ORIGINAL PAPER

Precipitation extremes in a karst region: a case studyin the Guizhou province, southwest China

Qiang Zhang & Chong-Yu Xu & Zengxin Zhang &

Xi Chen & Zhaoqing Han

Received: 16 January 2009 /Accepted: 17 August 2009 /Published online: 2 September 2009# Springer-Verlag 2009

Abstract We analyzed the changing properties of precip-itation extremes in the Guizhou province, a region oftypical karst geomorphology in China. Precipitationextremes were defined by the largest 1- and 5-dayprecipitation total. Trends of precipitation extremes weredetected by using Mann–Kendall trend test technique.Besides, we also investigated moisture flux variationsbased on the National Centers for Environmental Predictionand the National Center for Atmospheric Research reanal-ysis dataset with the aim to further explore the possiblecauses behind the changes in precipitation extremes. Theresults of this study indicated that: (1) Although thechanges in precipitation extremes at most of the stations

were not significant, enhanced precipitation extremes werestill detected after the early 1990s mainly in the middle andwest parts of the Guizhou province; (2) In winter, east andsouth parts of the Guizhou province were characterized byincreasing precipitation extremes; In summer, enhancedprecipitation extremes were observed mainly in the middleand east parts of the Guizhou province; (3) A significantincrease of moisture flux was observed after the 1990swhen compared to that before the 1990s. Cumulativedeparture analysis results of moisture flux and precipitationextremes confirmed the influences of moisture flux on thevariations of precipitation extremes in the study region.This study clarified the changes of weather extremes andtheir linkages with large-scale atmospheric circulation inthe karst region of China, which will definitely enhancehuman mitigation to natural hazards in the fragile ecolog-ical environment under the influences of changing climate.

1 Introduction

Global warming, characterized by increasing temperature,has the potential to cause higher evaporation rates andtransport larger amounts of water vapor into the atmo-sphere, probably having accelerated the global hydrologicalcycle (Semenov and Bengtsson 2002; Labat et al. 2004; Xuet al. 2006). One of the most significant consequences ofglobal warming would be an increase in the magnitude andfrequency of precipitation maxima brought about byincreased atmospheric moisture levels and/or large-scalestorm activities (Shouraseni and Robert 2004). Significantlydecreasing number of rainy days and significantly increas-ing precipitation intensity were identified in many places ofthe world, such as China (Ren et al. 2000; Gong and Ho2002; Zhai et al. 2005; Zhang et al. 2008a, b), USA (Karl et

Q. Zhang (*)Department of Water Resources and Environment,Sun Yat-sen University,Guangzhou 510275, Chinae-mail: zhangqnj@gmail.com

C.-Y. XuDepartment of Geosciences, University of Oslo,Sem Saelands vei 1,Blindern 0316 Oslo, Norway

Z. ZhangJiangsu Key Laboratory of Forestry Ecological Engineering,Nanjing Forestry University,Nanjing 210037, China

X. ChenState Key Laboratory of Hydrology-Water Resourcesand Hydraulics Engineering, Hohai University,Nanjing 210098, China

Z. HanCenter for Chinese Historical Geography Studies,Fudan University,Shanghai 200433, China

Theor Appl Climatol (2010) 101:53–65DOI 10.1007/s00704-009-0203-0

al. 1996), and India (Goswami et al. 2006). Recently,Goswami et al. (2006) reported significantly increasingfrequency and magnitude of extreme rain events, rain-storms, over central India during the monsoon season in awarming environment. Global warming might give rise toincrease and intensification of extreme events, such asprecipitation extremes defined by various indices (WMO2003). Due to the tremendous influences of climaticextremes, public awareness has risen sharply in recentyears and as a result, catastrophic floods, droughts, storms,and heat waves or cold spells have been receivingtremendous attention (e.g., Beniston and Stephenson2004; Zhang et al. 2006a, b, 2008c). Suppiah and Hennessy(1998) have pointed out that heavy precipitation events inmost parts of Australia have increased. Groisman et al.(1999) indicated that the probability of daily precipitationexceeding 50.8 mm in midlatitude countries (the USA,Mexico, China, and Australia) have increased by about20% in the later 20th century.

