A water and salt balance model for the polders and islands in the Ganges delta

Cropping in the polders and islands of the Ganges delta is difficult, particularly in the dry season when water is limited and salinity can damage plant growth. Cropping is generally restricted to the wet season. The limited cropping contributes to the high incidence of poverty and limited livelihood prospects in the region. Increasing crop production is desirable, and will depend upon management of the water and salt in a polder. We have developed a novel time-series model of water and salt stores and flows in an idealised polder to investigate water and salt management strategies for crop production, including strategies to cope with climate change and sea level rise. We know of no other model that simulates the overall water and salt balance of a polder, and that can assess a wide range of polder water and salt management strategies. The model adequately reproduces the dynamic water and salt behaviour observed in field data, and also agrees with results produced by a more detailed crop model. A sensitivity analysis of the model shows the results are sensitive to the key model parameters. In general, the results are more sensitive to parameters governing the transfers of salt, and less sensitive to those governing the availability of water. The crop evapotranspiration is particularly sensitive to the model parameters. The main practical consideration resulting from this simplified but robust representation of the polder salt and water balance processes is the importance of management (including drainage) to remove salt. Strategies to improve water availability, while useful, are secondary to the management of salt.