Adapting the thermal-based two-source energy balance model to estimate daytime turbulent fluxes in a complex tree-grass ecosystem

The thermal-based Two-Source Energy Balance (TSEB) model has accurately simulated energy fluxes in a wide range of landscapes. However, tree-grass ecosystems (TGE) have notably complex heterogenous vegetation mixtures and 20 dynamic phenological characteristics presenting clear challenges to earth observation and modeling methods. The TSEB model was tested in a TGE ecosystem and an adaptation was proposed to consider spatial and temporal complexity. This was based on sensitivity analyses (SA) conducted on both primary remote sensing inputs (local SA) and model parameters (global SA). The model was subsequently modified taking into account phenological dynamics and assuming a dominant vegetation structure and cover (i.e. either grassland or broadleaved trees) for different seasons (TSEB-2S). The adaptation 25 was compared against the default (i.e. non-seasonally changing) model and evaluated against eddy covariance (EC) flux measurements and lysimeters over a TGE experimental site in central Spain. TSEB-2S vastly improved over the default TSEB performance decreasing the mean bias and RMSD of LE from 34 and 77 W m -2 to 0 and 59 W m -2 , respectively during 2015. TSEB-2S was further validated for two other EC towers and for different years (2015, 2016 and 2017) obtaining similar error statistics with RMSE of LE ranging between 51 and 63 W m -2 . The results presented here 30 demonstrate the important role that vegetation, through its structure and phenology, has in controlling ecosystem level energy fluxes, which become important considerations for the modeling procedure. Additionally, TSEB was shown to be most sensitive to parameters related to radiation partitioning between canopy and soil, such as characterizing vegetation clumping, and parameters related to vegetation structure involved in quantifying the resistance to turbulent flow. 35