Variability of winter rye grain yield in a glacial plain catchment — modelling and observation

Crop growth of winter rye (Secale cereale) was simulated on sandy soils in northwest Germany at the meso-scale (6 km² catchment, 5 year rotation). Based on site-specific soil data, simulated grain yields (Y_MOD) were compared to observed yields (Y_OBS), and the results were discussed with respect to the N balance. The mean annual yields simulated for rain-fed (3.8–7.3 t/ha) and irrigated conditions (5.2–8 t/ha) varied more strongly than those observed in the region (4.6–6.0 t/ha). Excluding years with strongly deviating simulation results, the ratio Y_OBS/Y_MOD decreased from 0.82 to 0.73 under irrigation. Spatially, the effect of plant available water (PAW) on simulated yields varied considerably over the years, explaining 5–75% of the yield variability. In reality, yields depended on water availability (PAW+irrigation) in a year with spring drought, suggesting a time-specific water stress function. In the simulation scenario, there was ample N supply, and variable N mineralization explained 10% of the yield variability. Simulated mean N uptake and export were greater than the observed (160 vs. 140 and 112 vs. 100 kg N/ha, respectively), which may explain the overestimated growth rates. The mean harvest index varied less (0.36–0.55) than the fraction of N returned with the residues (0.17–0. 50), and irrigation decreased the variation of both. With respect to scaling methods, soil data could be aggregated, and simulations based on distributed inputs could be substituted by the weighted mean of simulations based on mean inputs. Down-scaling from the EU map created the difficulty of selecting appropriate soil units. The simple factor Y_OBS/Y_MOD was unreliable for scaling simulated yields to assess regional yields because of its temporal variation in response to climatic variables. The results suggest that changes in model structure and parameters are required to describe water stress, sink limitations, and N diffusion or influx rates better.