Coupling of Groundwater with Atmosphere: A New Anthropogenic Component in the Global Hydrological Cycle

One of the challenges in climate change studies is understanding the moisture source, oceanic or terrestrial, responsible for the recent increase in global land precipitation evident by ERA-5 reanalysis data.  To explain the moisture source responsible for increase in the land precipitation we have used the d-excess value of precipitation which distinctly inherits the signature of various processes in the hydrological cycle and has been widely used for the validation of general circulation models. We created a global monthly time series of d-excess value (1978-2021) using precipitation isotope data (n= 62,665) from 913 sites. Since 1996, the enigmatic surge in the d-excess value aligns with the rise in the global land precipitation which cannot be explained by the already known moisture source of the global water cycle. Such a huge spike in d-exces value suggests an increase in contribution from the terrestrial moisture, especially from the evaporation of irrigated groundwater, a component which was not considered in the global hydrologic cycle. To feed the growing population, introduction of multiple cropping seasons and a decrease in the frequency of global land precipitation led to an increase in the groundwater-dependent agricultural practice. Previous studies have shown that the extraction of groundwater for irrigation is so huge and significant that it has been held responsible for global sea level rise  and drift of Earth’s rotation axis. In Spite of that, the remotely sensed data and land surface models partition the moisture sources of water cycle in the various components; such as transpiration, open water evaporation,  canopy interception, bare soil evaporation and  snow sublimation, however, never considered the coupling of groundwater with atmosphere. Therefore, to understand the global hydrological cycle, the moisture and energy exchange between groundwater and atmosphere, via evaporation of irrigated water, should be considered. Here, the enigmatic rise in d-excess value, equivalent to glacial-interglacial scale variation, signifies human domination in the global hydrological cycle.