sustainable conjunctive use of surface and ground water: modelling on the basin scale

farmers in the indus basin, pakistan have generally switched to groundwater for additional water supplies due to the irregular supply of irrigation water; currently over 50% of the agricultural land in the basin is at least partially irrigated by tube-wells. these wells pump fresh groundwater, which essentially is the result of massive leakage from irrigation canals into the originally saltwater aquifer since the inception of modern irrigation around 1870. resalinization of the aquifer now threatens long-term prospects of this new groundwater resource. since building new dams has become ever more complicated, water resources planning now focuses on sustainable conjunctive use of surface and ground waters. the paper evaluates the raising of the mangla dam, its effects on long-term groundwater balance and water-logging using an irrigation-economic model. it suggests guidelines to optimize the surface and sub-surface reservoirs by considering the farmers' action in response to government policies. recently the government of pakistan decided to raise the height of the mangla dam to substantially increase the storage capacity of the basin. this decision was based on basin-wide modelling of conjunctive use by using the general algebraic modelling system (gams)-based indus basin model revised (ibmr), which was updated for this purpose in 2000 and supplied with new data in 2002. the results of the analysis reinforced the decision to raise the dam height by 9 m instead of 12 m, which would increase water availability by 68% in the basin. one of the objectives of raising the dam height was to increase the sustainability of beneficial groundwater use in the basin by saving about 2 km3/a of groundwater abstractions.