wetter soil layers to drier root-zone via plant roots, driven by moisture gradient. Likewise, reduction in root water uptake (RWU) that occurs in one part of root zone due soil water stress can be compensated for by enhanced extraction from wetter parts through root compensation mechanism (RCM). A numerical model is developed in this study to understand the effect of RWU mechanisms in multi-layered soil profile through coupled soil-water-root zone system. Soil water flow equation and root biomass are solved numerically using implicit finite difference method coupled with an iterative technique. Modeling of HR requires a sink/source term that allows flow of water in both directions, i.e. from soil to root and vice versa. For this a realistic non-linear function of root density distribution is incorporated in the soil moisture flow equation for simulating the rate of water removal with and without considering HR and RCM. The model was first tested for a barren layered soil profile before applying to Triticum aestivum plot. Model results indicate that in favourable soil moisture conditions, RWU is higher in the region close to ground surface as compared to the mid or lower root zone. However, after meeting the transpiration demand during day time, moisture stress is developed in the upper soil profile which is compensated by HR during the night time. Results also show that under favorable soil moisture conditions, plants extract water at the maximum rate according to the root distribution pattern and when the moisture stress is developed in a part of root zone, the diminished water uptake rate in the water scarce region is compensated by an enhanced water uptake from wetter zones. An underestimation of RWU in wet soil zone and slight overestimation in dry soil part has been observed during dry days when the RCM is not considered. This shows that the optimal availability of water in some part of root zone and little moisture stress in top soil can significantly reduce evaporation loss without significantly affecting water uptake by plants and, therefore, the plant growth. Thus, the developed HR model can be used for planning better irrigation scheduling and for accurate quantification of groundwater recharge and solute load to underlying groundwater resources.