Groundwater rivals aridity in determining global photosynthesis

Understanding the role of groundwater in regulating photosynthesis is key in land-climate interactions. However, the impact of groundwater on terrestrial ecosystem productivity remains poorly understood. Here, we use satellite observations of solar-induced fluorescence as a proxy for photosynthesis, together with estimates of water table depth (WTD) and aridity as quantified by the annual moisture index with reanalysis data, to investigate the relationships between groundwater and photosynthesis. Using explainable machine learning (Shapley additive explanations or SHAP), we demonstrate that groundwater plays a crucial role in determining the spatial patterns of global photosynthesis, with varying importance across different ecosystem types, and that this effect is comparable to aridity. We show that in forests across the contiguous USA (CONUS), the relative importance of groundwater represents 89% of the effect attributed to the moisture index in modulating photosynthesis, and this impact is even more pronounced globally (105% relative importance). The relative importance of groundwater compared to the moisture index remains substantial in grasslands (37%), savannas and shrublands (25%), and croplands (15%) in CONUS. Global analysis confirms these trends for grasslands (41%) and croplands (15%) but indicates a 60% importance in savannas and shrublands. Our findings highlight the key global role of groundwater in driving ecosystem productivity.