Groundwater recharge in irrigated agriculture: the theory and practice of inverse modelling

Quantifying the rate of natural groundwater recharge is a prerequisite for drainage design in waterlogged areas. The present study was aimed at determining the seasonal net recharge in the Schedule I-B area of the Fourth Drainage Project, Faisalabad, Pakistan. A numerical groundwater model was developed for the said area and was run in inverse mode to determine the seasonal net recharge. Net recharge values were determined for the period June 1985–June 1990, resulting in five monsoon and five non-monsoon estimates. The data used by the model were the geometry of the aquifer system, the aquifer parameters, and historical water table elevations. The methodology is unique in that it gives information on temporal and areal recharge variations, and requires fewer data than the traditional water-balance method. The maximum average net recharge to the aquifer was calculated as 0.6 mm day−1 during monsoon 1986; the corresponding monsoon rainfall had a return period of some 2.8 years.