Jinsong Wang 1, Suping Wang 1 *, Qiang Zhang 1, Yiping Li 1, Jing Wang 1, Jing Zhang 2

Transcription

1 Pol. J. Eviro. Stud. Vol. 24, No. 5 (2015), DOI: /pjoes/58764 Origial Research Characteristics of Drought Disaster-Causig Factor Aomalies i Southwester ad Souther Chia agaist the Backgroud of Global Warmig Jisog Wag 1, Supig Wag 1 *, Qiag Zhag 1, Yipig Li 1, Jig Wag 1, Jig Zhag 2 1 Istitute of Arid Meteorology, Chia Meteorological Admiistratio, Key Ope Laboratory of Arid Climatic Chage ad Disaster Reductio of CMA, Key Laboratory of Arid Climatic Chage ad Reducig Disaster of Gasu Provice, Lazhou ,Chia 2 Lazhou Cetral Meteorological Observatory, Lazhou , Chia Received: May 21, 2015 Accepted: July 6, 2015 Abstract I this study, takig drought disaster-causig factors like precipitatio, temperature, referece evapotraspiratio, ad the degree of drought severity based o the drought idex as study objects, the characteristics of these above elemets agaist the backgroud of global warmig were aalyzed by usig the method of climate statistics. The data iclude a variety of climate observatios of 129 meteorological statios i southwester ad souther Chia from 1961 to The results show that over the past half cetury sustaied temperature icrease has bee preseted i the study area agaist the backgroud of global warmig. However, there is still a sharp warmig poit (mutatio poit) of temperature time series that occurred i 1994 durig 1961 to 2012 accordig to the Ma-Kedal test. I other words, the past half cetury could be divided ito two episodes with the year 1994 as the boudary: the first period (i.e. the period before sigificat temperature rise) from 1961 to 1994, ad the secod period (i.e. the period after sigificat temperature rise) from 1995 to I order to highlight the effects that result from obvious warmig, a compariso of the above factors betwee the two episodes ad the tred of these factors i the two episodes are aalyzed. This shows that, durig the period after sigificat temperature rise, the study area has experieced a sigificat dowward tred i precipitatio ad a declie i referece evapotraspiratio, but a rise i its chage tred, ad more serious degree of drought severity due to the impact of less precipitatio ad higher temperatures. Keywords: drought disaster-causig factors, global warmig, aomaly, southwester ad souther Chia Itroductio Global warmig has bee a hot topic ad has draw widespread attetio of scietists. Multiple data aalysis results have verified global warmig i the 20 th cetury [1-4]. * wagjs02@lzu.edu.c The 20 th cetury witessed global warmig, which is idisputable, although whether it witessed the highest temperature i the recet milleium eeds to be further cofirmed [5]. Sice 1950, multiple observed chages have show that global warmig i the past 60 years is beyod doubt [6]. Global warmig has brought about severe evirometal problems, icludig a rise i sea level ad

2 2242 Wag J., et al. icreased atural hazards [7]. For example, Oerlemas [8] estimated that the total area of the world s moutai glaciers would decrease by 1/3-2/3 i the 21 st cetury as a result of global warmig. Agaist the backgroud of global warmig, extreme weather ad climate evets ad accompayig disasters have icreased i frequecy ad itesity. Losses caused by meteorological disasters especially droughts have cosequetly bee icreasig, seriously threateig the sustaiable developmet of the social ecoomy ad eviromet worldwide [9]. Dai [10] poited out that recet warmig has icreased atmospheric moisture demad ad likely altered atmospheric circulatio patters, both cotributig to the dryig. I the past decade droughts have occurred more frequetly over the world. Drought becomes a mai threat to crops, which compromises food productio capacity ad food security [11]. Research also shows that social, ecoomic, ad evirometal sustaiability ca be ehaced by disaster risk maagemet ad adaptatio approaches [12]. The icreased risk of drought duratio, severity, ad extet is a direct cosequece of global warmig [13]. Drought risk maagemet ca trasfer egative exposure ad passive adaptatio to positive prevetio ad proactive respose, favorig efficiet allocatio of drought cotrol resources ad elimiatio of the adverse effects of drought [14]. The drought hazards assessmet is essetial for makig mitigatio plas to