Watershed-Scale Runoff Efficiency Response to Climate Variability

The fraction of precipitation converted to stream discharge within a watershed, termed as runoff efficiency, may shift as climate changes. Runoff efficiency is known to be temperature-sensitive in some watersheds, but temperature sensitivity is unquantified in many other watersheds. We identify regions where runoff efficiency is temperature-sensitive using 942 watersheds, minimally influenced by anthropogenic activity, across the continental United States and Canada. Stepwise regression using historical discharge and climate records shows that runoff efficiency in 10 of 16 hydrologically similar hydro-regions is sensitive to temperature, expanding the number of locations expected to experience temperature-driven water stress, particularly in the North American continental interior. Runoff efficiency in all hydro-regions demonstrates sensitivity to precipitation, but during wet years, runoff efficiency temporarily decreases, likely reflecting increasing groundwater storage. The temporary decrease in runoff efficiency is followed by an increase in the following year, likely due to the release of stored groundwater. This effect suggests changes in runoff efficiency help to stabilise watersheds, making it more difficult to both enter and leave drought as climate changes. The latter effect may partially explain observations of hydrologic drought persistence after meteorological drought ends. Understanding regional temperature sensitivity and the multiple-year effect of precipitation will improve the ability to forecast runoff efficiency.