Cropland expansion is threatening terrestrial water storage in dryland watersheds

Water is crucial for the long-term development in dryland watersheds, but unregulated cropland expansion exacerbates losses of terrestrial water storage (TWS). The current understanding of the quantitative impacts of cropland expansion on TWS remains limited. Here, taking the Bogda watershed in the dryland of Northwest China as a case, we examined changes in cropland area, cropland gross primary productivity (GPP), evapotranspiration (ET), and net TWS consumption from 2002 to 2022. Furthermore, we identified the key impact factors of net TWS consumption and predicted its future values for three distinct land development scenarios under climate change. We found that the Bogda watershed experienced a cropland expansion of 522 km2, accompanied by increases in total cropland GPP (0.38 million t C), ET (0.87 billion m3), and net TWS consumption (0.86 billion m3). Since 2004, TWS in the watershed has been unsustainable. Further analysis indicated that cropland was the main driver of net TWS consumption, with cropland expansion indirectly increasing net TWS consumption by enhancing ET. Significant positive correlations were found between cropland area and both ET and net TWS consumption. Each additional square meter of cropland increased ET by 1.23 m3 and net TWS consumption by 0.59 m3 on average. Future predictions suggest that TWS may become sustainable if cropland is reduced to 912 km2, whereas continued cropland expansion will maintain TWS unsustainability. This study reveals a significant conflict between cropland expansion and water conservation in drylands, and provides critical insights for developing agricultural and water management strategies to support long-term water sustainability.