Examining the role of groundwater in the spatio-temporal variation of Ecosystem Water Use Efficiency in India

Ecosystem Water Use Efficiency (WUE_e) is an ecohydrological indicator that shows the strength of coupling between the carbon cycle and water cycle, and it is calculated as the net carbon gain (estimated as Net Primary Productivity, NPP) per unit of water consumed by ecosystems (estimated as Evapotranspiration, ET). The carbon and water cycles, as well as the WUE_e is influenced by groundwater (GW) as it is one of the sources for supplying moisture to the terrestrial ecosystem. Various factors like meteorological factors (precipitation, temperature, aridity), vegetation types, irrigation, etc. affect the interaction between GW and WUE_e. Thus, the influence of GW on the dynamics of WUE_e varies spatially and temporally. In this study, remote sensing-based datasets of NPP, ET and Potential ET (PET) along with precipitation data from India Meteorological Department (IMD) and the mean annual water table depth (WTD) values taken from the equilibrium WTD model of Eurasia were utilized to analyze the response of WUE_e to GW fluctuations at yearly temporal scale across India. The concept of climatic elasticity of NPP (ε_NPP), ET (ε_ET) and WUE_e (ε_WUE), which is calculated as the ratio of normalised WUE_e (or NPP and ET) to Aridity Index (AI) was used to quantify the effect of climate change on the terrestrial ecosystem’s productivity. Our results showed that in general, WUE_e fluxes, i.e., NPP and ET, were higher in the regions having lower WTD, such as Western Ghats and north-eastern areas. Further, we examined the responses of vegetations to climatic changes at different WTD, and the effect of irrigation on WUE_e and its fluxes were studied. In general, the interactions between WTD and WUE_e (and its fluxes) were stronger in the irrigated croplands. Shrublands in arid regions of India (i.e., north-western states of India) were found to be more sensitive to aridity as compared to the wet and humid regions dominated by forests and croplands type of land cover. This study gives an insight into the interaction between the ecological performance of terrestrial ecosystem and groundwater, which will support reasonable land use and groundwater management over the entire country of India.