As mentioned above, increasing precipitation extremescan be observed in some regions of China. Changingproperties of precipitation events are different from regionto region due to the inhomogeneous distribution ofprecipitation variations over China (e.g., Zhang et al.2008d). Zhai et al. (1999) have indicated increasedintensive precipitation events in west China since 1950.Wang and Zhou (2005) investigated the spatial distributionof extreme precipitation during 1961–2001 and found thatthe annual mean precipitation increased significantly insouthwest, northwest, and east China, and significantlydecreased annual mean precipitation was observed incentral, north, and northeast China. The increasing trendswere observed mainly in summer in east China, while inboth spring and autumn, the decreasing trends wereidentified mainly in central, north, and northeast China.Besides, increasing precipitation maxima can also beidentified in the southeast China (e.g., Zhang et al.2008a). Located in the east parts of the Yunnan–GuizhouPlateau, China, the Guizhou province is characterized bytypical karst geomorphology termed as “karst rockydesertification” and by the extremely fragile ecologicalenvironment (Song et al. 1983; Wang et al. 2004). The karsttopography in the Guizhou province gives rise to largeslopes in mountainous areas with thin soil thickness,leading to frequent landslides and serious soil erosion. Inthis case, occurrence of precipitation maxima has greatpotential to trigger appearances of natural hazards, such asflash floods, serious soil erosion, landslides, and so on. Luoet al. (2006) indicated that precipitation in flooding seasonaccounted for about 75% of the annual total precipitation.Besides, they also found abrupt increase of rainstorm daysafter 1991. Wu and Wang (2006) analyzed relationsbetween summer precipitation and wind field in the

Guizhou province, indirectly addressing significant influ-ences of moisture flux on the summer precipitationchanges. Therefore, changes of precipitation maxima andpossible underlying causes in the Guizhou province havedrawn considerable concerns. However, so far, studies ofprecipitation changes were mainly found in Chineseliteratures. Besides, studies focusing on the seasonalchanges of precipitation maxima and associated linkageswith atmospheric circulation, particularly the moisture flux,were not found. Now that the currently well-evidencedglobal warming is expected to accelerate the hydrologicalcycle and would cause more climatic extremes, and theresults of studies illustrated increasing frequencies ofprecipitation extremes in east and south China (e.g., Zhaiet al. 1999; Zhang et al. 2008a, b). Thus, it is natural to askthe question as to whether extreme weather and climateevents are truly increasing under the changing climate inthe Guizhou province, a typical karst area, and what couldbe the circulation patterns behind the changes in extremeclimate events, if any, in the study region. This constitutedthe major motivation for this study.

Thereby, the objectives of this study were: (1) to detectchanging properties of precipitation extremes defined byvarious indices and (2) to study large-scale atmosphericcirculation patterns behind the changes in precipitationextremes with the aim to understand possible physicalmechanisms causing changing properties of precipitationextremes in the karst region of China. The results of thisstudy would be of practical significance in the localecological environment conservation and the natural hazardmanagement in the karst region of China.

2 Study region

Located in the southwest China, the Guizhou province(Fig. 1) is dominated by a typical karst geomorphology.The karst area covers 17,600 km2, being one of the largestkarst regions of the world with a population of 32.4 million.The karst area covers about 73% of the Guizhou provinceand is characterized by soluble carbonate rocks (Zeng1994). The mountainous area occupies 92.5%, and themountain ridges account for only 7.5% of the total area ofthe Guizhou province (Wu et al. 2003). Typical cone andcockpit karst geomorphology gives rise to sharp relief andsteep slopes with an average slope of 17.8°. Large terrainslopes, thin soil thickness, and vegetation degradation dueto human activities result in fragile ecological environment.Frequent natural hazards, such as floods, droughts, land-slides, debris flow, and so on, have caused significant lossof economy and human life in the study region. Specifical-ly, in 1954, serious floods occurred in the Guizhouprovince, about 0.16 million hm2 fields were affected, and

54 Q. Zhang et al.

more than 180 people died (Yang and Xu 1999). In 1998,the economic losses due to rainstorm-induced floodsreached more than 0.16 billion US dollars (Liu et al.1999). With respect to climate, the study area is character-ized by subtropical monsoon climate with the meansummer temperature of 20–25°C and the mean wintertemperature of 4–9°C. The annual mean precipitation is1,100–1,300 mm. Precipitation mainly occurs in summerwith a large variability. This kind of climate and topo-graphical properties easily trigger natural hazards such asflash floods or droughts.