reduce the impact of drought i Chia [15, 16]. As a foudatio for drought risk maagemet, drought hazard assessmet should make clear drought characteristics, icludig variatios i itesity caused by drought disaster-causig factors such as precipitatio, temperature, ad evaporatio, with precipitatio as the most direct. It is ecessary to aalyze characteristics of drought disaster-causig factors ad their combied effect so as to achieve drought risk maagemet ad ehace the sustaiable developmet of society, ecoomy, ad the eviromet. Northwester ad orther Chia have suffered log ad frequet drought i the past. However, i recet years southwester Chia ad the middle ad lower Yagtze River regios that used to have abudat precipitatio have experieced severe drought, causig eormous ecoomic losses [17-19]. Especially i southwester Chia, drought has bee icreasig ad the frequecy of extreme drought has rise sigificatly i the past 50 years [20-24]. The research of He et al. [20] also showed that southwester Chia is oe of the high drought hazards areas. Agaist the backgroud of global warmig, there is a upward tred i serious ad frequet drought, especially i the past decade i souther Chia [25, 26]. All of the above research idicate that drought has occurred frequetly ot oly i orther Chia, but also i the south. I fact, drought occurs over most parts of the world, eve i wet ad humid regios [10]. Although drought ca occur aywhere, a primary cause of dramatic icrease i drought i the past decade i southwester ad souther Chia may have some coectio with global warmig. Uder global warmig coditios, what are the evolutio characteristics of drought disaster-causig factors? I this paper, we take southwester ad souther Chia as the case area, beig the area of the greatest occurrece of drought i recet years. Rice plated i souther Chia accouts for 94% of the total i sow acreage ad 88% of the total i productio i Chia [11]. Although global temperature icreases have become proouced after the 1970s [27], resposes to global warmig i regios are differet. Degrees of global warmig experiece are obvious spatial differeces i regioal distributio [3]. This kid of differet regioal distributio i temperature is also reflected i Chia [28]. I order to aalyze evolutio characteristics of drought disaster-causig factors agaist the backgroud of global warmig, it is ecessary to first cosider the characteristics of resposes to global warmig of the study area. I this paper, the period from 1961 to 2012 has bee selected as the aalysis period, cosiderig the characteristics of time periods ad regioal differeces of respodig to warmig. This paper first aalyzed the temperature mutatio poit of the study area i the past half a cetury ad the gave the compariso results of the evolutio characteristics of drought disaster-causig factors betwee the period before ad after sigificat temperature rise. Data ad Method Descriptio Study Area ad Data The study area is situated i souther Chia (Fig. 1), icludig Sichua, Chogqig, Yua, Guizhou, Guagxi, ad Guagdog provices. This area is maily located i the subtropical climate zoe, except for orthwester Sichua ad the orthermost part of Yua, which are located i the plateau climate zoe. I additio, the souther part of Yua ad Guagdog provices also belog to a tropical climate zoe. O the whole, the precipitatio of the study area icreases from the orthwest to the southeast. The mea aual precipitatio varies from about 500 mm i the orthwest i Sichua to more tha 2,000 mm i Guagxi ad Guagdog alog the souther coast. It is worth otig that there is a high-value precipitatio area i cetral Sichua, where Emei Moutai is located, due to the moutaiprecipitatio effect. Drought hazards occur frequetly i the study area, with oe distict feature i recet years that droughts geerally exted over a large area ad they ca cotiue for moths or years, causig serious ad lastig impacts o society ad the ecoomy. This paper selected a variety of climate observatios from 129 meteorological statios i the study area (Fig. 1) durig 1961 to 2012, icludig mothly precipitatio ad mothly average temperature, daily maximum temperature ad miimum temperature, sushie hours, relative humidity, ad average wid speed. All the data i this paper are obtaied from the Natioal Meteorological Iformatio Ceter ad have passed quality cotrol.

