Groundwater regulates the spatial pattern of photosynthesis in the contiguous United States

Abstract

Abstract Plants can access water stored in aquifers to sustain their activity, releasing moisture into the atmosphere—a critical survival mechanism during drought. Understanding the role of groundwater in regulating photosynthesis is thus key for modelling land-atmosphere interactions. However, the contribution of groundwater to spatial variation in mean annual photosynthesis remains poorly quantified, particularly relative to well-known factors like climatic aridity. Here, we use satellite observations of solar-induced fluorescence together with model estimates of water table depth and aridity to investigate the relative role of groundwater and aridity in determining the spatial pattern of photosynthesis in the contiguous United States. Using causality-guided explainable machine learning, we show that the relative importance of groundwater accounts for 48 to 101% of the effect attributed to aridity in modulating forest photosynthesis across the contiguous United States. The relative importance of groundwater compared to aridity remains substantial in savannahs and shrublands (30-58%), grasslands (22-42%), and croplands (15-32%). Our findings reflect the role of water tables in regulating the water available in the rooting zone, with an effect comparable in magnitude to climatic aridity.