Fei YU,Xiaoping GU,Hua XIONG

1.Guizhou Institute of Mountainous Climate and Environment,Guiyang 550002,China;2.Guizhou Key Laboratory of Mountain Climate and Resources,Guiyang 550002,China;3.Guizhou Forest Inventory and Planning Institute,Guiyang 550003,China

Potential evapotranspiration,a measure of atmospheric evaporation capacity, indicates there is a capacity to control the evaporation process of humid underlying surface in the atmosphere,and it is an important indicator for assessment of drought-wet degree of climate,reservoir design,water consumption of crops,and calculation of crop productivity potential,as well as the fundamental basis for establishment of irrigation system and a plan of the irrigation requirement for water[1-3],playing a significant role in global water cycle and climate evolution. Guizhou Province,the center of southwestern China’s karst areas,is rich in precipitation,but drought occurred at different levels every year.On the one hand,water is lost through potential evapotranspiration besides surface leakage,so studying the temporal and spatial variation of potential evapotranspiration in Guizhou Province is worthy of attention.On the other hand,under the background of global warming,potential evapotranspiration all over the world has tended to reduce in recent 50 years,which is called“evaporation paradox”and hasattracted broad attention in the world[4-5].Changes in potential evapotranspiration are mainly affected by meteorological factors such as temperature,wind speed,sunshine hours,relative hu-midity,saturated vapor pressure deficit and cloud cover,and these factors influence each other,so the reasons for changes in potential evapotranspiration are very complex[6-7].Here based on daily meteorological data at each meteorological station in Guizhou Province from 1961 to 2010,the changes of potential evapotranspiration and its sensitivity to meteorological factors in Guizhou Province were discussed,so as to provide important references for assessment of water resources,research of agricultural water conservancy and climate change.

Data and Methods

Daily meteorological data at each meteorological station in Guizhou Province from 1961 to 2010 are from Guizhou Meteorological Bureau.Based on the daily meteorological data,daily potential evapotranspiration in Guizhou Province was calculated using Penman-Monteith formula recommended by the Food and Agriculture Organization(FAO).The formula is shown as follows:

where PE is potential evapotranspiration,mm/d;Rnis surface net radiation,MJ/(m·d);G is soil heat flux,MJ/(m2·d);Tmeanis daily mean temperature,℃;μ2is wind speed at a height of 2 m,m/s;esis saturation vapor pressure,kPa;eais actual vapor pressure,kPa;△is the slope of a saturation vapor pressure curve,kPa/℃;γ is psychrometer constant,kPa/℃.Each component was calculated according to related studies[8].

Results and Analysis

Temporal and spatial distribution characteristics of potential evapotranspiration

As shown in Fig.1,on the whole,multi-year average potential evapotranspiration in Guizhou Province was higher in the southwest and the west compared with the northeast and the east.Multiyear average potential evapotranspiration in Guizhou Province ranged mostly from 850.0 to 950.0 mm,and it was higher than 1 000.0 m in the southwest marginal areas.It was higher than 900.0 mm in the west and most areas in the south.It varied from 804.1 to 900.0 mm in the middle and most areas in the north-central part and lower than 850.0 mm in the north.

According to Table 1,among the representative stations in Guizhou Province,the maximum ofannual average potential evapotranspiration(1 067.32 mm)appeared in Xingyi City,followed by Guiyang(942.30 mm)and Kaili City(931.71 mm);the annual average potential evapotranspiration was the lowest in Bijie (846.71 mm),Zunyi(857.30 mm)and Tongren City(877.43 mm).

As shown in Fig.2,in most areas of Guizhou Province,potential evapotranspiration was the highest in July but the lowest in January.In various seasons,it was the highest in summer,followed by spring and autumn,while it was the lowest in winter.

Table 1 The maximum,minimum and average of annual potential evapotranspiration in the representative stations in Guizhou Province mm

Changing tend of potential evapotranspiration

In recent 40 years,annual potential evapotranspiration showed an obvious decreasing trend in most stations of Guizhou Province,but the decreasing trend was not obvious in Tongren and Zunyi City(Fig.3).Generally speaking,with the increase of temperature,evaporation and evapotranspiration become stronger on the surface of the ground(including vegetation)and water,but evapotranspiration in the Northern Hemisphere tended to reduce in recent 50 years[9],which is called “evaporation paradox”.“Evaporation paradox”also appeared in Guzhou Province.

Analysis of sensitivity of potential evapotranspiration to meteorological factors

The correlation between potentialevapotranspiration and meteorological factors(daily average temperature,daily maximum temperature,daily minimum temperature,sunshine duration,daily average wind speed,and daily average relative humidity)in representative stations in Guizhou Province was analyzed.The results show that the main meteorological factors influencing changes of potential evapotranspiration in Guizhou Province were sunshine duration,daily maximum temperature,and daily average relative humidity(Table 2).

Table 2 Correlation coefficients between potential evapotranspiration and meteorological factors various representative stations in Guizhou Province

Conclusions and Discussion

On the whole,potential evapotranspiration in Guizhou Province was higher in the southwest and the west compared with the northeast and the east.Multi-year average potential evapotranspiration in Guizhou Province ranged mostly from 850.0 to 950.0 mm,and it was higher than 1 000.0 m in the southwest marginal areas and lower than 850.0 mm in the north.In recent 40 years,annual potential evapotranspiration showed an obvious decreasing trend in most stations of Guizhou Province,but the decreasing trend was not obvious in Tongren and Zunyi City.In most areas of Guizhou Province,potential evapotranspiration was the highest in July but the lowest in January.

Correlation analysis shows that the main meteorological factors influencing changes of potential evapotranspiration in Guizhou Province were sunshine duration,daily maximum temperature,and daily average relative humidity.The decrease in potentialevapotranspiration in Guizhou Province was closely related to significant drop of sunshine duration.With the increase of daily average temperature,potential evapotranspiration in Guizhou Province did not increase,revealing that daily average temperature was not the main meteorological factor affecting changes of potential evapotranspiration in Guizhou Province.

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