3 Characteristics of Drought Disaster-Causig Climatic variability is actually a liear regressio coefficiet betwee climatic factor ad time usig the least square to estimate: k 1 ( x t ) ( i i i i i1 i1 i ti ( ti) i1 i1 x )( t ) (2) Fig. 1. The locatio of southwester ad souther areas i Chia, the spatial distributio of mea aual precipitatio (mm), ad 129 meteorological statios (solid dots) i the study area. Methods Ma-Kedall Method The Ma-Kedall tred test method (referred to as M-K) was adopted to detect sequece treds of climatic elemets ad to evaluate whether there is a sigificat discotiuity i data collected over a period of time. It is oe of the most widely applied tests to detect the tred ad abrupt chage of a climate variable. As a oparametric statistical test, this method is free from compliace with a certai distributio of samples ad outlier iterferece. Characterized by a wide rage of detectio, less artificiality, ad high quatificatio, focusig o climate sequece mutatios, M-K has bee widely adopted [29]. The positive value of the M-K test meas a icreasig tred, ad the egative value deotes a decreasig tred. The basic steps ca be foud i the literature [30]. Tred Coefficiet ad Variability This paper adopted tred coefficiet ad variability to represet the ature ad amplitude of climatic elemet chage, i other words icrease or decrease ad their respective degree expressed with magitude, i geeral, as far as precipitatio ad temperature are cocered, with the uit of mm decade -1 ad ºC decade -1. The tred coefficiet is calculated as follows: r xt i1 i1 x xi t 2 2 x x i t i i i1...where r xt is tred coefficiet, is the legth of temporal series (i.e. umber of year), x i is the value of climate factor i year i, x is the mea value of the samples, ad t =(+1)/2 [30]. (1)...where x i is temporal series of oe climatic factor, t i deotes the correspodig time for x i, is the quatity of samples (i.e. umber of year), ad k is variability [30]. Pema-Moteith Method Referece evapotraspiratio refers to the case of evapotraspiratio at the sufficiet soil moisture coditio, reflectig the uderlyig surface s evapotraspiratio ability determied by weather ad climate coditios servig as a mai factor to be cosidered i the aalysis of drought. Referece evapotraspiratio is geerally obtaied through estimatig, ad there are may methods for doig so. The FAO Pema-Moteith method was recommeded as the sole stadard method for the defiitio ad computatio of referece evapotraspiratio [31]. Also, accordig to the studies of Gao et al. [32], the FAO Pema-Moteith formula, which is based o the eergy balace ad water vapor diffusio theory, ca reflect the combied effects of various climatic elemets applicable to evapotraspiratio calculatios of areas of differet climate types. This paper adopted the FAO Pema-Moteith method to calculate referece evapotraspiratio. The specific formula of the FAO Pema-Moteith method as well as the selectio of some parameters ca be foud as follows: The equatio of referece evapotraspiratio from Alle et al. [31] is defied as: ET ( R G) U T 273 (1 0.34U )...where ET 0 is the referece evapotraspiratio (mm day -1 ), R is the et radiatio at the crop surface (MJ m -2 day -1 ), G is the soil heat flux desity (MJ m -2 day -1 ), T is the mea daily air temperature at 2 m height (ºC), U 2 is the wid speed at 2 m height (m s -1 ), e s is the saturatio vapour pressure (kpa), e a is the actual vapour pressure (kpa), e s e a is the saturatio vapour pressure deficit (kpa), Δ is the slope of saturatio vapour pressure curve at air temperature T (kpa ºC -1 ), ad γ is the psychrometric costat (kpa ºC -1 ). Sigificatly, as the magitude of the day soil heat flux beeath the grass referece surface is relatively small, it may be igored ad thus: (3) G 0 (4) For the other details, expressios are give by: 2 2 e e s a