3 Data and methodology

Daily precipitation data for 1960–2005 were collected from 19national standard rain stations in the Guizhou province (Fig. 1;Table 1). There are missing data in the daily precipitationdataset. The missing precipitation data at a station were filledin by the average value of its neighboring days (Zhang et al.2008a). We consider the gap filling method will have noinfluence on the long-term temporal trend (Zhang et al.2008b). The consistency of the data was checked by thedouble-mass method, and the results revealed that all the

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2508.7-27982219.3-2508.71930-2219.3

1640.7-19301351.3-1640.71062-1351.3772.7-1062483.3-772.7194-483.3

# Stations

Rivers

103 E 107 E105 E 109 E

0 400 km

Altitude (m)

Guizhou Province

29 N

27 N

25 N

Fig. 1 Location of the studyregion and the meteorologicalstations. The solid dots in theright panel show the locationsof the meteorological stations

Station name Longitude Latitude Altitude (m) Mean (mm) Maximum (mm) IQR (mm)

Weining 26°52'N 104°17'E 2237.5 899.17 1263.45 196.9

Panxian 25°43'N 104°28'E 1800 1395.54 2106.03 260.37

Tongzi 28°08'N 106°50'E 972 1036.68 1335.67 186.95

Xishui 28°20'N 106°13'E 1180.2 1117.27 1461.06 211.36

Bijie 27°18'N 105°17'E 1510.6 892.13 1285.44 119.88

Zunyi 27°42'N 106°53'E 843.9 1084.90 1452.73 154.54

Meitan 27°46'N 107°28'E 792.2 1140.85 1428.72 230.44

Sinan 27°57'N 108°15'E 416.3 1139.30 1673.43 271.02

Tongren 27°43'N 109°11'E 279.7 1267.90 1608.68 244.69

Qianxi 27°02'N 106°01'E 1231.4 979.01 1415.05 160.38

Anshun 26°15'N 105°54'E 1431.1 1349.17 1898.72 272.36

Guiyang 26°35'N 106°44'E 1223.8 1118.07 1441.71 193.92

Kaili 26°36'N 107°59'E 720.3 1213.34 1641.68 281.96

Sansui 26°58'N 108°40'E 626.9 1116.15 1548.39 225.26

Xingren 25°26'N 105°11'E 1378.5 1337.39 1888.23 320.24

Wangmo 25°11'N 106°05'E 566.8 1233.49 1743.45 202.43

Luodian 25°26'N 106°46'E 440.3 1140.12 1624.07 318.45

Dushan 25°50'N 107°33'E 1013.3 1319.49 1730.42 213.79

Rongjiang 25°58'N 108°32'E 285.7 1196.65 1580.04 329.39

Table 1 Locations of rain gaug-ing stations; precipitationmean, maximum, and IQR(interquantile range) at eachstation for 19 rain stations

Precipitation extremes in a karst region 55

precipitation series used in the study were consistent. Variousextreme precipitation variables were defined by usingdifferent indices (Table 2). The definitions of these precipi-tation indices are based on the previous studies (e.g., Tebaldiet al. 2006; Zhang et al. 2008b; Fatichi and Caporali 2009).In this study, rainy days were defined as those days withprecipitation of greater than or equal to 2 mm. The thresholdof 2-mm rainfall in the definition of “rainy days” was used toavoid artificial trends, which can arise from a tendency ofsome observers failing to report small rainfall amounts(Lavery et al. 1992). To understand possible physicalmechanisms behind the changing properties of precipitationextremes, we analyzed moisture flux by using the NationalCenter for Atmospheric Research and the National Centersfor Environmental Prediction (NCAR/NCEP) reanalysis data-set. In the actual atmosphere, the moisture is very low over

300 hPa. Thus, the moisture content and related transportfeatures, also the moisture flux in the following text, of thewhole Ps layer (surface pressure) −300 hPa were studied withthe NCAR/NCEP reanalysis data covering 1960 to 2005(Miao et al. 2005; Zhang et al. 2008e).

