Potential yield simulated by global gridded crop models: using a process-based emulator to explain their differences

How global gridded crop models (GGCMs) differ in their simulation of potential yield and reasons for those differences have never been assessed. The GGCM Intercomparison (GGCMI) offers a good framework for this assessment. Here, we built an emulator (called SMM for simple mechanistic model) of GGCMs based on generic and simplified formalism. The SMM equations describe crop phonology by a sum of growing degree days, canopy radiation absorption by the Beer-Lambert law, and its conversion into above ground biomass by a radiation use efficiency (RUE). We fitted the parameters of this emulator against gridded above ground maize biomass at the end of the growing season simulated by eight different GGCMs in a given year (2000). Our assumption is that the simple set of equations of SMM, after calibration, could reproduce the response of most GGCMs so that differences between GGCMs can be attributed to the parameters related to processes captured by the emulator. Despite huge differences between GGCMs, we show that if we fit both a parameter describing the thermal requirement for leaf emergence by adjusting its value to each grid-point in space, as done by GGCM modellers following the GGCMI protocol, and a GGCM-dependent globally uniform RUE, then the simple set of equations of the SMM emulator is sufficient to reproduce the spatial distribution of the original above ground biomass simulated by most GGCMs. The grain filling is simulated in SMM by considering a fixedin-time fraction of net primary productivity allocated to the grains (frac) once a threshold in leaves number (n(thresh)) is reached. Once calibrated, these two parameters allow for the capture of the relationship between potential yield and final above ground biomass of each GGCM. It is particularly important as the divergence among GGCMs is larger for yield than for above ground biomass. Thus, we showed that the divergence between GGCMs can be summarized by the differences in a few parameters. Our simple but mechanistic model could also be an interesting tool to test new developments in order to improve the simulation of potential yield at the global scale.