4 2244 Wag J., et al. 4 4 Table 1. The categories of drought severity about K idex. T max, k Tmi, k R (1 0.23) Rs Category Drought descriptio Percetile chace (%) 2 1 No drought > 30 (5) Rs ( ea )( ) 2 Icipiet drought 15 to 30 Rso 3 Moderate drought 5 to 15 e s e T ) e ( T 2 ( max mi ) 4 Severe drought 2 to 5 (6) 5 Extreme drought 2 U e T ( T) exp T U e 10 a RH 100 mea...where σ is Stefa-Boltzma costat (MJK -4 m -2 day -1 ), T max,k ad T mi,k are the maximum ad the miimum daily air temperatures i kelvi uits at 2 m height (K), respectively; R s is the solar or shortwave radiatio (MJ m -2 day -1 ); R so is the clear-sky solar radiatio (MJ m -2 day -1 ); T max ad T mi are the maximum ad the miimum daily air temperature at 2 m height (ºC) respectively; RH mea is the daily relative humidity (%); ad U 10 is the wid speed at 10 m height (m s -1 ). K Drought Idex Calculatio The degree of drought severity is expressed with K drought idex, which has bee well applied i orthwester Chia ad the Yellow River basi [33, 34], while its applicatio i souther Chia is aalyzed i aother paper [35]. I the above literature, comparative aalysis of drought moitorig effects of differet drought idices highlighted the advatages of K drought idex i drought moitorig of differet areas. The K drought idex is defied as: K R / E i, j i, j i, j (7) (8) (9) (10)...where K i,j is the K drought idex; R i,j ad E i,j are the relative variability rate of precipitatio ad referece evapotraspiratio i a appoited period time, respectively; i deotes the year umber, ad j deotes the meteorological statio umber. The R i,j is give by: R R R i, j i, j/ pj (11)...where R i,j is the precipitatio i a appoited period time ad R pj is the latest 30-year average value of precipitatio (i.e. ormal climate) of a appoited time period. e 4.87 l( ) s The E i,j is give by: E E E i, j i, j/ pj (12)...where E i,j is the referece evapotraspiratio i a appoited period ad E pj is the latest 30-year average value of referece evapotraspiratio (i.e. climate ormal) of a appoited period. Followig the approach of Svoboda et al. [36], the classificatio of drought severity of K idex is divided ito five levels accordig to the percetile method (Table 1). Each category is associated with its percetile chace of happeig i ay give year out of the study period from 1961 to Results ad Aalysis Characteristics of Resposes to Global Warmig i Southwester ad Souther Chia Usig the M-K method, this paper tested the tred ad mutatio of the aual average temperature sequece of 129 meteorological statios i southwester ad souther Chia durig 1961 to As show i Fig. 2, the overall temperature of the study area has icreased i the past half cetury. Before 1970, fluctuatios appeared i temperature, with alterate rises ad falls i iter-aual terms. After 1970, temperature experieced a rise i the fluctuatio situatio with the latter particularly sigificat after Clearly, the respose to global warmig of the study area is obvious, amely cosistet with the global average temperature rise. Although Fig. 2 shows that the past half cetury has displayed a costat temperature rise i southwester ad souther Chia, a mutatio occurred i 1994 (UF ad UB itersectio i Fig. 2) ad a more sigificat temperature rise after 1994, servig as a period with sigificat temperature rise i the past half cetury. Takig 1994 as a boudary, we divided the period of 1961 to 2012 ito two episodes: the first from 1961 to 1994 (i.e., the period before sigificat temperature rise) ad the secod from 1995 to 2012 (i.e., the period after sigificat temperature rise). Thus, i order to highlight the impact of sigificat warmig, the followig will use cotrastive aalysis to study the ew evolutio features of drought disaster-causig factors uder climate warmig ad describe the impact of the ew characteristics o the developmet of drought.

