Shifting water scarcities: Irrigation alleviates agricultural green water deficits while increasing blue water scarcity

Abstract. Agricultural areas often experience green water scarcity – i.e. soil moisture limitation on crop growth – due to e.g. unfavourable soil texture, high potential evapotranspiration rates, poor or inefficient crop management, and fluctuations in meteorological conditions. Driven by the growing effects of climate change and the rising water and food demands of an increasing world population, agricultural green water scarcity is becoming an increasingly important phenomenon. In this global modelling study, a plant-physiology based indicator of green water stress is applied, that quantifies the ratio between soil moisture limitation and atmospheric water demand on agricultural areas. Results show that currently (2015–2019 average) 37 % of the global agricultural area is green water stressed, where this ratio is >0.2. Hotspots are characterized by a high seasonal variability in stress conditions, and are mainly located in India and Pakistan, northern Sub-Saharan Africa, North Africa and southwestern Asia. Using an analogous blue water stress indicator – which relates human water use for households, industry and agriculture to available blue water resources in rivers, reservoirs and aquifers – current irrigation is shown to alleviate green water stress on 13 % of the total agricultural area (207 Mha) but simultaneously increases the share of areas experiencing blue water stress by 12 % (199 Mha). Moreover, on average 585 km³ yr^-1 of water used for irrigation (22 % of the total water use) is found to stem from surface water resources at the expense of rivers' environmental flow requirements. This shift in water stress types highlights the importance of jointly considering the interconnected green and blue water resources and stresses in pathways towards sustainable water use in agriculture.