Theor Appl Climatol DOI 10.1007/s00704-013-1073-z ORIGINAL PAPER Evolution of meteorological drought characteristics in Vietnam during the 1961–2007 period Hang Vu-Thanh & Thanh Ngo-Duc & Tan Phan-Van Received: April 2013 / Accepted: December 2013 # Springer-Verlag Wien 2013 Abstract The drought conditions over the seven subclimatological regions in Vietnam are examined using three meteorological drought indices: de Martonne J, PED, and Standardized Precipitation Index (SPI) According to the seasonal probabilities of drought occurrence estimated by the de Martonne index, droughts mainly occur between November and March in all the sub-regions The PED index and the SPI index generally show high probabilities of drought occurrence from April to August and from May to October, respectively In the southern sub-regions of Vietnam, droughts more frequently occur in El Niño years and wet conditions are more frequently observed in La Niña years However, such El Niño–Southern Oscillation influences are not clearly observed in the northern sub-regions During 1961–2007, droughts significantly increased in the northern part of Vietnam In the southern regions, PED shows increasing drought conditions while J and SPI show decreasing drought trends for almost all the stations Introduction Drought is a temporary natural disaster, which originates from a lack of precipitation over an extended period compared to longterm average conditions and can bring significant economic losses Generally, droughts can be classified into four types: (1) meteorological drought; (2) agricultural drought; (3) hydrological drought; and (4) socioeconomic drought (Heim 2002) Among them, meteorological droughts are usually related to the deficiencies of precipitation over a specific region Although it is still impossible to avoid meteorological droughts, they can be H Vu-Thanh (*) : T Ngo-Duc : T Phan-Van Department of Meteorology, Hanoi College of Science, Vietnam National University, 334 Nguyen Trai, ThanhXuan, Hanoi, Vietnam 10000 e-mail: hangvt@vnu.edu.vn predicted and monitored so that their impacts can be alleviated Assessing and monitoring droughts are normally performed using drought indices A number of drought indices have been developed and widely used to date Drought indices are usually defined as functions of rainfall and/or river discharge or other measurable hydrometeorological variables Among them, rainfall is the main factor controlling the drought formation and its duration (Palmer 1965) Oladipio (1985) showed that the indices that were based only on precipitation data performed well when compared to more complex hydrological ones Friedman (1957) identified four basic criteria that any drought index should meet: (1) the timescale should be appropriate to the problem at hand; (2) the index should be a quantitative measure of large-scale, longcontinuing drought conditions; (3) the index should be applicable to the problem; (4) a long accurate past record of the index should be available or computable In operational drought monitoring, one should add a fifth criterion: (5) the index should be able to be computed on a near real-time basis Several reviews of drought indices can be found in Alley (1984), Wu et al (2001), and Smakhtin and Hughes (2004) The inherent complexity of drought phenomena implies that no drought index is ideal for all regions (Morid et al 2006) Therefore, to assess the drought conditions of a specific region, it is useful to consider different indices The choice of drought indices is normally based on the climate data availability and the index ability in detecting drought characteristics (Morid et al 2006) During an El Niño–Southern Oscillation (ENSO) event, droughts can occur virtually anywhere in the world in varying degrees of magnitude The impacts of ENSO on meteorological quantities and drought indices around the globe have been reported in several previous studies (e.g., Ropelewski and Halpert 1987; Rajagopalan et al 2000; Lyon 2004) For example, Ropelewski and Halpert (1987) pointed out different regions in the world that are prone to droughts during ENSO They concluded that ENSO events seem to have a stronger influence on H Vu-Thanh et al regions in the lower latitudes, especially in the equatorial Pacific and bordering tropical areas Juneng and Tangang (2005) showed that the Southeast Asia rainfall anomalies depend strongly on phases of