5 Characteristics of Drought Disaster-Causig Ma-Kedall statistic Fig. 2. The aual mea temperature of southwest ad south Chia betwee 1961 ad 2012 by the Ma-Kedall test. The dashed at y= ±1.96 are the sigificace stadard borderlies of α=0.05. UF: aalysis of the temperature aomaly values from 1961 to UB: aalysis of the temperature aomaly values from 2012 retrograde to Graphical aalysis is coducted for UF ad UB to idetify the itersectio of the curves, thus allowig detectio of the iitiatio of a tred or chage. The poit of itersectio of the two curves UF ad UB i 1994 idicates iitiatio of a sharp warmig tred. Aalysis of Drought Disaster-Causig Factor Characteristics before ad after Sigificat Temperature Rises The direct cause for drought occurrece (or relief) is the shortage (or icrease) of precipitatio. I case the icrease i precipitatio fails to offset the icrease i evaporatio, it will be difficult to relieve the drought. I the case of a decrease i precipitatio accompaied by a icrease i evaporatio, it will exacerbate drought. Therefore, i cosiderig drought disaster-causig factors, precipitatio ad evaporatio should be the most basic elemets. To highlight the impact of temperature rise, as described i sectio 3.1, this paper took the year 1994 as the boudary to divide the period from 1961 to 2012 ito 2 periods: the first period (1961 to 1994), ad the secod period (1995 to 2012), with the first as the pre-sigificat temperature rise period ad the secod as the post-temperature rise period, aalyzig the differeces betwee aual precipitatio, temperature, ad referece evapotraspiratio of the two periods so as to explore the variatio characteristics of drought disaster-causig factors after the sigificat temperature rise ad the impact of sigificat temperature rise o drought disaster-causig factors. of the study area clearly experieced less precipitatio after the temperature rise. Fig. 4 shows the precipitatio variatio treds of the study area before ad after the period of sigificat temperature rise. I terms of distributio, the rage of precipitatio decrease was more tha that of precipitatio icrease before the period of sigificat temperature rise (Fig. 4a). However, the precipitatio decrease tred of most areas failed to pass the cofidece level test with credibility of 95%. Compared with the period prior to the sigificat temperature rise, precipitatio variatio i most areas i the period after the sigificat temperature rise showed a dowward tred (Fig. 4b), except for orthwest Yua, ortheast Chogqig, ad souther Sichua ad Guagdog, which witessed the cotrary, with ortheast Sichua experiecig a slight rise, cosistet with the precipitatio variatio i the period prior to the sigificat temperature rise. Differet characteristics lie i the fact that after the sigificat temperature rise, a dowward tred i precipitatio i most areas passed the cofidece level test with credibility of 95%. Precipitatio i a small area i ortheast Sichua experieced a obvious icrease. Variatio Characteristics of Precipitatio before ad after Sigificat Temperature Rise Most areas of southwester ad souther Chia experieced less aual average precipitatio durig the period after sigificat temperature rise from 1995 to 2012 tha that durig the period before sigificat temperature rise from 1961 to 1994 (Fig. 3), ceterig o cetral ad souther Sichua, orth Chogqig, east Yua, west Guizhou, cetral Guagdog, orth ad southwest Guagxi, with decrease values from 120 to 220 mm, while west Sichua, cetral Yua, east Guagxi, ad south ad orth Guagdog saw a certai icrease. Fig. 3 shows that most Fig. 3. Differece of mea aual precipitatio betwee periods of ad i southwester ad souther Chia (uit: mm).

