Jin-liang REN, Qiong-fang LI*, Mei-xiu YU, Hao-yang LI

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Hydrology and Water Resources, Hohai University, Nanjing 210098, P. R. China

Variation trends of meteorological variables and their impacts on potential evaporation in Hailar region

Jin-liang REN1,2, Qiong-fang LI*1,2, Mei-xiu YU1,2, Hao-yang LI1,2

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Hydrology and Water Resources, Hohai University, Nanjing 210098, P. R. China

Evaporation, which is an important factor in the water balance at the basin scale, is a critical variable in the determination of local available water resources. Since the potential evaporation is mainly influenced by meteorological variables, it is necessary to investigate the extent to which different meteorological variables affect the potential evaporation. The aim of this study was to explore the variation trends of different meteorological variables, and their impacts on the potential evaporation. This study selected the Hailar Meteorological Station of the Hailar region, which is situated in a cold, semi-arid, and sub-humid region, as a case study site. Based on observed daily meteorological data from 1951 to 2009, the potential evaporation was calculated with the Penman formula, and the variations of meteorological variables were investigated with the nonparametric Mann-Kendall test. The correlation between the potential evaporation and each meteorological variable at annual and seasonal scales was also analyzed. The results show that the annual and seasonal potential evaporation and air temperature present increasing trends, whereas the wind speed, sunshine duration, and relative humidity present decreasing trends. Among the meteorological variables, the air temperature and relative humidity are the key factors that affect potential evaporation at different time scales, and the impacts of other meteorological variables on the potential evaporation are not significant and vary with time scales.

potential evaporation; meteorological variable; variation trend; correlation analysis; Hailar region

1 Introduction

Evaporation, a critical component of the water cycle, is an important variable in the determination of the amount of local available water resources (Allen 1996; Kay and Davies 2008; Li et al. 2008). The alteration in water availability is closely related to the variation of evaporation. The accurate estimation of the potential evaporation (PE), used to calculate the actual evaporation, is of vital significance to hydrologic simulation (Lindsey and Farnsworth 1997). Since the potential evaporation is mainly influenced by meteorological variables, it isnecessary to investigate the extent to which different meteorological variables affect the potential evaporation (Li et al. 2010).

In the past decades, climate change resulted from global warming has occurred in China (Yang et al. 2003; Qian et al. 2004; Ren and Guo 2006), producing an effect on the potential evaporation and further on the assessment of water resources. Li et al. (2009), Qiu et al. (2003), Liu et al. (2004), and Xu et al. (2006) found that the areal potential evaporation showed a decreasing tendency in the Huaihe River Basin, the Yellow River Basin and the Yangtze River Basin of China. They also explored the driving forces for the decreasing trend by analyzing the variations of a number of relevant meteorological variables.

This study selected the Hailar Meteorological Station as a case study site. It is located in the cold high-latitude region. Few studies of hydrological variation have been carried out in the Hailar region. In this study an attempt was made to analyze and compare the temporal variation of the potential evaporation and the impacts of related meteorological variables on it at different time scales in the Hailar region. The outputs of this study can provide references for regional hydrological studies and water resources planning and management in cold, high-latitude, semi-arid, and sub-humid regions.

2 Case study site

The study area is situated in the northeast part of the Inner Mongolia Autonomous Region, in China. Daily meteorological variables are observed at the Hailar Station (at a latitude of 49.12°N and longitude of 119.45°E), which is a national basic meteorological station founded in 1951. The Hailar River Basin, with an area of 22 516 km2, is situated in the semi-arid and sub-humid continental monsoon climate zone with an average annual precipitation of 350 mm, an average annual evaporation of 780 mm (E601), and an average annual air temperature of–5°C, and with a minimum air temperature ranging from –45°C to –25°C. Four seasons in the basin were studied, according to Wu et al. (2009): winter (November to March), spring (April to May), summer (June to August), and autumn (September to October).

