To assess the drainable surplus of an irrigated area, a methodology based on a groundwater-balance approach was developed and applied in Schedule I-B of the Fourth Drainage Project near Faisalabad in Pakistan. To determine the seasonal net recharge in this area, a numerical groundwater model was run in inverse mode. The data input for the model consisted of the geometry of the aquifer system, the aquifer parameters, and historical watertable elevations. The seasonal net recharge values, calculated from the individual recharge and discharge components, were "tuned" with the results of the inverse modelling. The advantage of such an integrated approach is that all these components are linked. The design net recharge was estimated from the historical net recharge of the wettest monsoon in the study period. Its rainfall recharge values were then substituted for those of a design monsoon. In this substitution procedure, the rainfall recharge methodology and parameters were adopted from the tuning procedure. From this design net recharge, estimates could be made of the required drainable surplus, with and without drainage simulation.
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