6 2246 Wag J., et al. was clearly greater tha that after the sigificat temperature rise, idicatig that over the past half cetury, the overall temperature rise of the study area maily depeded o a sigificat temperature rise after We ca see from Fig. 5a that after the sigificat temperature rise, ortheast Sichua, southwest Guagxi, ad most parts of Guagdog illustrated a temperature drop. Ad the rest of the areas displayed a obvious temperature rise. The differece i temperature durig 1995 to 2012 ad 1961 to 1994 (Fig. 6) showed the temperature i the whole area as a positive value, demostratig that the temperature of southwest ad south Chia was clearly higher i the period after sigificat temperature rise tha that i the period before sigificat temperature rise, idicatig that southwest ad south Chia have experieced serious impacts of global warmig sice Referece Evapotraspiratio Variatio before ad after the Sigificat Temperature Rise Fig. 4. Tred ratio of aual precipitatio durig the period before (a) ad after (b) 1994 i southwester ad souther Chia (uit: mm decade -1 ). (Symbols,, ad i figures mea the tred ratio passed the sigificat test at 0.001, 0.01, ad 0.05 levels, respectively.) It is show i Fig. 7a that there is a dowward tred of referece evapotraspiratio i the period before sigificat temperature rise i southwest ad south Chia, except a small area of southwest ad orth Sichua ad west Yua, ad the dowward tred of referece evapotraspiratio i most areas passed the cofidece level test with However, all other areas of southwest ad south Chia witessed a decrease, ceterig o orth Guagxi (as little as 450mm decade -1 ), ad the south Sichua, east-cetral ad south Yua, ad west ad orth Guizhou, as little as mm decade -1, i other words, after the sigificat temperature rise, precipitatio i most areas of southwest ad south Chia experieced a sigificat decrease. Aalysis i combiatio with Figs. 3 ad 4 shows that temperature rise made the precipitatio decrease more sigificat i the study area. East Yua, west Guizhou, ad orth Guagxi experieced a large precipitatio decrease amplitude ad a obvious dowward tred i precipitatio, i other words makig these areas show the most serious precipitatio decrease. Distributio of Average Temperature before ad after Sigificat Temperature Rise Over the past half cetury, the average temperature of the study area has show a rise as a whole, with mutatio i 1994 ad the illustratig a sigificat temperature rise (Fig. 2). However, i terms of the spatial variatio distributio before ad after the sigificat temperature rise (Fig. 5), i either case temperature i some areas also showed a dowward tred. The rage of the dowward tred of temperature before the sigificat temperature rise Fig. 5. Tred ratio of aual mea temperatures durig the period before (a) ad after (b) 1994 i southwester ad souther Chia (uit: ºC decade -1 ). (Symbols,, ad i figures mea the tred ratio passed the sigificat test at 0.001, 0.01, ad 0.05 levels, respectively.)

7 Characteristics of Drought Disaster-Causig distributio of the areas with larger referece evapotraspiratio determied the distributio rage of referece evapotraspiratio rise of the study area over the past half a cetury. However, most of the areas i Fig. 8 showed a egative value, idicatig that the average aual referece evapotraspiratio i the period after the sigificat temperature rise, i most areas of southwest ad south Chia, was less tha that of the period before the sigificat temperature rise. Fig. 8 shows the drought magitude decreased by mm, ceterig o orthwest Sichua, which decreased by 110 mm. Fig. 6. Differece of average aual mea temperatures betwee periods of ad i southwester ad souther Chia (uit: ºC). credibility of 99%, presetig that before the sigificat temperature rise, referece evapotraspiratio had sigificatly dropped i the study area, with the largest decreasig value of 50 mm decade -1, ceterig o the easter part of the study area. I the period after sigificat temperature rise (Fig. 7b), except for a few places, the referece evapotraspiratio variatio experieced growth that passed the cofidece level test with credibility of 95%, ad 99% i ortheast ad west Yua ad south Guagxi, showig a sigificat temperature rise i referece evapotraspiratio variatio, with the largest value reachig 90 mm decade -1, ceterig o Guagxi i the west ad Yua i the east of the study area ad orth Guagdog. Over the past half cetury, referece evapotraspiratio has geerally witessed a dowward tred (Fig. 7c) except i southwest Yua, southwest ad orth Sichua, ad east Guagdog displayig a upward tred, i other