ENSO, and those anomalies are linked with the evolution of sea surface temperature anomalies as well as with the anomalous low-level circulation Lyon (2004) also showed that severe droughts developed during El Niño events in the tropics and the severity and extent of tropical droughts clearly have a relationship to the strength of the El Niño events The climate of Vietnam is strongly affected by monsoons and its complex topography Based on the differences in radiation, temperature, and rainfall conditions, Vietnam is classified into seven sub-regions, consisting of the North West (B1), North East (B2), North Plain (B3), North Central (B4), South Central (N1), Central Highland (N2), and the South Plain (N3) regions (Fig 1) (Phan et al 2009) Over the sub-regions, the rainy season usually starts in May and ends in December The total precipitation amount in the rainy season accounts for approximately 75–85 % of the annual precipitation (Ngu and Hieu 2004) The average annual rainfall over the country is approximately 1,400–2,400 mm and can vary from 700 up to 5,000 mm, depending on each region The linkages between ENSO and some climate elements in Vietnam have been documented Yen et al (2011) showed that Central Vietnam has less (more) rainfall in El Niño (La Niña) years Nguyen-Thi et al (2012) analyzed tropical cyclone rainfall ratio and concluded that during El Niño (La Niña) years, the tropical cyclone rain ratio in Central Vietnam has a significant decrease (increase) in October–November Presently, there are few publications dealing with the drought characteristics in Vietnam This study focuses on analyzing variations and trends of droughts in the seven subregions in Vietnam during 1961–2007 Performance of different indices in representing drought conditions is examined Besides, the influences of ENSO on droughts in Vietnam are discussed Data and drought indices 2.1 Data Daily near surface temperature and rainfall data of 50 meteorological stations during 1961–2007 were provided by the National Hydro-Meteorological Service of Vietnam Locations of the stations are displayed in Fig A list of the seven sub-regions and their stations are described in Table Missing data is replaced by −99.00 and is excluded from the calculation 2.2 Drought indices In this study, three drought indices based on temperature and precipitation data are used Details of the indices are described below (1) The de Martonne J index (de Martonne 1926) The de Martonne index (mm/°C) for monthly values is defined as 24N B2 22N B1 J¼ B3 20N B4 m 18N 3000 2500 2000 16N 1500 N1 14N 500 12N 10N N3 104E 106E 108E ð1Þ where P and T are monthly total precipitation (mm) and monthly mean near surface air temperature (°C), respectively To estimate the annual values of J, the below expression is applied: Jẳ Pa ; T a ỵ 10ị 2ị 1000 N2 8N 102E 12P ; T ỵ 10ị 110E 112E Fig Spatial distribution and topography (m) of stations included in the analysis in the seven sub-regions in Vietnam where Pa and Ta are annual total precipitation (mm) and annual mean near surface air temperature (°C), respectively The J index was introduced by de Martonne (1926) to characterize the aridity of a given climate Low values of J represent dry conditions while higher values represent wet conditions When J is less than 30, moderate drought conditions can be observed A value of less than 20 for J typically indicates severe drought (2) The Standardized Precipitation Index (SPI) (McKee et al 1993) Evolution of meteorological drought characteristics in Vietnam Table List of the meteorological stations in the seven sub-regions in Vietnam Table (continued) Number Latitude (N) 108.43 107.80 11.95 11.47 106.67 107.08 105.78 105.08 105.28 106.60 10.82 10.33 10.03 10.00 9.17 8.23 20.0 b Nov Dec Oct Aug Sep Jul Jun May 0.0 Ped index 35.0 B1 B2 B3 B4 N1 N2 N3 30.0 25.0 20.0 c Nov Dec Oct Sep Aug Jul Jun 15.0 SPI index 20.0 B1 B2 B3 B4 N1 N2 N3 15.0 10.0 5.0 Nov Dec Oct Sep Aug Jul 0.0 Jun 14.33 13.98 13.42 12.68 12.00 40.0 Apr 10.52 60.0 May 37 Phuquy 108.93 Sub-region 6: Central Highland (N2) 38 Kontum 107.62 39 Pleiku 108.00 40 Ayunpa 108.43 41 Bmthuot 108.05 42 Daknong 107.68 B1 B2 B3 B4 N1 N2 N3 May 16.03 15.35 14.77 13.77 13.08 12.25 10.93 J index 80.0 Mar 108.18 108.22 108.72 109.22 109.28 109.20 108.10 a 100.0 Apr 20.37 19.82 19.28 18.67 18.35 18.18 17.47 16.83 16.40 16.15 ð3Þ Apr 105.12 105.77 104.43 105.67 105.90 