Meteorological Factors Affecting Pan Evaporation in the Haihe River Basin, China

Abstract

Pan evaporation (Epan) is an important indicator of regional evaporation intensity and degree of drought. However, although more evaporation is expected under rising temperatures, the reverse trend has been observed in many parts of the world, known as the “pan evaporation paradox”. In this paper, the Haihe River Basin (HRB) is divided into six sub-regions using the Canopy and k-means (The process for partitioning an N-dimensional population into k sets on the basis of a sample is called “k-means”) to cluster 44 meteorological stations in the area. The interannual and seasonal trends and the significance of eight meteorological indicators, including average temperature, maximum temperature, minimum temperature, precipitation, relative humidity, sunshine duration, wind speed, and Epan, were analyzed for 1961 to 2010 using the trend-free pre-whitening Mann-Kendall (TFPW-MK) test. Then, the correlation between meteorological elements and Epan was analyzed using the Spearman correlation coefficient. Results show that the average temperature, maximum temperature, and minimum temperature of the HRB increased, while precipitation, relative humidity, sunshine duration, wind speed and Epan exhibited a downward trend. The minimum temperature rose 2 and 1.5 times faster than the maximum temperature and average temperature, respectively. A significant reduction in sunshine duration was found to be the primary factor in the Epan decrease, while declining wind speed was the secondary factor.

Effects of Urbanization on Watershed Evapotranspiration and Its Components in Southern China

“…However, we showed that watershed-level ET had a significant decreasing trend in the QRB, coincident with previous studies [2,42]. This was in contrast to the global warming trend and ET was expected to increase under increasing air temperature in the study region [73][74][75]. Declines in ET have also been reported in the literature around the world, such as eastern China [76], Haihe River Basin [77], Liaohe River Delta [78], and the southern hemisphere in Africa [79].…”

Identification of Parameters of Evaporation Equations Using an Optimization Technique Based on Pan Evaporation

“…However, either data from advanced techniques that use sensors to measure evaporation are not yet easily available in developing countries, or the available data are not sufficient for statistical analyses and the development of equations. Pan evaporation is considered a reasonably good indicator of the regional evaporation intensity that is usually used for estimations of lake evaporation and water resource planning and management [14]. Pan evaporation and meteorological variables, including solar radiation, sunshine duration, atmospheric pressure, vapor pressure, wind velocity, air temperature, and relative humidity, were used in a study on lake evaporation by Yan et al [14].…”

“…Pan evaporation is considered a reasonably good indicator of the regional evaporation intensity that is usually used for estimations of lake evaporation and water resource planning and management [14]. Pan evaporation and meteorological variables, including solar radiation, sunshine duration, atmospheric pressure, vapor pressure, wind velocity, air temperature, and relative humidity, were used in a study on lake evaporation by Yan et al [14]. A monthly pan evaporation dataset was used for monitoring and estimating important aspects of evaporation in various parts of the world [11][12][13][14].…”

“…There are many meteorological factors affecting the rate of evaporation, including (i) the temperature of the water and air, (ii) the difference in vapor pressure between water and air, (iii) the wind speed, and (iv) solar radiation [14,15]. The evaporation rate is also influenced by the surrounding air humidity.…”

“…The evaporation rate diminishes when the moisture content of the adjacent air reaches the maximum limiting value. The existence of airflow removes the vapor of the air surrounding the water source, which reduces the humidity and thus increases the evaporation rate [14,16].…”