Groundwater Storage as a Key Driver of Subannual Streamflow Variability

Precipitation is the largest flux of the global land hydrological cycle and is widely regarded as the dominant contributor to catchment streamflow. For decades, it has been the primary basis for attributing streamflow variability at annual to decadal timescales. Yet, the relative importance of other hydrological processes in controlling streamflow across different timescales remains unclear. As a measure of the sensitivity of streamflow to changes in its driving variables, streamflow elasticity has been used to quantify streamflow sensitivity to environmental changes. Here, using data from 1628 catchments distributed across the globe, we show that the elasticity of streamflow to precipitation at monthly and seasonal scales is far smaller than the annual elasticity because groundwater storage substantially buffers streamflow variability at subannual timescales, especially in water-limited regions. Therefore, groundwater storage must be considered alongside precipitation and potential evaporation to provide a comprehensive understanding of streamflow responses to climate variability and change. Our results underscore the important need for improving modeling of groundwater storage variability for robust projections of hydrological impacts under a changing climate.