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Estimates of the global extent of irrigation-induced soil salinity vary, but there is widespread agreement that the twin menaces of waterlogging and salinisation represent serious threats to the sustainability of irrigated agriculture in many arid and semi-arid regions. In certain circumstances, the conventional drainage solution may be questionable due to economic and/or environmental limitations and �dry drainage� has been postulated as an alternative. It involves the allocation of areas of fallow land, which operate as evaporative sinks drawing a stable flux of water and salt from irrigated areas. An evaluation of the merit of this approach requires answers to three key questions: (i) What is the limiting crop intensity? (ii) What is the limiting watertable depth? (iii) What is the long-term impact of salt accumulation in the drainage sink area? A simulation model was developed to investigate these questions for a dry-drainage system with a wheat�cotton cropping pattern using published data for the Lower Indus Basin in Pakistan, where shallow saline watertables, intensive irrigation, high evaporative demand and natural dry drainage exist. The simulation results showed that dry drainage could satisfy the necessary water and salt balance when the cropped area and sink area were approximately equal and watertable depth was around 1.5 m. The long-term impact of salt accumulation on the performance of the system was also considered.