There are many statistical techniques available to detecttrends within the time series, including moving average,linear regression, Mann–Kendall trend test, filtering technol-ogy, etc. Each method has its own strengths and weaknessesin trend detection. However, nonparametric trend detectionmethods are less sensitive to outliers than are parametricstatistics, such as Pearson's correlation coefficient. Moreover,the rank-based nonparametric Mann–Kendall test (Kendall1975; Mann 1945) can test trends in a time series withoutrequiring normality or linearity (Wang et al. 2008) and is,therefore, highly recommended for general use by the WorldMeteorological Organization (Mitchell et al. 1966). It waswidely used in detection of trends in hydrological series (e.g., Gao et al. 2007; Zhang et al. 2008a). This paper also usedthe Mann–Kendall test method to detect trends within theprecipitation series.

4 Results

4.1 Annual variations in precipitation extremes

In terms of the largest 1-day precipitation, 11 out of 19stations showed increasing trends in the largest 1-day

Table 2 Definitions of the indices of precipitation extremes

Indices ofprecipitationextremes

Descriptions

Precipitation days Frequency of days with at least 2 mm ofprecipitation

Precipitation total Total precipitation of the rain days with atleast 2 mm of precipitation

Largest 1-day pre-cipitation

The maximum daily precipitation in 1 year,in summer, or in winter

Largest 5-day total Greatest precipitation sum for 5-day interval

a b

c d

Fig. 2 Spatial distribution ofannual trends of a largest 1-dayprecipitation, b largest 5-daytotal, c rain days (greater than orequal to 2 mm), and d precipi-tation intensity. Precipitation in-tensity is defined as the averageprecipitation of rain days withprecipitation greater than orequal to 2 mm. Filled triangledenotes significant increase,inverted filled triangle denotessignificant decrease, invertedunfilled triangle denotes notsignificant decrease, and unfilledtriangle denotes not significantincrease. The same symbols inthe following figures denote thesame meanings

56 Q. Zhang et al.

precipitation (Fig. 2a), and these stations are located mainlyin the middle and east parts of the Guizhou province. Withrespect to changes in the largest 5-day total, 12 out of 19stations indicated decreasing trends, although the increaseswere not significant at the 95% confidence level. Thus,Fig. 2a, b indicated no significant trends in the largest 1-and 5-day precipitation total. It was observed that all thestations studied in the study region showed decreasing rainydays, and only three stations showed significant decreasingrainy days (Fig. 2c). The precipitation intensity at most ofthe stations in the study region was increasing (Fig. 2d),specifically, 16 out of 19 stations, accounting for 84.2% ofthe total stations, showed increasing precipitation intensity.

4.2 Seasonal variations of precipitation extremes

Generally, heavy precipitation events in the Guizhou provinceoccur mainly in summer. Therefore, the changes in the largest1-day precipitation and the largest 5-day total should be thesame as annual variations. Thus, we did not analyze thechanging properties of these precipitation variables. Figure 3aindicated that the majority of stations showed increasingnumber of rain days. Fourteen out of 19 stations showedincreasing rainy days, accounting for 73.7% of the totalstations, and these stations were found mainly in the middleand east parts of the Guizhou province. With respect to theprecipitation intensity in summer (Fig. 3b), most of thestations displayed increasing trends. It should be noted herethat only three stations showed significantly increasing rainydays or precipitation intensity in summer.

As for the precipitation changes in winter, we analyzedthe largest 1-day precipitation, the largest 5-day total, raindays (greater than or equal to 2 mm precipitation), andprecipitation intensity (greater than or equal to 2 mmprecipitation). Figure 4 showed the spatial distribution oftrends in the changes of the largest 1- and 5-dayprecipitation total, rainy days, and precipitation intensity.In winter, the largest 1-day precipitation at 14 stations wasincreasing, but was not significant at the 95% confidencelevel; these stations accounted for 73.7% of the total

stations considered in the study and were found mainly inthe east, south, and north parts of the Guizhou province(Fig. 4a). Most of the stations showed increasing largest 5-day precipitation total (Fig. 4b), and these changes were notyet significant at the 95% confidence level. Figure 4b alsoindicated that the stations showing increasing largest 5-daytotal were observed mainly in the east, south, and northparts of the Guizhou province. It can be identified in Fig. 4cthat the increasing number of rainy days can be observed atall the stations. Increasing precipitation intensity can alsobe identified at most of the stations. Figure 4d demonstratedthat 18 out of 19 stations showed increasing precipitationintensity, and only one station showed significantly in-creasing precipitation intensity.

