Modeling solute transport in sub-surface drained soil-aquifer system of irrigated lands

Abstract

A two-dimensional finite element model of solute transport in a tile — drained soil — aquifer system has been applied to study the effects of the depth of impervious layer and quality of irrigation water on salt distribution during drainage of an initially highly saline soil. The model assumes steady state water movement through partially saturated soil and to drains in the saturated zone. The exact in time numerical solution yields explicit expressions for concentration field at any future time without having to compute concentrations at intermediate times. The model facilitates predictions of long-term effects of different irrigation and drainage practices on concentration of drainage effluent and salt distribution in the soil and groundwater. The model results indicated that the depth of impervious layer from drain level, dI, does not significantly influence the salt distribution in the surface 1 m root zone of different drain spacings (drain spacing (2S)=25, 50, 75 m; drain depth (dd)=1.8 m), its effect in the aquifer becomes dominant as drain spacing increases. It was also observed that dI significantly governs the quality of drainage effluent. The salinity of drainage water increases with increasing dI in all drain spacings and this effect magnifies with time. The model was also applied to study the effects of salinity of irrigation water in four drain spacing-drain depth combinations: (2S=48 m, dd=1.0 m; 2S=67 m, dd=1.5 m; 2S=77 m, dd=2.0 m; 2S=85 m, dd=2.5 m). The results indicated that a favorable salt balance can be maintained in the root zone even while irrigating with water up to 5 dS/m salinity in drains installed at 48 to 67 m spacing and 1.0 to 1.5 m depth. Further, irrespective of the quality of irrigation water, the deep, widely spaced drains (dd=2.5 m, 2S=85 m) produced much saline drainage effluent during the initial few years of operation of the drainage system than the more shallow, closely spaced drains, thus posing a more serious effluent disposal problem.