Downward trend of potential evaporation accompanied with upward of air temperature which is denoted as evaporation paradox has been reported in many regions over the past several decades in the world. In this paper, evaporation paradox and key factors attributed to ET0 changes are systematically analyzed based on data from 599 meteorological stations during 1960–2013. Results show that (1) Evaporation paradox exists in all regions in1960–2013 and 1960–1999 except SWRB in 1960–2013 but no evaporation paradox in 2000–2013. (2) Evaporation paradox exists in large areas in spring and summer, the extent and range fall in autumn, and there is no evaporation paradox in winter. (3) The evaporation paradox area accounts for 73.7% of China in 1960–2013 and 91.2% in 1969–1999. (4) Sunshine hours, humidity, wind speed, and maximum temperature appear to be the most important variables which contributed to ET0 change in China.
Climate change characterized by global warming has been the focus of diversified research fields such as water resource, agriculture, ecosystem, and human health. It is widely accepted that global air temperature had been increasing in recent decades, it has risen by about 0.85 (0.65–1.06)°C from 1951 to 2012, and the average rising rate was 0.12 (0.08–0.14)°C (IPCC [
Potential evapotranspiration (ET0) is one of the most important components of the hydrological system which refers to “the quantity of water evaporated per unit area, per unit time from an idealized, extensive free water surface under existing atmospheric conditions.” It is an important indicator of atmospheric evaporative demand for estimating terrestrial evaporation and crop water requirements. There have been many discussions on methods of calculating ET0 (Penman [
In fact pan evaporation observations mostly ended in 2001 in China; evaporation paradox was concluded based on annual pan evaporation of 1960–2000 (H. Yang and D. Yang [
Daily meteorological data were obtained from 754 stations from the China Meteorological Administration (CMA) and National Meteorological Information Center of China (NMIC); 599 stations (Figure
Spatial distribution of meteorological stations and first-order basin in China.
In the data set, the 10 river basins are the first-order basin in China (Figure
In this paper, potential evapotranspiration (ET0) was estimated using the Penman-Monteith (PM) method (Allen et al. [
The simple linear regression method was used to estimate the trend magnitudes (slope) in ET0 and other climatic variables. The linear equation is
The basic idea is to introduce the influencing factors into regression equation one by one. Significant test is carried out when introducing one variable into the model, retaining the significant factors and rejecting the insignificant ones until there are no variables introduced into the model and no one rejected. This method can eliminate the variables which contribute little to principal component or those existing linear relations and can overcome the multicollinearity based on guaranteeing the regression effects.
In previous researches, regional value was obtained by using an arithmetic mean method from meteorological station. However, meteorological stations are not distributed evenly but dense in the east and sparse in the west in China. Therefore, it is necessary to assign different weights for different stations when evaluating climate change accurately for different regions. When calculating the average value of an area, the weight of a station is determined by the percentage of the Thiessen polygon in the whole area. Thiessen polygon method was more accurate than simple mean method and less workload grid data set method.
In 1960–2013, 98.2% of the 599 stations show upward trend (91.2% of all stations are at 95% significance level). The average daily temperature in China as a whole (Figure
Interannual changes of temperature and ET0 in 1960–2013.
According to Figure
Change trends of temperature and potential evapotranspiration (ET0) in China.
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China | −3.9* | 59.1 | −14.78** | 75.1 | 10.08 | 43.7 | 0.24** | 97.8 | 0.21** | 90 | −0.08 | 41.4 |
YaRB | −2.51 | 58.7 | −17.64** | 86 | 52.27 | 16.8 | 0.18** | 96.5 | 0.07 | 74.3 | 0.16 | 62.2 |
SERB | 3.61 | 60.7 | −20.93** | 89.3 | 19.37 | 21.4 | 0.21** | 100 | 0.1 | 92.9 | −0.14 | 46.4 |
HaRB | −9.21** | 75.8 | −18.08** | 84.8 | −17.05 | 72.7 | 0.28** | 97 | 0.3** | 97 | −0.41 | 39.4 |
HuRB | −5.95 | 60.5 | −8.16 | 65.8 | −17.24 | 60.5 | 0.2** | 97.4 | 0.17* | 97.4 | −0.1 | 39.5 |
YeRB | 1.02 | 61.2 | −5.94 | 55.2 | −6.22 | 53.7 | 0.27** | 97 | 0.23** | 98.5 | −0.1 | 13.4 |
LRB | −10.68* | 52.8 | −12.67 | 80.6 | −78.2 | 86.1 | 0.23** | 100 | 0.31** | 100 | −0.51 | 2.7 |
SRB | −4.04 | 70.9 | −3.77 | 58.2 | −74.26 | 90.9 | 0.33** | 100 | 0.44** | 98.2 | −0.36 | 27.3 |
NWRB | −11.13** | 69.1 | −27.65** | 81.4 | 26.69 | 39.2 | 0.34** | 99 | 0.31** | 94.8 | 0 | 45.4 |
SWRB | 3.89 | 48.6 | −3.26 | 60 | 102.4 | 5.7 | 0.28** | 100 | 0.22** | 88.6 | 0.4 | 97.1 |
PRB | −1.01 | 44.8 | −14.65** | 76.1 | 5.74 | 43.3 | 0.14** | 98.5 | 0.11** | 88.1 | −0.28 | 25.4 |
The mean annual temperature in the 10 river basins all rose at 95% significance level and ET0 all showed downward trend except in SERB, YeRB, and SWRB in 1960–2013; all river basins indicated upward in temperature and downward in ET0 in 1960–1999. The maximum downward in ET0 and upward in temperature appeared in NWRB and SRB with values being −27.65 mm per decade and 0.44°C per decade, respectively, in 1960–1999. In 2000–2013, ET0 in YaRB, SERB, SWRB, NWRB, and PRB increased while temperature in SWRB and PRB decreased. In other basins the trend in temperature and ET0 was the same. In this period temperature only in YaRB and SWRB increased, whether the increase was a fluctuation in the whole upward process or the beginning of decrease needs further investigation.
