Using Natural Experiments and Counterfactuals for Causal Assessment: River Salinity and the Ganges Water Agreement

The effect of environmental policy on water resources is often challenging to evaluate due to dynamic interactions between people and water, particularly in data‐scarce watersheds. Increasing interactions between society and hydrology present a need to understand causal relations for improved assessment and prediction in complex human‐water systems. Conventional approaches to causal assessment in hydrology are sometimes insufficient due to data scarcity or system complexity. We argue that natural experiments present a promising and complementary avenue for assessing causal relations in such systems. In this spirit, we describe a natural experiment to assess causal effects of the Ganges water treaty, between India and Bangladesh, on streamflow and channel salinity in the Ganges delta in Bangladesh. We apply causal inference to assess the effect of the treaty on streamflow and use the results to generate synthetic ensembles of streamflow and salinity under a realistic scenario (with the treaty) and a counterfactual scenario (without the treaty). We then use synthetic streamflow ensembles to model salinity ensembles. The Ganges water treaty increased dry‐season streamflow in Bangladesh by approximately 18% and decreased channel salinity by approximately 10%. The treaty has the greatest effect on salinity in Bangladesh in the driest years, but the overall effect is small compared with natural variability. We show that our approach accounts for natural hydrologic variability to accurately assess the causal effect of the treaty, compared with a naive approach that greatly overestimates the effect. This research demonstrates the value of natural experiments for causal assessment in coupled human‐water systems.