Deep soil moisture reveals hidden water stress in African rainfed maize systems

Abstract Weather variability strongly influences rainfed maize production across sub-Saharan Africa, affecting food security for hundreds of millions. Disentangling how weather affects yield from observations is difficult, because soil water limitation and high atmospheric demand often co-occur and cannot be independently constrained from surface observations. Here we show that depth-resolved soil moisture enables this separation because coupling between soil moisture and atmospheric demand weakens from the surface (r = -0.54) to the root zone (r = -0.13). Subnational maize yields increase with root-zone soil moisture, whereas surface soil moisture shows no consistent relationship with yield. Including root-zone soil moisture in yield models reduces estimated damage from atmospheric demand by one-fifth, and machine-learning attribution of satellite-derived photosynthesis identifies root-zone soil moisture as the dominant driver across African croplands in all climate zones. These results highlight the importance of soil moisture depth for drought assessment in rainfed systems under rising atmospheric demand.