Abstract
The Priestley–Taylor equation (PTE) is widely used with its sole parameter (α) set as 1.26 for estimating the evapotranspiration (ET) of water bodies. However, variations in α may be large for ephemeral lakes. Poyang Lake, which is the largest freshwater lake in China, is water-covered and wetland-covered during its high-water and low-water periods, respectively, over a year. This paper examines the seasonal and diurnal variations in α using eddy covariance observation data for Poyang Lake. The results show that α = 1.26 is overall feasible for both periods at daily and subdaily scales. No obvious seasonal trend was observed, although the standard deviation in α for the wetland was larger than that for the water surface. The mean bias in evaporation estimations using the PTE was less than 5 W·m−2 during both periods, and the root mean square errors were much smaller than the average evaporation measurements at daily scale. U-shaped diurnal patterns of α were found during both periods, due partly to the negative correlation between α and the available energy (A). Compared to the vapor pressure deficit (VPD), wind speed (u) exerts a larger contribution to these variations. In addition, u is positively correlated with α during both periods, however, VPD was positively and negatively correlated with α during the high-water and low-water periods, respectively. Subdaily α exhibited contrasting clusters in the (u, VPD) plane under the same available energy ranges. Our study highlights the seasonal and diurnal course of α and suggests the careful use of PTE at subdaily scales.
Highlights
Inland water bodies cover less than 4% of the total terrestrial land [1]
Eeq can be theoretically determined as the available energy flux (A) times ∆/(∆+γ) because the Bowen ratio (B = H/LE) equals γ/∆ in this case, where ∆ is the slope of the saturated vapor pressure to the air temperature and γ is the psychrometric constant
The high-water and low-water periods (See Figure in Section 3) for the EC site used in this study reveal the times when the water levels at Xingzi station were higher than 14 m, and lower than 12 m, respectively, according to the study that quantified the relationship between the water surface areas of the Poyang Lake and the water level at the Xingzi station [38]
Summary
Inland water bodies cover less than 4% of the total terrestrial land [1] They exert huge impacts on the terrestrial water cycle, energy cycle, and climate system [2,3,4] because of their higher heat capacity and lower albedo compared to the land surface [5,6]. Eeq can be theoretically determined as the available energy flux (A) times ∆/(∆+γ) because the Bowen ratio (B = H/LE) equals γ/∆ in this case, where ∆ is the slope of the saturated vapor pressure to the air temperature and γ is the psychrometric constant. Due to the high heat capacity of the water body [5,6], the G of lake systems usually takes up a large proportion of the surface energy balance [31,32,33]
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