Irrigation scheduling resulting in minimum percolation loss was optimized for a wheat crop under deep water table (15 m) conditions through the simulation model SWASALT (Soil, Water, Atmosphere, SALT), using a field experimental study of an area underlain by poor-quality groundwater. The line source sprinkler irrigation technique was adopted to create different irrigation regimes at different critical crop growth stages. The model was calibrated and validated by comparing observed and simulated soil moisture content at different depths (0-50 cm and 50-100 cm), and observed and predicted crop yields. The concept of water management response indicators (WMRIs), i.e. relative transpiration (T-r = E-t/E-tp), transpiration efficiency (T-eff = E-t/(Irr + P)), evapotranspiration efficiency (ETeff = ET/(Irr + P)), moisture storage change (MSC = (W-harvest - W-sowing)/W-sowing) and percolation index (P-i = Perc/(Irr + P)), was used to evaluate on-farm irrigation performance under different irrigation scenarios by carrying out simulations for two soil textures, i.e. sandy soil underlain by loamy sand (Type I) and sandy clay loam underlain by sandy loam (Type II). The WMRIs were based on water balance components for each simulation study (after calibration and validation of the model). The intersection of T,and T,, with minimum percolation loss was the criterion applied for selecting the optimum irrigation schedule. On this basis, the optimum irrigation schedules for soil Types I and II were found to be pre-sowing irrigation depth of 4 cm, ten subsequent post-sowing irrigations of 3 cm each with an interval of 11 days, and pre-sowing irrigation of 8 cm and four subsequent post-sowing irrigations (critical crop growth stages) of 6.5 cm each, respectively, and resulted in no percolation loss. For these schedules, the relative transpiration for soil Types I and II were 91% and 79%, respectively, and the corresponding soil water storage (deficit) values were 26.9% and 13% cent, respectively, with irrigation supply of 34 cm. The simulation study further indicated that the percolation loss increased with greater irrigation application depths and resulted in inefficient use of soil moisture storage. Thus the on-farm water management strategy for areas underlain by brackish groundwater should be light but frequent irrigations for light textured soil as compared with medium textured soil to maintain optimum crop yield and hydrodynamic equilibrium. (C) 1997 Elsevier Science B.V.