words, the cotiual temperature rise over the past half cetury has failed to icrease referece evapotraspiratio i southwest ad south Chia. Accordig to the study results of Re et al. [37], the direct ifluecig factors o evaporatio are elemets such as sushie hours, average wid speed, ad daily rage of temperature evaporatio, ad the the sigificat temperature rise. Liu et al. [38] poited out that the observed pa evaporatio ad simulated actual evaporatio also demostrated a dowward tred from 1960 to 2005 i southwest ad south Chia. Accordig to the comparative aalysis betwee the results of this paper ad that of the previous research, we ca see that referece evapotraspiratio variatio characteristics of the study area over the past half cetury obtaied by this paper through aalysis are cosistet with the results obtaied by previous studies. As ca be see from Fig. 8, the differece of referece evapotraspiratio betwee the period before ad after sigificat temperature rise was positive i southwest Yua (borderig Yua ad Sichua), ad east ad south Guagdog, with its distributio basically cosistet with the rage of referece evapotraspiratio rise over the past half a cetury (Fig. 7c), idicatig that i the period after 1994, amely the sigificat temperature rise period, the Fig. 7. Tred ratio of aual referece evapotraspiratio durig the period before (a) ad after (b) 1994, ad (c) from 1961 to 2012 i southwester ad souther Chia (uit: mm decade -1 ). (Symbols,, ad i figures mea the tred ratio passed the sigificat test at 0.001, 0.01, ad 0.05 levels, respectively.)

8 2248 Wag J., et al. Fig. 8. Differece of mea aual referece evapotraspiratio betwee ad i southwester ad souther Chia (uit: mm). Degree of Drought Severity Evolutio Characteristics before ad after Sigificat Temperature Rise Global warmig ot oly raises temperatures, but also ehaces dryig ear the surface, as is captured by the PDSI [39]. How about the drought evolutio i southwest ad south Chia agaist global warmig? Based o the K drought idex, this paper calculated the mothly K value ad aalyzed the variatio tred of the degree of drought severity from Jauary to December i the period before ad after sigificat temperature rise (Fig. 9). The greater the drought grade is, the higher the degree of drought severity. The positive variatio i the figures expresses a icreasigly high degree of drought severity. The differece betwee the degree of drought variatio treds before ad after sigificat temperature rise was characterized by the factors outlied below. Accordig to the characteristics of temporal chage, we ca see from Fig. 9 that there has bee a marked shift i drought itesity betwee the two episodes, amely the period after 1994 (i.e. the period after sigificat temperature rise) ad the period before 1994 (i.e. the period before sigificat temperature rise). These distict differeces were demostrated i all moths except April ad September, which oly showed a slight differece, with February, March, July, August, November, ad December showig the greatest chage. The degree of drought became more severe i most areas after the temperature rise, ceterig o February, March, July, ad August i the souther part of the study area. As far as November ad December are cocered, the degree of drought was more serious i the orther ad wester parts of the study area. I Jauary, May, Jue ad October, drought itesity variatio was sigificatly differet before ad after sigificat temperature rise ad the distributio of drought aggravatio ad mitigatio was also differet. No cosistet distributio variatio rule was available. From the characteristics of spatial chage, we ca also see from Fig. 9 that there was a upward tred i drought itesity i most parts of the study area after the sigificat temperature rise. There are differet features of the chage i the tred i the degree of drought severity i differet regios i the study area. I Sichua ad Chogqig, drought itesity showed a upward tred all year roud after the sigificat temperature rise, except for May ad July. I Yua, drought itesity showed a marked icrease i all moths after the sigificat temperature rise except for Jauary ad April. I Guizhou, drought itesity experieced a chage i the tred upwards i all the moths after the sigificat temperature rise except for April ad December. I Guagxi, drought itesity rose i all moths after the sigificat temperature rise except for Jue, November, ad December. I Guagdog, drought itesity showed a dowward tedecy after the sigificat temperature rise i Jauary, May, Jue, October, ad November, ad upward i the rest of the moths. So we ca see from the