4.3 Cumulative departure changes of precipitation extremes

The areal average rainy days in the Guizhou provincewere increasing before the mid-1980s and were decreas-ing thereafter (Fig. 5a). Increasing rainy days wereobserved during 1990–2000, and decreasing rainy dayswere identified after 2000. Changes in rainfall amount(greater than or equal to 2 mm precipitation) displayedsimilar properties when compared to those of rainy days(Fig. 2). Rainfall intensity (greater than or equal to 2 mmprecipitation) was in slight increase during 1960–1980,decrease during 1980–1990, and increase again after 1990.Figure 5b displayed cumulative departure variations ofprecipitation variables defined by the 2-mm precipitationthreshold. Decreasing (decreasing) rainy days with pre-cipitation of greater than or equal to 2 mm precipitationwere detected during 1960–1990 (after 1990). Similarchanging characteristics can also be identified for therainfall intensity (greater than or equal to 2 mm precipi-tation). Figure 5c indicated that the number of rainy daysin winter was decreasing during 1960–1980 and wasincreasing during 1980–2005. However, consistently in-creasing rainy days were observed after 1990. Theprecipitation intensity in winter was decreasing during1960–1990 and was increasing during 1990–2005.

a bFig. 3 Spatial distribution oftrends of a rain days (greaterthan or equal to 2 mm) and bprecipitation intensity in sum-mer. The precipitation intensityin summer is defined as theaverage precipitation of the raindays with precipitation greaterthan or equal to 2 mm

Precipitation extremes in a karst region 57

4.4 Moisture flux and possible correlations with changesin precipitation extremes

The results of analysis indicated that a trend in daily rainfallvariance was related to a trend in large-scale moistureavailability (Goswami et al. 2006). Zhang et al. (2008b)also found relationships between moisture budget andprecipitation variations in the Yangtze River basin. Ouranalysis indicated that changes in precipitation extremesindicated enhanced precipitation extremes after 1990. Tofurther understand the possible causes behind the changingproperties of precipitation extremes, we analyzed the trendsin the moisture flux in the longitudinal and latitudinaldirections and also the difference between moisture fluxbefore and after 1990. Figure 6 displayed trends in theannual variations in moisture flux in the latitudinal andlongitudinal directions before and after 1990. Gray areasindicated the areas covered by significant trends. Figure 6indicated that the moisture flux in the latitudinal direction(the range of the study region can be referred to Fig. 1) wasincreasing before 1990, but was not significant (Fig. 6a). Themoisture flux in the latitudinal direction was in significantincreasing trend (Fig. 6b). Similar phenomena were identi-fied in terms of moisture flux changes in the longitudinal

direction (Fig. 6c, d). The moisture flux in the longitudinaldirection was increasing (Fig. 6c), but the increase wassignificant after 1990 (Fig. 6d). Figure 7 displayed cumula-tive departure variations in the areal average moisture flux,which showed that, after the 1980s, the areal averagemoisture flux was increasing.

Figure 8a, b displayed changes in the moisture flux insummer in the longitudinal and the latitudinal direction,respectively. Comparison between Fig. 8a and b indicatedthat the moisture flux before 1990 was increasing, but wasnot significant; the moisture flux after 1990 was insignificantly increasing trend. Different results can beobtained for the changes of moisture flux in the longitudinaldirection. The moisture flux in the longitudinal direction wasdecreasing both before and after 1990. Figure 8d indicatedthat part of the study region was dominated by significantlyincreasing moisture flux. The cumulative departure of theareal average moisture flux is displayed in Fig. 9 whichindicated that areal average moisture flux was decreasingbefore the end of the 1970s and was increasing thereafter.

