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

Abstract

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.

Life at the borderline: Responses of Ganges river dolphins to dry‐season flow regulation of river and canal habitats by the Farakka barrage

“…The India–Bangladesh treaty implementation has reportedly increased average dry season water flows towards Bangladesh by releases of more water in the Ganga during the 10 day allocation periods (Penny et al, 2020). Estimated water availability at Farakka in the study period was found to be consistently greater than the average value of 3,000 m 3 s −1 (Thomas, 2017; Rahman et al, 2019), which was also backed up by local reports, and probably related to the anomalous and intensive flooding in 2019 (Bhatt et al, 2020).…”

“…Similarly high flows also occurred in 2014, and even at that time no 10 day cycle in water sharing was detected (Rahman et al, 2019). Water sharing and regulation were expected to intensify during the period with the lowest flow (11 March to 31 May), when both countries are guaranteed to receive a minimum quantum of water in alternating 10 day periods (Government of India & Government of Bangladesh, 1996; Penny et al, 2020). However, hydrological observations could not be made after 23 March owing to the nationwide lockdown imposed by the COVID‐19 pandemic.…”

“…Constructed in 1975, it diverts water from the Ganga into an approximately 36 km long feeder canal to maintain navigable water flow in the Hooghly distributary and is also operated for irrigation and flood control (Gain & Giupponi, 2014; Kawser & Samad, 2016). According to the India–Bangladesh water treaty (1996), the barrage is operated to share water between India and Bangladesh in the dry season (January to May) in alternating 10 day periods, through the Ganga downstream and the feeder canal (Government of India & Government of Bangladesh, 1996; Penny et al, 2020; see Appendix A of the Supporting Information).…”

Interplay of changing irrigation technologies and water reuse: example from the upper Snake River basin, Idaho, USA

“…Defining the efficiency of agricultural water use is complicated because water lost nonproductively by one water user may be used productively elsewhere, downstream in the basin, making terms describing efficiency or resource sufficiency specific to the spatial scale considered. We describe irrigation efficiency using hydrologic fractions that describe the fate of the water abstracted from either surface or groundwater sources for the purpose of irrigation (Frederiksen and Allen, 2011;Haie and Keller, 2008;Lankford, 2012;Perry, 2011). Water abstracted as gross irrigation (G) can have three fates when added to irrigated pixels at the plot scale, namely (i) beneficial use (B), which is the irrigation water used for beneficial crop growth, (ii) nonbeneficial consumption (N), which is water evaporated nonbeneficially from soil or canals, or (iii) nonconsumptive loss (L, herein incidental returns or incidental recharge), which is runoff or percolation as a liquid that remains in the basin (Fig.…”

Climatic and anthropogenic drivers of a drying Himalayan river

“…In arid and semi-arid regions, these concerns are particularly alarming, given the concurrent challenges of increasing hydrological uncertainty and competition for scarce water resources (Flörke et al, 2018;Aeschbach-Hertig and Gleeson, 2012). In many such regions, mitigation and adaptation strategies are urgently needed but require accurate understanding of the underlying drivers of change (Thompson et al, 2013;Penny et al, 2020a). In order to address the management challenges of mitigation and adaptation, observed changes in hydrological processes must be correctly attributed to the corresponding drivers (i.e., the processes that cause changes in hydrology).…”