The Water and Salt Balance of Polders / Islands in the Ganges Delta

Abstract

We present a conceptual understanding of the polder salt and water balance processes with a simple model that encapsulates the understanding. The model simulates reasonably well the dynamics of water and salt measured in field experiments at three sites. A feature of both the field measurements and the model is the increase of salinity in the soil and surface water bodies during dry season, despite rainfall exceeding potential evapotranspiration by 0.5 to 1.3 m annually amongst the experimental sites, which should flush salt out of the canals and ponds fairly and quickly. The observed salt in the canals and ponds presumably results from continual re-supply of salt. Capillary rise from a salty groundwater table could supply some salt that would cycle through the soil and into the surface drainage system to the canals and ponds; this behaviour is shown in the model. To remove salt from the polder, canal drainage must be managed effectively. Drainage is also likely to be important at the crop or field level. Irrigation will involve the addition of salt as well as water to the soil surface; the amount will depend on the salt concentration in the water supply. Without adequate field drainage, some of the added salt is likely to be leached downward to the water table in the wet season, only to be brought back again to the surface by capillary rise in the next dry season; over several years, this would lead to high salt concentrations that could limit crop growth. Field drainage should therefore be used in polder cropping systems.