ABSTRACT
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.
ABSTRACT
Variations in lake evaporation have a significant impact on the energy and water budgets of lakes. Understanding these variations and the role of climate is important for water resources management as well as predicting future changes in lake hydrology as a result of climate change. This study presents a comprehensive, 10-year analysis of seasonal, intraseasonal, and interannual variations in lake evaporation for Lake Nasser in South Egypt. Meteorological and lake temperature measurements were collected from an instrumented platform (Raft floating weather station) at 2 km upstream of the Aswan High Dam. In addition to that, radiation measurements at three locations on the lake: Allaqi, Abusembel and Arqeen (respectively at 75, 280 and 350 km upstream of the Aswan High Dam) are used. The data were analyzed over 14-day periods from 1995 to 2004 to provide bi-weekly energy budget estimates of evaporation rate. The mean evaporation rate for lake Nasser over the study period was 5.88 mm day-1, with a coefficient of variation of 63%. Considerable variability in evaporation rates was found on a wide range of timescales, with seasonal changes having the highest coefficient of variation (32%), followed by the intraseasonal (28%) and interannual timescales (11.6%; for summer means). Intraseasonal changes in evaporation were primarily associated with synoptic weather variations, with high evaporation events tending to occur during incursions of cold, dry air (due, in part, to the thermal lag between air and lake temperatures). Seasonal variations in evaporation were largely driven by temperature and net energy advection, but are out-of-phase with changes in wind speed. On interannual timescales, changes in summer evaporation rates were strongly associated with changes in net energy advection and showed only moderate connections to variations in temperature or humidity.