Agriculture in the coastal plain of Bangladesh faces a number of serious challenges, among which the issue of seawater intrusion that is exacerbated by the effects of climate change. During the dry season most of the surface water bodies of the vast delta area become saline or brackish, making the water unsuitable for irrigation. Farmers are therefore limited to one or two harvests of rainfed crops (usually rice) per year. This project aims at improving livelihoods and climate-resilient agriculture by making irrigation water available during the dry season, which allows farmers to grow an off-season harvest of high-value crops, such as water melon or bitter gourd.
In 2018, a managed aquifer recharge (MAR) system for the irrigation of crops was implemented in the district of Bagerhat, in the coastal plain south of Khulna, Bangladesh. Constructed as an aquifer storage and recovery (ASR) system, it provides a low-cost solution for storing freshwater in a saline environment to be used for irrigation during the dry season. The abundant monsoon rains provide large amounts of freshwater and applying ASR has proven to be a suitable solution for bridging the water availability-demand gap between rainy season and dry season.
In the project area, rainwater is collected in shallow ponds that can also be filled with river water. The developed AgriMAR scheme features a horizontal drain that is installed parallel to a pond. To reduce the turbidity, water flows from the pond to the horizontal drain through a jute-lined chamber with filter sand. Using a motor pump, a 5 m3 reservoir is filled with water from the drain. The water then infiltrates under gravity in the shallow and deep 4” filters of the well. The infiltration capacity is around 9 m3/h with an overhead pressure of 1.5 to 3.0 m, depending on the water level in the reservoir. The infiltrated freshwater displaces the brackish groundwater and forms a lens at the top of the aquifer. As a considerable amount of infiltrated freshwater is lost to mixing at the boundary of the lens, only the shallow filter is used for recovery.
The simple, low-cost AgriMAR system was constructed with locally available materials and unskilled labor. The annual infiltration volume is estimated to be at least 5,400 m3. Assuming a recovery efficiency of 50% this allows for the irrigation of at least 0.3 ha of high-value crops. A detailed economic assessment showed that there is a positive business case for growing bitter gourd or water melon using the AgriMAR system.
By providing a low-cost source for irrigation water throughout the year the AgriMAR system can greatly improve the climate resilience of farmers in the delta of Bangladesh, one of the most vulnerable regions regarding climate change impacts.