Effects of management practices on water fluxes in winter wheat across different scales

Agricultural systems play a central role in the terrestrial water cycle, with crop transpiration representing a major component of land-atmosphere water fluxes. Over the past century, plant breeding and increasing nitrogen fertilization have substantially increased crop growth and productivity. However, their combined effects on crop water use remain insufficiently understood, particularly across spatial scales. This thesis investigates how cultivar traits and nitrogen fertilization interact to control crop growth and transpiration, combining field experiments, physiological measurements, and process-based modeling. Field experiments with winter wheat cultivars representing different breeding periods revealed that breeding has significantly altered biomass allocation, canopy development, and phenology. While no increase in biomass production was found, modern cultivars exhibited a stronger allocation of biomass to grains and faster canopy development, whereas historic cultivars showed greater investment in vegetative biomass and higher maximum leaf area index (LAI). These structural differences had important implications for crop water use. Measurements of transpiration using sap flow sensors demonstrated differences between cultivars, which were strongly influenced by nitrogen availability. Nitrogen fertilization was identified as a key driver of transpiration, primarily through its effect on biomass accumulation and canopy development. However, cultivar-specific responses resulted in significant genotype × nitrogen fertilization interactions. Sap flow measurements at the tiller level revealed that differences in transpiration cannot be explained by canopy structure alone, as physiological processes became more important under limiting conditions. Using a process-based crop model, these interactions were further analyzed at the continental scale. The simulations showed that cultivar differences led to substantial variation in cumulative transpiration across Europe, with average differences of approximately 17% even without considering nitrogen fertilization. In regions with high evaporative demand, these differences were particularly pronounced. The analysis further indicated that canopy development is the dominant control on transpiration across most environments, while root hydraulic processes become increasingly important under water-limited conditions. Overall, crop transpiration is governed by the interaction between genetic traits, management practices, and environmental conditions. These findings highlight the importance of explicitly considering genotype × management interactions in the assessment of agricultural water use and provide a basis for improving the representation of crop water fluxes in models.