105.70 106.62 107.83 107.68 107.72 P−P ; σ Feb 21.01 20.82 20.48 20.27 20.26 20.13 SPI ¼ Mar 105.48 105.33 106.38 105.98 106.09 107.72 Sub-region 4: North Central (B4) 20 Hoixuan 21 Thanhhoa 22 Tuongduong 23 Vinh 24 Hatinh 25 Huongkhe 26 Donghoi 27 Dongha 28 Hue 29 Namdong Sub-region 5: South Central (N1) 30 Danang 31 Tramy 32 Bato 33 Quynhon 34 Tuyhoa 35 Nhatrang 36 Phanthiet Longitude (E) The formula of SPI is described as Mar 22.82 22.48 22.33 21.83 21.58 21.49 21.42 20.98 20.95 Jan 104.98 104.08 103.83 106.77 105.83 105.13 104.52 107.77 107.07 Feb 20.85 Tansonhoa Vungtau Cantho Rachgia Camau Condao Feb 104.63 45 46 47 48 49 50 Jan 22.05 21.35 21.33 43 Dalat 44 Baoloc Sub-region 7: South Plain (N3) Jan 103.15 103.00 103.90 Mocchau Sub-region 2: North East (B2) Hagiang Bacquang Sapa Langson Thainguyen 10 Tuyenquang 11 Yenbai 12 Coto 13 Baichay Sub-region 3: North Plain (B3) 14 Lang 15 Hoabinh 16 Phulien 17 Ninhbinh 18 Namdinh 19 Bachlongvi Stations Latitude (N) Probability (%) Sub-region 1: North West (B1) Laichau Dienbien Sonla Longitude (E) Probability (%) Stations Probability (%) Number Fig Seasonal occurrence probability of drought conditions estimated by the a J index, b PED index, and c SPI index in the seven sub-regions in Vietnam for 1961–2007 H Vu-Thanh et al a J index Probability (%) Fig Occurrence probability of drought conditions estimated by the annual a J index, b PED index, and c SPI index in the seven subregions in Vietnam for 1961– 2007 100.0 80.0 60.0 40.0 20.0 0.0 B1 B2 B3 B4 N1 N2 N3 SD 0.0 0.0 0.4 MD 0.6 1.3 6.7 0.3 0.6 0.0 0.4 2.0 16.7 3.0 1.5 W 99.4 98.7 93.0 97.7 82.8 97.0 98.1 b PED index Probability (%) 100.0 80.0 60.0 40.0 20.0 0.0 B1 B2 B3 B4 N1 N2 N3 SD 4.9 4.2 4.0 3.2 0.0 1.8 1.2 MD 17.5 19.8 19.1 21.8 24.4 19.3 19.7 W 77.5 76.0 76.9 75.0 75.6 78.9 79.1 c SPI index Probability (%) 100.0 80.0 60.0 40.0 20.0 0.0 SD B1 B2 B3 B4 N1 N2 N3 6.0 6.1 2.5 4.6 2.0 5.3 7.3 MD 8.7 9.7 11.9 10.8 11.9 10.1 8.1 W 85.3 84.2 85.6 84.6 86.2 84.6 84.6 where P and are precipitation and average precipitation (mm) in a given time period, respectively, and σ is the standard deviation of precipitation According to Eq (3), SPI can quantify the degree of wetness in different time periods (e.g., month, months, months, year, years, and so on) depending on user application The SPI value normally ranges from (−2.0) to (+2.0) A value of (+2.0) indicates extremely wet conditions; (1.5) to (1.99) indicates very wet; (1.0) to (1.49) moderately wet; (0.99) to (−0.99) near normal; (−1.0) to (−1.49) moderately dry; (−1.5) to (−1.99) severely dry; and an SPI value of (−2.0) or (less) indicates extremely dry conditions (McKee et al 1993) (3) The PED index (Ped 1975) The PED index is defined as PED ẳ T P ; T P 4ị where ΔT and ΔP are anomalies of near surface air temperature (°C) and precipitation (mm) in a given time period and σT and σP are standard deviations of temperature and precipitation, respectively A negative PED value represents a wet period An insignificant drought may occur if PED is between and When PED is between and 3, a moderate drought Evolution of meteorological drought characteristics in Vietnam occurs A greater value of PED (PED>3) indicates severe drought conditions Results and discussions 3.1 Seasonal drought conditions The probabilities of drought occurrence estimated with the three indices in the seven sub-regions in Vietnam during 1961–2007 are represented in Fig According to the J index (Fig 2a), the occurrence probabilities of drought conditions over Vietnam are highly concentrated from November to March, which is in accordance with the dry seasons in almost all sub-regions The high drought probabilities by more than 90 % are found in January in N2 and N3 It is due to little rainfall at some stations during this month Except for B4 and N1, drought occurrence has a low probability from May to September compared to other months During summer, Central Vietnam, including the B4 and N1 sub-regions, experiences the foehn effect leading to more frequent hot days and aridity According to the PED index (Fig 2b), the occurrence probabilities of drought conditions are smaller compared to those of the J index These values are in the range of over 15 % to more than 30 % It can be seen that the high probabilities of drought occurrence are found in April except for B2 and B3 sub-regions During the period of June–August, the probabilities