T&C-CROP: Representing mechanistic crop growth with a terrestrial biosphere model (T&C, v1.5): Model formulation and validation

Abstract. Cropland cultivation is fundamental to food security and plays a crucial role in the global water, energy, and carbon cycles. However, our understanding of how climate change will impact cropland functions is still limited. This knowledge gap is partly due to the simplifications made in Terrestrial Biosphere Models (TBMs), which often overlook essential agricultural management practices such as irrigation and fertilizer application, and simplify critical physiological crop processes.

Here we demonstrate how with minor, parsimonious enhancements to the TBM T&C it is possible to accurately represent a complex cropland system. Our modified model, T&C-CROP, incorporates realistic agricultural management practices, including complex crop rotations, irrigation and fertilization regimes, along with their effects on soil biogeochemical cycling. We successfully validate T&C-CROP across four distinct agricultural sites, encompassing diverse cropping systems such as multi-crop rotations, monoculture, and managed grassland.

A comprehensive validation of T&C-CROP was conducted, encompassing water, energy, and carbon fluxes, Leaf Area Index (LAI), and organ-specific yields. Our model effectively captured the heterogeneity in daily land surface energy balances across crop sites, achieving coefficients of determination of 0.77, 0.48, and 0.87 for observed versus simulated net radiation (Rn), sensible heat flux (H), and latent heat flux (LE), respectively. Seasonal, crop-specific gross primary production (GPP) was simulated with an average absolute bias of less than 10 %. Peak season LAI was accurately represented, with an r² of 0.67. Harvested yields (above-ground biomass, grain, and straw) were generally simulated within 10–20 % accuracy of observed values, although inter-annual variations in crop-specific growth were difficult to capture.