Bangladesh has always been vulnerable to flooding; monsoons flood about 80% of the country annually[1], destroying residences and infrastructure alike, and due to climate change, destruction by flooding is only expected to worsen. Average freshwater floods are expected to increase by 3-9% and up to 30% in mega-floods in some areas[2]. One indigenous technique used to mitigate damage due to flooding as well as provide areas for agricultural production is the construction of man-made ponds. Small ponds, called dighis or khals in the local language, have long since been an important abiotic, or nonliving, aspect of the Bangladesh ecosystem[1]. Not only do these ponds maintain ecological diversity through habitats for freshwater organisms, but they also provide many beneficial effects in terms of mitigating the effects of climate change. Traditionally, before building a residence structure, Bangladeshis manually dig a hole the size of a small pond nearby, using the excavated earth to raise the land on which their house will sit.
Figure 1: A farmer in northwestern Bangladesh points to one of the newly dug ponds that are helping to boost food production
Source: Naimul Haq/IPS[3]
During floods, the ponds fill, and the combination of this extra water catchment area as well as the raised land for housing dramatically reduces the relative strength and damage of the flood to buildings[1]. Agriculturally, these man-made ponds are very important because they could also provide fresh water for irrigation, fishing, and alternative water-based agriculture.
In the past, these ponds were frequently found in rural Bangladesh and were effective in mitigating flooding damage. However, in recent years, the total number and size of small ponds has decreased dramatically. Between 1981 and 2013, total net area of bodies of water in Bangladesh decreased by 36%[4]. This decrease is due to sediment naturally settling in ponds after being carried from other areas by floods, but also from increased land development by humans. Smaller and fewer freshwater habitats results a lack of ecological diversity, decrease in fish for food production, lack of freshwater and decrease in water quality, and more serious damage from flooding. Natural and man-made ponds have myriad benefits in adapting to climate change, and this solution is enhanced by its indigenous origins, meaning it can easily be integrated into communities.
However, a recent study done by MIT researchers determined that standing water, like the non-flowing water in these ponds, actually catalyses arsenic pollution of the water and soil[5]. When soil is excavated to dig these ponds, organic compounds from the soils are freed from the stable soil structure, so when the ponds flood, these compounds are free to sink to the bottom of the pond. There, microbes metabolize the compounds, effectively creating a chemical reaction that releases arsenic from the soil, which then sinks into the groundwater below[5]. Bangladesh already has problems with arsenic pollution in groundwater sources, and this pollution can be sourced back to ponds in the past. However, according to one researcher, most arsenic dates back to ponds hundreds of years ago, ponds that have since naturally dried up or shifted locations[5]. Because it takes many decades for arsenic to seep into the groundwater from these ponds, he suggests that the benefits in water catchment and the material they provide for elevated land are worth the risk of arsenic pollution in the distant future, especially as by then, much of Bangladesh will likely be underwater and/or have better water treatment systems in place.
From this data, we propose that farmers and rural residents continue the ancient practice of digging small ponds on their land and using the material to raise the elevation of their homes. Although this may take up a small amount of land from agriculture or other development, the benefits they provide in mitigating flood damage are more important in the long term. Educating locals about the benefits, as well as the eventual risks, of these ponds should be a priority task for NGO’s or government agencies working on educating the Bangladeshi people on climate change. Further, when developing land for industry or infrastructure, companies should take into account the current locations of small bodies of water and should generally avoid development in areas that would lead to destruction of these ponds. Satellite imagery, as well as data from GIS, should be used to determine where these small ponds are located, and as a result, where excessive development should be avoided. The cost of this solution is low as constructing these ponds can be achieved with manual labor and minimal supplies. Overall, rebuilding the system of small bodies of water is a good short-term solution to reduce the destructive impacts of severe flooding, and should be prioritized by Bangladesh.
By Sarah Weidman
References
- Dewan, T. H. (2015). Societal impacts and vulnerability to floods in Bangladesh and Nepal. Weather and Climate Extremes 7, 36-42. Retrieved October 4, 2017.
- Bangladesh Delta Plan 2100 (pp. 1-714, Rep.). (2017). Bangladesh: General Economics Division, Bangladesh Planning Commission.
- Ummeh, Saika. (2016). Detecting Temporal Changes of Small Water Bodies Using GIS Techniques: Northern part of Bangladesh as a Case Study. Int. Journal of Applied Sciences and Engineering Research 5, no. 6, 458-464.
- Haq, Naimul. (2013). Small Ponds Bring Bumper Harvests. Inter Press Service. Retrieved from http://www.ipsnews
- Neumann, Rebecca, Khandaker N. Ashfaque, A. B. M. Badruzzaman, M. Ashraf Ali, Julie K. Shoemaker and Charles F. Harvey. (2010). Anthropogenic influences on groundwater arsenic concentrations in Bangladesh. Nature Geoscience 3, 46-52.