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| DOI：10.13522/j.cnki.ggps.2020306 |
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| Calculating Daily Reference Evapotranspiration in Erhai Irrigated District with the Evaporation Paradox in Consideration |
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ZHAO Zhong, ZHOU Mi, ZHANG Liudong, GU Shixiang, LI Jing
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1. Key Laboratory of Water Security and Water Saving and Emission Reduction in Yunnan Agricultural University, Kunming 650021, China;2. Yunnan Institute for Investigation Design and Research of Water Resources & Hydropower Engineering, Kunming 650021, China; 3. State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China
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| Abstract: |
| 【Background】Evapotranspiration (ET) is an important process in the hydrological cycle and has been increasing, as shown by both field measurements and satellite remote sensing, at 0.63 mm/year over the past 30 years due to the increased leaf area index which resulted in a 0.72 mm increase in transpiration. In contrast, evaporation from soil has been decreasing at 0.32 mm/year, a phenomenon known as evaporation paradox.【Objective】The purpose of this paper is to compare the daily reference evapotranspiration (ET0) calculated from the Penman-Monteith formula with the evaporation paradox in consideration with data measured from a 20cm pan in the Erhai lake irrigation district after modifying the measurements by a pan conversion coefficient.【Method】 Meteorological data measured from Dali weather station at upper stretch of the Erhai Lake irrigation district in central Yunnan plateau was used to analyze the occurrence of the evaporation paradox. We then predicted ET0 using four models: linear regressions using the meteorological factors only, multiplying measurements from the 20cm pan by a pan conversion coefficient, multiple regression model using the meteorological factors in combination with the pan measurements, and the normal copula model. The ET0 predicted by these models was compared with that calculated from the Penman- Monteith formula.【Result】①Data measured from the 20 cm evaporation pan showed that the evaporation had been in decline from 1954 to 2018, while ET0 and temperature had been in increase although the former did not increase as fast as the latter. ET0 and evaporation measured from the pan trend with time in opposite ways, but the difference between their annual means was not significant. Both seasonal and annual average evaporations showed paradox in the 1960s and the 2000s, while in the 1970s only did the annual average evaporation and seasonal average evaporation in summer, autumn and winter showed paradox. In the 1990s and 2010s, the paradox occurred only seasonally in summer and autumn, respectively. ②Without the evaporation paradox, the daily ET0 predicted by the multiple regression model using both the meteorological factors and the pan measurements was most accurate compared to the standard ET0, followed by the linear regression model using the meteorological factors only; the worst was that calculated by multiplying the pan measurements by a pan conversion coefficient. Among all data we calculated by the model considering both the pan measurements and the meteorological factors, the fractions that have relative errors less than 15%, 20% and 25% were 79.18%~90.16%, 89.32%~97.23%, 94.79%~98.36%, respectively. ③In the presence of evaporation paradox, the evaporation measured from the pan and the ET0 trended differently with time, and the ET0 can only be calculated using the copula joint distribution function model. The constructed T-Tmax model was most accurate, and among all data calculated using it, the fractions that have relative errors less than 15%, 20% and 25% were 73.70%~86.56%, 82.51%~92.95%, 89.89%~98.52%, respectively.【Conclusion】We used the M-K test to determine the presence of evaporation paradox. We proved that in the absence of the paradox, the multiple regression model considering both meteorological factors and the pan measurements worked better, while in the presence of the paradox, the T-Tmax two-dimensional normal copula model was most accurate for predicting ET0. |
| Key words: ET0; pan evaporation; Mann-Kendall test; Copula function; Erhai irrigated district |
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