Global Water Scarcity Assessment Incorporating Green Water in Crop Production

Over the past decades, water scarcity has become prevalent in many regions of the world. Previous water scarcity assessments largely considered blue water (surface and groundwater) and overlooked green water (soil moisture), despite its key role in supporting crop production. Moreover, the long-term evolution of water scarcity remains unclear. Here, we propose an agricultural water scarcity index (WSIAW) that explicitly combines green and blue water. The WSIAW measures the degree to which both green and blue water availabilities can satisfy the agricultural water requirement. The WSIAW can directly indicate water scarcity in both rainfed and irrigated areas, which cannot be captured by the blue water scarcity index alone. Using the WSIAW, we assessed the evolution of water scarcity across regions of the world during 1971-2010 at a spatial resolution of 0.5 arc degree. We found that 22% of the 228 river basins with a WSIAW of >1 presented a much broader geographical coverage compared with estimates of water scarcity based on blue water only (8% of total basins). The WSIAW also exhibited a clear trend of increasing severity over the study period, particularly in major agricultural regions. Expanding cropland and increasing/competing water withdrawals by other sectors were the main reasons for the intensification of water scarcity, which was pronounced in some Asian and African river basins. The proposed WSIAW can be applied to different regions with various scales and is important for addressing global water scarcity issues.

Automated summary

Water scarcity has become a prevalent issue in many regions over the past decades. Previous assessments largely overlooked the role of green water in supporting crop production. This study proposes an agricultural water scarcity index (WSIAW) that combines green and blue water to measure water scarcity. The results show that the WSIAW provides a better understanding of water scarcity compared to blue water assessments alone, and reveals an increasing severity of water scarcity in major agricultural regions, mainly due to expanding cropland and competing water withdrawals. The proposed WSIAW is important for addressing global water scarcity issues.