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In support of climate change mitigation strategies and food security, there is an ever-increasing requirement to understand the key drivers of cropland yields and carbon (C) budgets. Process-based crop models, driven by meteorological observations, can provide realistic simulations and diagnosis of the variability in crop C budgets. However, the field-scale observations required to validate the models, particularly across multiple growing seasons and crop types, are seldom available.

This research evaluates the Soil-Plant-Atmosphere Crop (SPA-Crop) model for estimating the C fluxes of multiple crop types and seasons at the Swiss long-term flux site Oensingen (CH-OE2). SPA-Crop is a detailed parametric model, simulating ecosystem photosynthesis and water balance at fine temporal (half-hourly time steps) and vertical scales (multiple canopy and soil layers). We initialised the model at the point-scale using management information and meteorological observations recorded during ten crop seasons from 2004 to 2015. SPA-Crop has been developed for simulating winter cereals (wheat and barley), we further evaluated the model performance for these crops at CH-OE2 and extended the simulation to include a crop-specific calibration for winter rapeseed and pea crops. Model estimates of net ecosystem exchange (NEE) of carbon dioxide (CO2), ecosystem respiration (R-eco), and gross primary production (GPP) were evaluated based on gap-filled and partitioned eddy covariance (EC) fluxes. Further comparisons were made between modelled and observed yields.

The daily SPA-Crop estimates had a high agreement with the EC measurements across the different crop types with a mean R-2 of the NEE of 0.74 (wheat), 0.73 (barley), 0.71 (rapeseed) and 0.69 (pea). The modelled yield estimates had biases ranging from -22% (barley, measured-model difference -70 gC m(-2) ;0.31 t ha(-1)) to +56% (rapeseed, measured-model difference -85 gC m(-2);-0.38 t ha(-1)), with the largest bias of wheat being only -13% (measured-model difference 34 gC m(-2); 0.15 t ha(-1)) across four wheat seasons. We recommend future SPA-Crop developments-particularly with regards to improving the precision of Reeo estimates. Our research demonstrates that SPA-Crop is a reliable tool for simulating the C budget and yield across the four crop types: winter wheat, winter barley, winter rapeseed, and peas.