above aalyses that the moths with sigificat marked icreases i drought itesity are differet for the differet areas. However, it was commo that after the sigificat temperature rise i February, March, July, ad August the degree of drought severity was aggravated obviously i Yua, Guizhou, Guagxi, ad Guagdog, which is located i the south of the study area. Ad i Jauary, November, ad December, the degree of drought became more serious i Sichua ad Chogqig all located i the orther part of the study area. The above-metioed differet distributio of the degree of drought severity i the differet regios agaist the backgroud of global warmig revealed that a chage i the tred of the degree of drought severity is distict i orther ad souther regios i the study area. Although regioal differeces i the degree of drought severity exist, the uderlyig coditios of warmig more easily leads to droughts. This result is cosistet with the result obtaied by Sog et al. [40]. Coclusios The above cotrastive aalysis o drought disastercausig factors of the study area, icludig precipitatio, temperature, referece evapotraspiratio, ad drought itesity, variatio characteristics before ad after the sigificat temperature rise, idicate that the sharp rise of temperature has sigificatly decreased precipitatio, ceterig o east Yua, west Guizhou, ad orth Guagxi, ad makig referece evapotraspiratio magitude decrease ad variatio tred icrease i the period after the sharp temperature rise. The itesity variatio tred of the degree of drought severity expressed by the drought idex idicates that drought becomes serious with temperature rise. The basic defiitio of the drought idex shows that drought will be further aggravated uder the impact of less precipitatio ad higher temperatures. It should be emphasized that multiple research results show that agaist the backgroud of a rise i temperature, referece evapotraspiratio variatio has decreased [32, 37, 38]. Aalysis results of this paper also poit out that the referece evapotraspiratio variatio tred over the past half cetury has decreased, which is cosistet with the

9 Characteristics of Drought Disaster-Causig Fig. 9. Tred ratio of mothly degree of drought severity defied by K drought idex durig the period before (a1-a12) ad after (b1-b12) 1994 i southwester ad souther Chia. The symbols a1 to a12 o the top right corer of the figures idicate the differet moths i the period durig 1961 to 1994 (a1 meas i Jauary, a2 meas i February, a3 meas i March, etc.). The symbols b1 to b12 are the same as a1 to a12, but for 1995 to Red shade: the degree of drought severity has aggravated the tred. Blue shade: the degree of drought severity has alleviated the tred. Symbols,, ad i figures mea the tred ratio passed the sigificat test at 0.001, 0.01, ad 0.05 level, respectively.

10 2250 Wag J., et al. results obtaied by the previous studies. However, i the warmer period after 1994, the referece evapotraspiratio tred i the study area icreased, which is icosistet with the results obtaied by previous studies. We thik that two issues exist: Firstly, although the impact of the cotiual sigificat temperature rises o evaporatio is secodary [37], the referece evapotraspiratio icrease idicated i the study area origiates from the warmer period after the temperature sharp warmig poit. So, will the more dramatic temperature rise have a importat effect ad ifluece o referece evapotraspiratio? This problem should be discussed i the future. Secodly, this article reaches the coclusio that referece evapotraspiratio icreases i the sigificat temperature rise period, which may be closely related to the study time iterval. To highlight the temperature rise ifluece based o the M-K test, this paper foud the temperature mutatio poit occurred i Takig the year 1994 as the boudary, this paper took the period durig 1995 to 2012 as the sigificat temperature rise period. Thus, the referece evapotraspiratio i this paper refers to the sigificat temperature rise period, amely the period durig 1995 to 2012, i other words a very limited period. It is ecessary to coduct further aalysis with the follow-up data so as to determie the credibility of the upward tred i referece evapotraspiratio agaist the backgroud of a sharp rise i temperature. This problem has yet to be studied further. Ackowledgemets The authors are grateful to the Natioal Meteorological Iformatio Ceter of CMA for providig the data used i this study. We also thak the reviewers ad editors for helpful commets ad suggestios that improved the mauscript. 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