Climate versus demographic controls on water availability across India at 1.5 °C, 2.0 °C and 3.0 °C global warming levels

Water resources across the globe are projected to undergo significant changes as climate and socio-economic conditions change. A policy relevant question thus arises: whether climate change or population change exerts a greater control on future freshwater resources of a region? Understanding the relative importance of these factors in affecting water availability can help guide prioritization of policy level interventions. In this study, we quantify the changes in mean annual per capita water availability (PCWA) across India under 1.5 °C, 2.0 °C, and 3.0 °C levels of global warming. We utilize projections of future climate from several general circulation models (GCMs) under three different representative concentration pathways (RCPs) along with projections of future population from five socio-economic pathways (SSPs). Using the estimated PCWA from these GCM-RCP-SSP combinations, we perform a sensitivity analysis to ascertain the relative importance of climatic (precipitation and temperature change) and demographic (population) factors in affecting per capita freshwater availability in a region. Our analysis shows that PCWA over India will decrease across all warming scenarios. In addition, a transition from the 1.5 °C warmer world to the 2.0 °C warmer world leads to a reduction in PCWA for a majority (92.8%) of regions across India. The number of people likely to face severe water stress (PCWA < 500 m³/year/capita) under 1.5 °C, 2.0 °C, and 3.0 °C warming scenarios, are 354, 421, and 380 million, respectively. Sensitivity analysis indicated that changes in both population and mean annual precipitation are dominating factors controlling PCWA, depending upon the historical setting of the region. Regions with historically lower populations and lower aridity indices tend to be more sensitive to population changes. On the other hand, as historical population of a region increases, sensitivity to changing climate (mainly mean annual precipitation) increases. These results indicate the complex interactions between demographic and climatic changes that need to be accounted for in policies that aim to manage water scarcity by either controlling global warming or via socio-economic interventions.