Figure 10 illustrated changes in the moisture flux beforeand after 1990 in the latitudinal and longitudinal directions,respectively. It can be seen from Fig. 10a that the moistureflux in the study region was decreasing before 1990 and

a b

c d

Fig. 4 Spatial distribution of trends of a largest 1-day precipitation, b largest 5-day total, c rain days (greater than or equal to 2 mm), and dprecipitation intensity (greater than or equal to 2 mm) in winter

58 Q. Zhang et al.

turned into a significant increasing trend thereafter(Fig. 10b). The decrease can be identified in the moistureflux variations in the longitudinal direction (Fig. 10c, d). Thecumulative departure (Fig. 11) indicated an increasing trendin the areal average moisture flux in winter after the mid-1980. Moreover, we also analyzed the difference betweenmoisture flux before and after 1990, and the results aredemonstrated in Fig. 12. In terms of annual variations andchanges in summer, decreasing northward moisture fluxtransport can be observed (Fig. 12a, b). The study regionwas also characterized by a positive difference of moistureflux before and after 1990, indicating an increase in themoisture flux after 1990. In winter, however, the direction ofmoisture transport was not distinctly altered. Slightlyincreased moisture flux can still be identified.

5 Discussions and conclusions

The Guizhou province, the study region of this study, ischaracterized by typical karst geomorphology. The uniquegeographical and topographical characteristics of the studyregion, such as large terrain slopes, thin soil thickness, poorvegetation coverage, etc., leaded to fragile ecologicalenvironment, which is highly sensitive to weather extremesand precipitation extremes in particular. Frequent naturalhazards, such as flash floods, landslides, debris flow,droughts, and so forth, have caused tremendous loss ofhuman life and economy. Changing properties of precipi-tation extremes and precipitation intensity will exertconsiderable influences on hydrological processes, spatial,and temporal distribution of geologic hazards such as

1960 1970 1980 1990 2000 –50

0

50

100Rainy days (>=2mm)

1960 1970 1980 1990 2000 –500

0

500

1000Rainfall amount (>=2mm)

1960 1970 1980 1990 2000 –6

–4

–2

0

2Rainfall intensity (>=2mm)

(A)

1960 1970 1980 1990 2000 –600

–400

–200

0

200Rainy days (>=2mm)

1960 1970 1980 1990 2000 –20

–10

0

10Rainfall intensity (>=2mm)

(B)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 –20

–15

–10

–5

0

5Rainy days (>=2mm)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 –8

–6

–4

–2

0

2Rainfall intensity (>=2mm)

(C)

Fig. 5 Cumulative departure ofa the precipitation variables de-fined by the precipitationthreshold as 2 mm, b precipita-tion variables defined as precip-itation threshold as 2 mm insummer, and c rain days andprecipitation intensity in winter

Precipitation extremes in a karst region 59

landslides and debris flow. We analyzed changing charac-teristics of precipitation extremes and underlying causes byanalyzing moisture flux with NCAR/NCEP reanalysisdataset. We think it is the important step toward goodunderstanding of changes of weather extremes under theinfluences of global warming in the typical karst region ofChina.

The following important conclusions were obtained anddiscussed: (1) Analysis of precipitation extremes indicatedenhanced extreme precipitation in the Guizhou province,particularly in the middle and east parts. The enhancedprecipitation extremes are mainly reflected by decreasingrainy days and increasing precipitation intensity. In summer

(A) (B)

(C) (D)

Fig. 6 Spatial distribution of the annual trends of moisture flux (unit:kg/m·s). a Trends of moisture flux in latitudinal direction during1960–1990, b trends of moisture flux in latitudinal direction during

1991–2005, c trends of moisture flux in longitudinal direction during1960–1990, and d trends of moisture flux in longitudinal directionduring 1991–2005

1960 1970 1980 1990 20000

20

40

60

80

Time (years)

Moi

stur

e flu

x (k

g/m

.s)

Fig. 7 Cumulative departure of areal annual variations of moistureflux (unit: kg/m·s)