3 Data and methods

The time series of daily meteorological data from 1951 to 2009 at the Hailar Station were available from the international exchange station at the China Meteorological Data Sharing Service System (http://cdc.cma.gov.cn/shuju/index.jsp?current=2&tpcat=SURF). Four daily meteorological variables were recorded, including the air temperature, relative humidity, sunshine duration, and wind speed. The Penman method (Penman 1948, 1963) was selected to estimate the potential evaporation at different time scales (Shuttleworth 1993; Xu and Chen 2005; Valiantzas 2006; Romano and Giudici 2009). The formula is shown below:

whereEPis the potential open water evaporation (mm/d), andRnis the net radiation near thesurface (mm/d), which is calculated as follows:

whereRais the local astronomical radiation (mm/d);nis the actual sunshine duration;Nis the theoretical sunshine duration;Ais the reflectivity, andA= 0.05;σis the Stefan-Boltzmann constant;TKis the absolute temperature (K);eais the actual vapour pressure (kPa);Δis the slope of the saturation vapour pressure curve at the air temperature (kPa/℃);γis the psychrometric constant (kPa/℃); andEais the drying power of the air, which in general can be written as

wheref(u)=au+buuis the wind function,uis the wind speed at a height of 2 m (m/s),auandbuare wind function coefficients, andau=0.35,bu=0.175 (Rong 2004); andesis the saturation vapour pressure (kPa).

Consequently, variation trends of annual and seasonal potential evaporation and meteorological variables were studied using the nonparametric Mann-Kendall (M-K) test (Burn and Hesch 2007; Yue et al. 2002). Correlations between the potential evaporation and meteorological variables were analyzed via the linear regression analysis to help to identify causal mechanisms for the potential evaporation variation.

4 Results and discussion

4.1 Variations of annual meteorological variables and annual potential evaporation

4.1.1 Annual meteorological variables

The temporal variations of annual meteorological variables from 1951 to 2009 at the Hailar Station are plotted in Fig. 1. It can be seen that the annual air temperature shows a significant upward tendency at the Hailar Station (Fig. 1(a)), while the annual sunshine duration, annual relative humidity, and annual wind speed all show downward trends, with the annual sunshine duration decreasing most (Figs. 1(b)-1(d)). The nonparametric M-K test of the air temperature also showed a rising tendency with an M-K statistic value of 6.350 at the confidence level of 99%. However, the nonparametric M-K tests of the sunshine duration, relative humidity, and wind speed had M-K statistic values of –5.604, –2.635, and –4.617, respectively, verifying the decreasing tendency at the confidence level of 99%.

4.1.2 Annual potential evaporation

Fig. 2 illustrates the temporal variation of the annual potential evaporation at the Hailar Station. It can be seen from Fig. 2 that the annual potential evaporation at the Hailar Station, with a minimum value of 679 mm and maximum value of 1 030 mm, shows a distinct increasing trend with an M-K statistic value of 3.355 at the confidence level of 99%, which might decrease the amount of local water available.

Fig. 1 Temporal variations of annual meteorological variables at Hailar Station

Fig. 2 Temporal variation of annual potential evaporation at Hailar Station

Correlations between the annual potential evaporation and the annual mean meteorological variables at the Hailar Station are shown in Fig. 3. It can be seen in Fig. 3 that the annual potential evaporation is strongly positively correlated with the annual mean air temperature, with a correlation coefficient (r) of 0.543, while it is strongly negatively correlated with the annual mean relative humidity, with a higher correlation coefficient of 0.609. The annual mean wind speed and sunshine duration are slightly correlated with the annual potential evaporation. The results indicate that the relative humidity and air temperature are the most important factors influencing the annual potential evaporation at the Hailar Station.

Fig. 3 Correlations between annual potential evaporation and annual mean meteorological variables at Hailar Station

4.2 Variations of seasonal meteorological variables and seasonal potential evaporation

4.2.1 Seasonal meteorological variables

The M-K statistics of seasonal air temperature, seasonal sunshine duration, seasonal relative humidity, and seasonal wind speed at the Hailar Station are presented in Table 1.

