Relative Efficiency of Surface Energy Partitioning Over Different Land Covers

Aims: In this paper, we aim to assess different parameterization schemes for quantifying the surface energy portioning process, in particular, the latent and sensible heat fluxes, and their applicability to various surface cover types.
Study Design: This study intercompares theoretical models that predict the relative efficiency of the latent heat (evapotranspiration) with respect to the sensible heat flux. Model predictions are compared with field measurements over surface covers with different physical characteristics and soil water availability.
Place and Duration of Study: This study was carried out at the Arizona State University, Tempe, AZ, between August 2012 and December 2012.
Methodology: Three theoretical models for prediction of the relative efficiency of the latent heat were investigated, based on the lumped heat transfer (Priestley), the linear stability analysis (LSA) and the maximum entropy principle (MEP), respectively. Model predictions were compared against field measurements over three different land cover types, viz. water, grassland and suburban surfaces. An explicit moisture availability parameter β is incorporated in the MEP model, to facilitate direct comparison against the LSA and field measurements. Standard post-processing and quality control were applied to field measured turbulent fluxes using the eddy-covariance (EC) technique. To be consistent with the premise of all theoretical models, diurnal series of sensible and latent heat fluxes were filtered such that only data points under convective conditions were selected.
Results: Among all three models, the application of Priestley model is restricted to saturated land surfaces, and generally overestimates the relative efficiency of the latent heat for water-limited surfaces. The LSA and MEP models predict similar β ranges, i.e., 0.05-0.3 in summer and 0.1-0.7 in winter over suburban area, and 0.1 to 0.5 over lake surface. Over vegetated surfaces, the MEP model predicts a reasonable β range around unity by taking transpiration into consideration, while the LSA model consistently underestimated the relative efficiency.
Conclusion: Moisture availability plays an essential role in regulating the surface energy partitioning process. The introduction of the moisture availability parameter enables versatile theoretical models for latent heat (and evapotranspiration) predictions over a wide range of land cover types. This study provides a physical insight into the thermodynamics mechanism governing the surface energy balance, and the potential to develop novel surface energy parameterization schemes based on the concept of relative efficiency. The MEP model is found to have the greatest potential in terms of future theoretical model development.