Figure
Seasonal change of temperature and ET0 in different regions.
1960–2013
1960–1999
2000–2013
Spring (March to May): in 1960–2013, evaporation paradox existed in HaRB, LRB, SRB, NWRB, SWRB, PRB, and China as a whole. In 1960–1999, evaporation paradox existed in all regions except SRB; in 2000–2013, there was no evaporation paradox; change of temperature and ET0 was the same in eight river basins and temperature in 5 river basins dropped. The opposite changes of temperature and ET0 in 1960–1999 and 2000–2013 weakened the evaporation paradox of 1960–2013.
Summer (June to August): in 1960–2013, ET0 and temperature were the highest values in a whole year; the slope of ET0 and percent of downward stations were the highest values too. ET0 descended in YaRB, HaRB, HuRB, YeRB, LRB, NWRB, and China as a whole; temperature rose in all regions, except HuRB which was at 99% confidence level. In 1960–1999, evaporation paradox existed in all river basins except SWRB and HuRB; in China as a whole the percent of ET0 downward climaxed 77% and 8 river basins were more than 70%, and so evaporation paradox was the most prominent in all statistical periods. In 2000–2013, the variation of ET0 and temperature was the same except the HuRB.
Autumn (September to November): the slope and range of ET0 decline reduced in autumn comparing with that of spring and summer. In 1960–2013, ET0 in HaRB, LRB, SERB, SRB, and NWRB decreased and temperature increased significantly. The phenomenon existed in the 5 river basins in 1960–1999 too. In 2000–2013, HuRB, LRB, SRB, and PRB showed evaporation paradox.
Winter (December to February next year): change in temperature was the most severe compared with the other seasons. Except in PRB in 1960–1999, temperature in 1960–2013 and 1960–1999 all rose significantly. Opposite to the severe increase in temperature, decrease of ET0 in winter was moderate. In 1960–2013, ET0 only in HuRB, HaRB, LRB, and NWRB declined slightly and other regions showed upward trend. In 1960–1999, ET0 in HaRB, HuRB, PRB, YaRB, NWRB, and China as a whole insignificantly decreased. In 2000–2013, temperature showed biggest fall and only SWRB showed upward trend. In winter ET0 changed the smallest in the four seasons and evaporation paradox was moderate.
In 1960–2013 and 1960–1999, the percent of rising stations in temperature exceeded 90%, so stations in which ET0 decreased can be judged as where evaporation paradox existed (Figure
Distribution of evaporation paradox in different periods.
Precipitation and ET0 were two important segments of the hydrologic cycle; ET0 will decline with the increase of precipitation according to the Bouchet assumption. The annual average precipitation is 808 mm in China during 1960–2013; it rose insignificantly and the rising percent was 48.4%. The annual average precipitation was 811 mm in 1960–1999 and the insignificant rising percent was 55.9%; in 2000–2013 the average value was 799.5 mm; change of ET0 and precipitation in China were consistent with the Bouchet hypothesis and Figure
Change of precipitation and ET0 in China.
Meteorological factors change had profound impacts on ET0; in this paper stepwise regression was used to extract the influencing factors of ET0. Yearly ET0 and 7 meteorological factors such as mean temperature (
Results of the stepwise regression.
China | YaRB | SERB | HaRB | HuRB | YeRB | LRB | SRB | NWRB | SWRB | PRB |
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Table
Slope of climate variables and ET0 in China.
