| In this study, several important topics related to water flow and solute transport in soils under different irrigation conditions were studied using numerical modeling. Water flow and solute transport in irrigated soils are greatly dependent on different irrigation practices. Water flow and solute transport in soils were investigated under alternate-furrow irrigation, fixed-furrow irrigation, and the conventional every-furrow irrigation. To account for water and salt stresses to plants, a root-water uptake model was integrated into the water and solute movement model, modified CHAIN_IR, to simulate root-water uptake at various irrigation water use and salinity levels. Parameters used in the root-water uptake model were evaluated. Long-term irrigation with saline water predicted root-water uptake to be substantially reduced, even at very low irrigation application levels attributable to salt accumulation. To study the effects of spatial variability of saturated hydraulic conductivity ( Ks) on water and solute movement, two-dimensional random fields of Log Ks were generated with five different standard deviations (0.5, 0.75, 1.0, 1.25, and 1.5). Spatial variability of Ks was shown to have similar effects on mean values of water and solute leaching fractions for flood, drip, and sprinkler irrigation methods. Water movement using four root-water uptake models with different root density functions was evaluated in a loamy sand soil under furrow irrigation. Two main distributions of root systems, in cylindrical and conical shapes, were examined at two irrigation levels. The effects of random fields of Ks on these root-water uptake models were also discussed. The simulation results suggested that the shape of root systems had a greater effect on root-water uptake than the root distribution function. Numerical solutions of water flow in variably saturated porous media resulted in poor mass balance due to the inherent nonlinear characteristics of the governing equation. To improve mass balance under irrigation conditions with large outflow water flux, a hybrid method that utilized either the h-based or mixed formulations based on the local soil water condition was proposed. The h-based formulation should be adopted if the absolute change of pressure heads between time steps is smaller than 3 cm, otherwise, the mixed formulation should be applied. The hybrid method resulted in better mass balance than the mixed formation with the same tolerance levels for iterations within a time step. To achieve similar mass balance errors, the mixed formation required much more computation time than the hybrid method. |
