Climate change and industrialization as the main drivers of Spanish agriculture water stress

In recent decades, there has been growing concern regarding worsening global climate conditions for agricultural production, especially in Mediterranean climate and other semi-arid areas where the scarcity of water is a major problem. The objective of the study was to estimate the long-term evolution (in Spain, over 1922-2016) of the decoupling between crop's water requirements and the green water available, and to analyze the causes of the changes. The FA0-56 methodology was followed to estimate the green water, blue water and water stress of crops, via an explicit territorial distribution, using 0.5^o grid cells. We propose a new indicator called Violet Water. This indicator groups together the water stress of the rainfed croplands and the blue water of the irrigated croplands, representing the components of this decoupling. Our results show that crops´ water requirements increased by 21% and violet water by 30% over the study period. The decomposition analysis used to study the drivers of these changes suggests that increasing reference evapotranspiration, caused by global warming, was the major driver triggering the growth of crop water requirements. Meanwhile, the changes in violet water values were mainly driven by the coupling of the cropland's spatiotemporal structure and climate change. The sensitivity analysis performed showed that the increasing trends in water stress, blue water and violet water were consistent for almost all the tested scenarios, but significant differences on the scale aroused depending on the climate data used (climate normal or each year climate) and specially on the geographical and historical Kc adjustment (crop-fixed Kc versus adjusted Kc). This highlights the importance of considering simultaneously the effect of the different factors, but also the need for refining this type of adjustment in future research.