Energy balance closures in diverse ecosystems of an endorheic river basin

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The surface energy imbalance or non-closure problem, where the sum of turbulent heat fluxes measured by the eddy covariance method is systematically lower than the available energy, is one of the greatest challenges in micrometeorology. We examine the energy balance closure ratio (EBR) and its relationship with related variables over a wide variety of ecosystems including alpine meadow, desert, shrub, and crops in the Heihe River Basin (HRB), China, which is representative of endorheic river basins. The results show that half-hour EBR values over different ecosystems range from 0.53 to 0.99, with a mean of 0.84. In different HRB ecosystems, the trends of EBR values with friction velocity (u*), thermally induced turbulence (TT), atmospheric stability (z/L), correlation coefficients for vertical velocity and water vapor (Rwq) and potential temperature (Rwθ), and relative vertical turbulent intensity (RI) are consistent with previous results obtained from other ecosystems. A significant negative linear relationship exists between the EBR and variation values of surface temperatures at landscape and local scales. Additionally, the field data confirm our previous findings from large eddy simulations that there is a positive linear relationship between the turbulent kinetic energy (TKE) and EBR values. Because the TKE diurnal variations explain a larger fraction (71%) of EBR diurnal variations than other variables (u*, TT, z/L, RI, Rwθ and Rwq), we argue that TKE is mainly responsible for EBR diurnal variations and explains the different EBR diurnal variations observed here and in previous studies. The possible reason for this better performance of TKE than other variables is that the TKE includes comprehensive effects of shear, buoyancy, turbulent transport and dissipation from the TKE budget equation. Thus, TKE may be a promising indicator for potential energy balance closure correction. Our findings contribute to a better understanding of the surface energy imbalance problem.