Abstract
In the case of lakes, evaporation is one of the most significant losses of water and energy. Based on high-frequency eddy-covariance (EC) measurements between May and September of 2019, the offshore heat and water vapor exchanges are evaluated for the large (~600 km2) but shallow (~3.2 m deep) Lake Balaton (Transdanubian region, Hungary). The role of local driving forces of evaporation in different time scales (from 20 min to one month) is explored, such as water surface and air temperatures, humidity, atmospheric stability, net radiation, and energy budget components. EC-derived water vapor roughness lengths and transfer coefficients (Cq) show an apparent intra-seasonal variation. Different energy balance-based evaporation estimation methods (such as the Priestley-Taylor and the Penman-Monteith) confirm this observation. Furthermore, this has suggested the existence of an intra-annual variation in these parameters. This hypothesis is verified using ten years of water balance measurements, from which, as a first step, evaporation rates and, second, transfer coefficients are derived on a monthly scale. Cq is highly reduced in winter months (~1 × 10−3) compared to summer months (~2.5 × 10−3) and strongly correlated with net radiation. The application of time-varying Cq significantly increases the accuracy of evaporation estimation when the Monin-Obukhov similarity theory-based aerodynamic method is applied. The determination coefficient increases to 0.84 compared to 0.52 when a constant Cq is employed.
Highlights
Evaporation is often the major loss in the water budget of inland lakes, and thereby it directly influences the lake’s water resources management strategy such as the water level regulation
We have found that EC-derived roughness lengths and transfer coefficients for latent heat flux dispose of apparent intraseasonal variations
The energy balance (EB) of Lake Balaton was determined for five months in the warm season to analyze intra-seasonal variations
Summary
Evaporation is often the major loss in the water budget of inland lakes, and thereby it directly influences the lake’s water resources management strategy such as the water level regulation. As for the WB method, accurate monthly evaporation estimates can be achieved if source and loss terms, such as precipitation, inflow, runoff, water level, and outflow are adequately measured. It cannot be used at finer temporal scales like daily or sub-daily. The Penman-Monteith and Priestley-Taylor methods belong to these, and their applicability was proved by many studies [13,14], but they can only be used at daily or longer time scales In these methods, the heat storage term of the EB is crucial otherwise a strong seasonal bias may occur. The only methods that can work on sub-daily time scales are the aerodynamic ones
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