Blue and green water accounting for climate change adaptation in a water scarce river basin

Abstract

Understanding how climate change influences the portioning of rainfall into blue water (BW) and green water (GW) is vital for establishing adaptive water management strategies in water-scarce regions. This study presents a conceptual framework to investigate climate change impacts on blue and green water accounting by integrating a hydrological model with a multi-criteria decision-making approach to prioritize watersheds for climate change adaptation. A Soil and Water Assessment Tool (SWAT) is adopted to investigate the hydrologic variability of BW and GW for baseline (1995–2019) and future (2020–2100) periods in a semi-arid Dharoi catchment of the Sabarmati River basin, India. The future climate data, derived from five general circulation models, are ensembled using a multi-model ensemble mean approach under two RCP (representative concentration pathways) scenarios. Several non-parametric tests are employed to ascertain statistically significant changes in the hydrologic variables between future and baseline periods. Further, Analytic Hierarchy Process (AHP) is applied considering seven attributes related to climate change, land use, and demographic parameters for watershed prioritization. Higher weights are assigned to the climate change attributes, followed by land use and demographic attributes. The SWAT results indicate a decrease in rainfall, BW, and GWS (green water storage) and an increase in temperature and GWF (green water flow) while traversing from the beginning of 2040 till the end of the 21st century under both RCPs. Thus, 2040 is established as a ‘tipping point’ for climate-induced changes in the Dharoi catchment, wherein a distinct reversal in the hydroclimatic regime is evident before and after 2040. Further, significant warming is observed by the end of 21st century under RCP4.5 and RCP8.5, contributing to a reduction in rainfall (up to 24.8%) and a decline in BW (up to 78.8%) across two-thirds of the watersheds in the Dharoi catchment. The AHP analysis identifies three out of ten watersheds as vulnerable and assigned ‘high priority’ for implementing adaptation measures to counter climate change-induced water scarcity.