Table 1 M-K statistics of seasonal meteorological variables at Hailar Station

It can be found that the air temperature presents a significant upward tendency in spring with an M-K statistic value of 5.925 at the confidence level of 99%, while the wind speed, relative humidity, and sunshine duration show downward tendencies with M-K statistic values of –4.708, –2.760, and –2.917, respectively, at the confidence level of 99%.

In summer it also can be seen that the air temperature presents an increasing trend at the confidence level of 99%, while the other three meteorological variables (wind speed, relative humidity, and sunshine duration) also show obvious decreasing trends at the confidence level of 99%. It can be seen that the decreasing degrees of wind speed and sunshine duration in summer are not as remarkable as those in spring.

In autumn, the air temperature presents an increasing trend with an M-K statistic value of5.048 at the confidence level of 99%, while the wind speed, relative humidity, and sunshine duration show significant decreasing trends with M-K statistic values of –3.564, –3.211, and–3.152, respectively, at the confidence level of 99%.

In winter, it can be found that the air temperature presents an increasing trend at the confidence level of 99%, while the wind speed and sunshine duration show remarkable decreasing trends at the confidence level of 99%, and the decreasing trend of relative humidity is slight compared with those of the wind speed and sunshine duration.

This analysis shows that the seasonal air temperature presents a significant increasing trend and the other three meteorological variables present significant decreasing trends in all seasons, but the relative humidity’s decreasing trend in winter is slight.

4.2.2 Seasonal potential evaporation

The M-K statistics of seasonal potential evaporation at the Hailar Station were calculated. The seasonal potential evaporation showed an upward tendency in all four seasons at the Hailar Station. The seasonal potential evaporation in autumn presented an increasing trend with an M-K statistic value of 3.139 at the confidence level of 99%, while the seasonal potential evaporation in spring and summer presented increasing trends with M-K statistic values of 2.171 and 2.080, respectively, at the confidence level of 95%. The seasonal potential evaporation in winter showed an increasing trend with an M-K statistic value of 1.818 at the confidence level of 90%.

Correlations between the seasonal potential evaporation and the seasonal wind speed, relative humidity, air temperature, and sunshine duration at the Hailar Station are shown in Table 2. It can be found that the potential evaporation in spring is strongly positively correlated with the air temperature and sunshine duration, and significantly negatively correlated with the relative humidity. The potential evaporation in summer is strongly positively correlated with the air temperature and significantly negatively correlated with the relative humidity. The potential evaporation in autumn is positively correlated with the air temperature and sunshine duration and negatively correlated with the relative humidity and wind speed. In winter, it also can be seen that the potential evaporation is significantly positively correlated with the air temperature and negatively correlated with the relative humidity. This analysis suggests that the relative humidity and air temperature are the two key meteorological factors that influence the potential evaporation over four seasons in the Hailar region.

Table 2 Correlation coefficients between seasonal potential evaporation and seasonal meteorological variables at Hailar Station

5 Conclusions

Variation trends of different meteorological variables, and their impacts on the potential evaporation, were evaluated at different time scales in the Hailar region in this study. The following conclusions can be drawn: Annual and seasonal potential evaporation in the Hailar region shows a significant increasing trend, which may decrease the amount of the local water resources available if the potential evaporation and actual evapotranspiration present the same variation trends. The air temperature presents a remarkable upward tendency while the wind speed, relative humidity, and sunshine duration show downward tendencies at different time scales. The relative humidity and air temperature are the key factors affecting the potential evaporation at the Hailar Station. The outputs of this study provide a valuable reference for the forecast of future water resource available and the sustainable development and utilization of water resources in cold semi-arid and sub-humid regions.

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(Edited by Yan LEI)

This work was supported by the Special Fund for Public Welfare Industry of Ministry of Water Resources of China (Grant No. 200901045), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0717), and the 111 Project (Grant No. B08048).

*Corresponding author (e-mail:li_qiongfang@hotmail.com)

Received May 2, 2011; accepted Oct. 14, 2011