60 Q. Zhang et al.

and winter, rainy days were increasing and so were thevariations in precipitation intensity. From the viewpoint ofannual variations, rainy days were decreasing. Therefore,we can conclude that precipitation extremes in the Guizhouprovince were increasing with a shift of more precipitationto summer and winter. In summer, enhanced precipitationextremes were observed mainly in the middle and east partsof the Guizhou province; in winter, however, enhancedprecipitation extremes were identified mainly in the westand south parts of the Guizhou province. It should be notedhere that more high lands were found in the west parts thanin the east parts of the Guizhou province. In this case,enhanced precipitation extremes in winter may cause moreserious soil erosion in the west. Enhanced extreme

1960 1970 1980 1990 2000 –20

–10

0

10

20

30

Time (years)

Moi

stur

e flu

x (u

nit:

kg/m

.s)

Fig. 9 Cumulative departure of areal average moisture flux insummer (unit: kg/m·s)

(A) (B)

(C) (D)

Fig. 8 Spatial distribution of the trends of summer moisture flux (unit:kg/m·s). a Trends of moisture flux in latitudinal direction during 1960–1990, b trends of moisture flux in latitudinal direction during 1991–2005,

c trends of moisture flux in longitudinal direction during 1960–1990, andd trends of moisture flux in longitudinal direction during 1991–2005

Precipitation extremes in a karst region 61

precipitation was identified mainly after early 1990s, whichcould be attributed to influences of global warming; and (2)Studies show increasing extreme precipitation, particularlyover land areas in middle and high latitudes of the NorthernHemisphere, and a decrease in rainy days in the latitudinal beltaround 40°N during summer (Khon et al. 2007). Changes inprecipitation intensity would be attributed to relativecontributions of precipitation originating from convectivemechanisms (Gregory and Mitchell 1995). Under theincreasing CO2 scenarios, some global climate modelsdemonstrate enhanced midlatitude precipitation intensity(e.g., Osborn et al. 2000). Furthermore, altered atmosphericmoisture, temperature fields, and shifts in the strength ofAsian monsoon could have driven across the board changes

1960 1970 1980 1990 2000 –20

–10

0

10

20

30

Time (years)

Moi

stur

e flu

x (k

g/m

.s)

Fig. 11 Cumulative departure of areal variations of moisture flux inwinter (unit: kg/m·s)

(A) (B)

(C) (D)

Fig. 10 Spatial distribution of the trends of winter moisture flux (unit:kg/m·s). a Trends of moisture flux in latitudinal direction during 1960–1990, b trends of moisture flux in latitudinal direction during 1991–

2005, c trends of moisture flux in longitudinal direction during 1960–1990, and d trends of moisture flux in longitudinal direction during1991–2005

62 Q. Zhang et al.

in precipitation intensity, with no shift in the importance ofvarious mechanisms (Osborn et al. 2000). The trend analysisof moisture flux in the longitudinal and latitudinal directions,respectively, indicated significantly increasing moisture fluxafter the 1990s. The moisture flux was increasing before1990s, but the increase is not significant at the 95%

confidence level. The difference between the moisture fluxbefore and after the 1990s indicated more moisture flux afterthe 1990s and decreasing northward transport of moistureflux. Changes of moisture were in good line with those ofprecipitation extremes, corroborating considerable influencesof moisture flux on precipitation extremes. It should be

(A) (B)

(C)

Fig. 12 Spatial distribution of the moisture flux anomalies between 1991–2005 and 1961–1990 (unit: kg/m·s). a Annual, b in summer, and c inwinter

Precipitation extremes in a karst region 63

pointed out here that the results of this study indicatedenhanced precipitation extremes. More significant increaseof moisture flux after the 1990s may be the major drivingfactor triggering enhanced precipitation extremes after1990s. The results of this study will be of practicalsignificance in mitigation of the detrimental effects ofvariations of weather extremes, particularly in the Guizhouprovince characterized by the fragile ecological environment.

Acknowledgments The research was financially supported by theNational Basic Research Program (“973 Program”, grant number2006CB403200), National Natural Science Foundation of China(grant number: 40701015; 40771199), and by the “985 Project”(Grant No.: 37000-3171315). Thanks should be extended to theNational Climate Center and China Meteorological Administration,China for kindly providing the meteorological data. The last but notthe least, we are also indebted to two anonymous reviewers and themanaging editor, Dr. Hartmut Grassl, for their invaluable commentswhich greatly improved the quality of this paper.

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