CYGMA is a global gridded crop model. The model operates at 0.5° resolution in longitude and latitude and has a daily time step. In the model, crop development is modelled as a fraction of the accumulated growing degree days relative to the crop thermal requirements. For wheat, only spring wheat is considered, because the vernalization process is not currently incorporated into the model. Leaf growth and senescence are calculated according to the fraction of the growing season using the prescribed shape of the leaf area index curve. The yields are computed from the photosynthetically active radiation intercepted by the crop canopy, the radiation-use efficiency (RUE), the effects of CO2 fertilization on the RUE and the fraction of total biomass increments allocated to the harvestable component. The soil water balance sub-model, which is coupled with the snow cover sub-model, is used to calculate the actual evapotranspiration. In the model, crop development is modelled as a fraction of the accumulated growing degree days relative to the crop thermal requirements. For wheat, only spring wheat is considered, because the vernalization process is not currently incorporated into the model. Leaf growth and senescence are calculated according to the fraction of the growing season using the prescribed shape of the leaf area index curve. The yields are computed from the photosynthetically active radiation intercepted by the crop canopy, the radiation-use efficiency (RUE), the effects of CO2 fertilization on the RUE and the fraction of total biomass increments allocated to the harvestable component. The soil water balance sub-model, which is coupled with the snow cover sub-model, is used to calculate the actual evapotranspiration. Five different stress types, i.e., nitrogen (N) deficits, heat, cold, water deficits and water excesses are considered, and the most dominant stress type for a day decreases the daily potential increment in the leaf area for the vegetative growth period and in yield for the reproductive growth period. The growth and yield of soybeans in the model are less sensitive to N deficit stress than are theother crops considered here because the soybean is a legume that fix nitrogen. All of the stress types except N deficits are functions of daily weather, and the tolerance of each crop to these stresses increases as the knowledge stock increases. The knowledge stock is an economic indicator that is calculated as the sum of the public annual agricultural research and development (R&D) expenditures for each country since the year 1961 with a certain obsolescence rate, and it represents the average level of agronomic technology and management among farmers in a country. More details on the modelling are available in Iizumi et al. (2017).