China | YaRB | SERB | HaRB | HuRB | YeRB | LRB | SRB | NWRB | SWRB | PRB | ||
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ET0 (mm per decade) | 1960–2013 | −3.9* | −2.51 | 3.61 | −9.21** | −5.95 | 1.02 | −10.68* | −4.04 | −11.13** | 3.89 | −1.01 |
1960–1999 | −14.78** | −17.64** | −20.93** | −18.08** | −8.16 | −5.94 | −12.67 | −3.77 | −27.65** | −3.26 | −14.65** | |
2000–2013 | 10.08 | 52.27 | 19.37 | −17.05 | −17.24 | −6.22 | −78.2 | −74.26 | 26.69 | 102.4 | 5.74 | |
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1960–2013 | −0.11** | −0.11** | −0.16** | −0.23** | −0.18** | −0.08* | −0.08* | −0.09** | −0.04** | 0 | −0.13** |
1960–1999 | −0.13** | −0.16** | −0.28** | −0.2** | −0.16** | −0.1* | −0.12** | −0.11** | −0.06* | −0.01 | −0.19** | |
2000–2013 | −0.04 | 0.09 | −0.05 | −0.17 | 0.08 | −0.13 | −0.14 | −0.43 | 0.07 | 0.23 | −0.25 | |
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RH (% per decade) | 1960–2013 | −0.43** | −0.53** | −0.83** | −0.6** | −0.87** | −0.5* | −0.11 | −0.46** | −0.12 | −0.34 | −0.63** |
1960–1999 | −0.02 | 0.12 | 0 | −0.33 | −0.26 | −0.17 | −0.11 | −0.42* | 0.21 | 0.21 | −0.23* | |
2000–2013 | −2.08 | −3.18 | −2.8 | −2.41 | −5.22 | −1.93 | 2.14 | 1.35 | −2.58 | −5.9 | −1.3 | |
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1960–2013 | 0.18** | 0.16** | 0.18** | 0.19** | 0.08 | 0.25** | 0.16** | 0.19** | 0.25** | 0.24** | 0.09* |
1960–1999 | 0.11 | 0 | 0.03 | 0.18* | 0.1 | 0.19 | 0.2* | 0.27** | 0.18* | 0.11 | 0.02 | |
2000–2013 | −0.11 | 0.25 | −0.21 | −0.41 | −0.46 | −0.02 | −0.72 | −0.69 | 0.02 | 0.74 | −0.41 | |
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1960–2013 | −0.11** | −0.08** | −0.13** | −0.16** | −0.1** | −0.07** | −0.18** | −0.18** | −0.14** | −0.05** | 0.04 |
1960–1999 | −0.12** | −0.09** | −0.15** | −0.19** | −0.11** | −0.07** | −0.18** | −0.17** | −0.18** | −0.02 | −0.07** | |
2000–2013 | −0.02 | 0.03 | −0.13 | 0.01 | −0.2 | −0.06 | −0.14 | −0.21 | −0.02 | 0.12 | 0.12 |
In 1960–2013, temperature in 98.2% stations of 599 stations increased in China. The decline rate of annual national ET0 was −3.9 mm per decade so evaporation paradox existed. In 1960–1999, ET0 of 75.1% stations was downward and temperature of 90% stations was upward which indicated the most prominent evaporation paradox. In 2000–2013 there was no evaporation paradox. The opposite change of temperature and ET0 in 1960–1999 and 2000–2013 weaken the evaporation paradox in 1960–2013 compared with that of 1960–1999. In 1960–2013, evaporation paradox existed in spring, summer, and autumn in China as a whole; it existed in 6, 6, 5, and 4 river basins in spring, summer, autumn, and winter; the decline rate of ET0 and percent of temperature downward climaxed in summer. In 1960–1999, except SRB in spring, SWRB in summer, HaRB, HuRB, and YeRB in autumn, HuRB, YeRB, LRB, and SRB in winter evaporation paradox exists in other times. In 2000–2013, there was no evaporation paradox. There was no evaporation paradox in the southeastern coastal areas south of 37°N, most of areas in 30°N–40°N, 90°E–110°E, northwestern in SRB, and southeastern of SWRB in China in 1960–2013; the area accounted for 26.3% of the 10 river basins. No evaporation paradox area was only in NWRB, northeastern of SRB, middle reach of YeRB, three river source regions, and northeastern of HuRB which accounted for 8.8% merely. Precipitation in NWRB, SERB, three river source regions, lower reaches of YaRB, northwestern of SRB, northwestern of HuRB, and lower reaches of PRB increased and in such regions ET0 decreased in 1960–2013. Most of stations in which ET0 and precipitation change inversely were located south of 27°N and north of 32°N; the number of the stations was 346 in 1960–1999. In 2000–2013, the stations which precipitation increased were located in north of 32°N and the number of stations in which ET0 and precipitation change inversely was 352.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by the National Natural Science Foundation of China (Grant no. 51279063); Program for Innovative Research Team (in Science and Technology) in University of Henan Province (15IRTSTHN030); and Program for New Century Excellent Talents in University (Grant no. NCET-13-0794).