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Water simulation (WaSim), a recent addition to drainage models, was evaluated for its capability to predict ground water table behaviour for non-drained and drained conditions under a semi-arid monsoon climate in Haryana (India). The model is a one-dimensional soil water balance model that is capable of predicting ground water table behaviour, drainage discharge, soil salinity and drainage effluent quality. It is accompanied by WaSimET that calculates daily reference evapotranspiration, which is used as input to the WaSim simulation model. The evaluation of Wasim using pre-drainage test data (Mundlana, Haryana, India) revealed that, when used in conjunction with WaSimET, it has a potential to predict the ground water behaviour under non-drained situations. Also under drained conditions (Sampla, Haryana, India), a close agreement was obtained between the measured data and simulated values of water table depth, drain discharge, soil salinity and drainage effluent quality. Deviations between WaSim simulated and observed values were 0.13 m for water table, 0.48�1.06 mm/day for drain discharge, 6.32 dS/m for soil salinity and 5.45 dS/m for discharge water quality. The large deviations in the case of soil salinity were attributed to long duration of simulation that included few salinization�desalinization cycles. Moreover, since the salt transport simulation in the WaSim does not take into account adsorption�desorption, dissolution and dead end pores, some discrepancies are bound to occur. The calibrated and validated model was used to predict excess water stress utilizing SEW30 index, which is the sum of daily exceedences (cm) of the water table above the depth of 30 cm below soil surface for different spacings under three rainfall situations. SEW30 exceeded 100 cm day for drains spaced at 75 m while it remained within 50 cm day for drains spaced at 50 m and within 0 cm day for drains spaced at 25 m. Clearly, the model has the potential for application in drainage design. Long-term simulations revealed that residual effects of 1 year do not affect the water table during the next year. It appears that WaSim is a simple tool to evaluate the hydraulic performance of the subsurface drainage systems or to design a subsurface drainage system for semi-arid monsoon climates. Since the model is a recent addition to the literature, it would be appropriate that more evaluation studies are undertaken before concrete recommendations are made in regard to its applicability to simulate leaching and drainage effluent quality (salt transport) in drained lands
