Impact of a large-scale replacement of maize by soybean on water deficit in Europe

The EU imports large quantities of soybeans, mainly for livestock feed. However, there is a trend to increase domestic soybean production and reduce imports. In this study, we investigate the potential impact of an increased EU soybean cultivation on evapotranspiration (ET), water deficit, and irrigation needs. We focus on the consequences of replacing maize with soybeans, as both crops have similar cropping periods and high water demands. We implement a simple, well established crop water model that estimates crop water deficit (ETd) as the difference between simulated potential (ETc) and actual (ETa) ET. We apply this model over the EU from 2001 to 2020, using data on daily reference ET and precipitation, soil hydrological properties and three different crop calendars. Results indicate that a maize-to-soybean conversion would result in an average ETd increase of 49.0 +/- 22.1 mm season(-1) across the EU. In the four countries of France, Italy, Hungary and Romania, where most of the additional soybean production would be allocated, crop water deficits would increase on average by 21-34 % compared to that of maize, following an increased ETc and/or decreased ETa. However, the decrease in ETa is largely due to an assumed shorter root depth for soybean, while recent empirical results suggest that both crops may actually have comparable root depths. Using the same root depth for maize and soybean, the simulated average increase in ETd amounts to only 28.2 +/- 18.3 mm season(-1). Our results are sensitive to the choice of crop calendar, with reduced ETd for later sowing dates.