Zahirul Haque Khan, Md Saiful Islam, Shume Akhter, MD RAQUBUL HASIB , Asish Sutradhar, Dr. Jagadish Timsina, Urs Christoph Schulthess , Timothy Krupnik 

In Bangladesh’s south-central coastal zone, there is considerable potential to intensify crop production by growing dry winter season ‘Boro’ rice, maize, wheat, pulses and oilseeds using irrigation from southward flowing and predominantly freshwater rivers. However, the impacts of surface water withdrawal for irrigation and its safe operating space remain unclear. We used field measurements and simulation modeling to investigate the effects of irrigation water withdrawal for Boro rice – the most water-consumptive crop that can be grown by farmers – on river water flow and salinity under different climate change and river flow scenarios. Under the baseline conditions of 2015, about 250,000 ha could potentially be irrigated with river water that has salinity levels below 2 dS/m. The impact on river water salinity would be minimal, and only between 0.71 to 1.12% of the cropland would shift from the 0-2 dS/m class to higher salinity levels. Similarly, a minor change in water flow and salinity was simulated for the moderate climate change scenario (RCP 4.5) that forecasts a sea level rise of 22 cm in 2050. Only under the extreme climate change scenario (RCP 8.5), resulting in a sea level rise of 43 cm by 2050, and low flow conditions that are exceeded in 90 percent of the cases, the 2 dS/m isohaline would move landward by 64 to 105 km in March and April for the Tentulia and Buriswar Rivers. This would expose an additional 36.6% of potentially irrigable cropland to salinity levels of 2 to 4 dS/m.  However, Boro rice is already well established by that time and can tolerate greater levels. We conclude that there is considerable scope to expand irrigated crop production without negatively exposing the cropland and rivers to detrimental salinization levels while preserving the ecosystem services of the rivers.