Systematic underestimation of type-specific ecosystem process variability in the Community Land Model v5 over Europe

Abstract. Evapotranspiration (ET) and gross primary production (GPP) are critical fluxes contributing to the energy, water, and carbon exchanges between the atmosphere and the land surface. Land surface models such as the Community Land Model v5 (CLM5) quantify these fluxes, contribute to a better understanding of climate change's impact on ecosystems, and estimate the state of carbon budgets and water resources. Past studies have shown the ability of CLM5 to model ET and GPP magnitudes well but emphasized systematic underestimations and lower variability than in the observations.

Here, we evaluate the simulated ET and GPP from CLM5 at the grid scale (CLM5_grid) and the plant functional type (PFT) scale (CLM5_PFT) with observations from eddy covariance stations from the Integrated Carbon Observation System (ICOS) over Europe. For most PFTs, CLM5_grid and CLM5_PFT compared better to ICOS than publicly available reanalysis data and estimates obtained from remote sensing. CLM5_PFT exhibited a low systematic error in simulating the ET of the ICOS measurements (average bias of -5.05 %), implying that the PFT-specific ET matches the magnitude of the observations closely. However, CLM5_PFT severely underestimates GPP, especially in deciduous forests (bias of -43.76 %). Furthermore, the simulated ET and GPP distribution moments across PFTs in CLM5_grid and CLM5_PFT, reanalyses, and remote sensing data indicate an underestimated spatiotemporal variability compared to the observations across Europe. These results are essential insights for further evaluations in CLM5 by pointing to the limitations of CLM5 in simulating the spatiotemporal variability of ET and GPP across PFTs.