of drought occurrence vary from 25 to 30 % and have no big differences between sub-regions The lower values are normally seen in November and December in almost all subregions The probabilities of drought occurrence according to the SPI index are smaller to those of the J and PED indices (Fig 2c) The high probabilities of drought occurrence are from May to October in almost sub-regions The lower probabilities of drought occurrence are often found from November to January Droughts not occur in December in B1, from November to January in B3, and from January to March in N3 A comparison of the three indices shows that the J index, which is calculated directly from monthly precipitation and temperature, indicates high probabilities of drought occurrence mostly in dry seasons in almost all sub-regions However, the PED and SPI indices are examined including the variations of precipitation (and temperature) during the period so that the high probabilities of drought occurrence are even found in wet seasons In general, the high probabilities of drought occurrence are from November to March according to the J index, from April to August according to the PED index and from May to October according to the SPI index 3.2 Annual drought conditions The occurrence probabilities of all conditions estimated by the three indices on an annual timescale are represented in Fig The conditions are grouped only in wet (W), moderate drought (MD), and severe drought (SD) conditions It can be seen in Fig 3a that the annual J index often shows wet conditions over the country because of the high total annual precipitation in the tropics, which leads to high values in the J index according to Eq (2) MD is more frequently observed in N1 and B3 while SD rarely occurs over the country According to the PED index (Fig 3b), one can note that the MD condition can occur in all sub-regions in which the highest probability of MD is 24.4 % in N1 and the lowest probability of MD is 17.5 % in B1 SD can occur with low probabilities in all sub-regions except N1 The occurrence probability of drought conditions estimated by the SPI index (Fig 3c) shows that MD may occur with lower probabilities but SD occurs with higher probabilities compared to those of the PED index in all sub-regions during 1961–2007 The probabilities of MD and SD occurrences not have significant differences among the sub-regions Generally, the J index is not useful to determine drought conditions on annual timescales due to the high annual precipitation in the tropics The occurrence probabilities of MD estimated by the PED index are generally greater than those estimated by the SPI index are, while it is vice versa with the occurrence probabilities of SD Table List of the ENSO years and their intensities in the period of 1961–2007 El Niño La Niña Year 1963 1965 1968 1969 1972 Intensity Weak Strong Moderate Weak Strong Year 1964 1970 1971 1973 1974 Intensity Weak Moderate Weak Strong Weak 1976 1977 1982 1986 1987 1991 1994 1997 2002 2004 2006 Weak Weak Strong Moderate Moderate Strong Moderate Strong Moderate Weak Weak 1975 1983 1984 1988 1995 1998 1999 2000 2005 2007 Strong Weak Weak Strong Weak Moderate Strong Weak Weak Moderate H Vu-Thanh et al 3.3 Linkages with ENSO In this section, the relationships between drought features and ENSO will be examined Information on ENSO years and intensities based on the Oceanic Niño Index (ONI) (Kousky and Higgins 2007) are available on the Climate Prediction Center's (CPC's) website (http://www.cpc.ncep.noaa.gov) (Table 2) The J index in the seven sub-regions for the El Niño, La Niña, and non-ENSO years are displayed in Fig 4a It is shown that all annual J values in the four sub-regions from B1 to B4 indicate wet conditions (i.e., J≥30) because of a relatively high annual precipitation in these regions The ENSO years not have a clear effect on J especially in the B1 and B2 sub-regions J had a slightly decreasing tendency Fig The a J index, b PED index, and c SPI index in ENSO years in the seven sub-regions of Vietnam for 1961–2007 El Niño years are marked with the tiny square pattern; La Niña years are marked with the up left to down right pattern Areas without patterns represent non-ENSO years during 1961–2007 In B3 and B4, it is found that there is less precipitation in some El Niño years resulting in less wet conditions Some small values of J mainly occur in the El Niño years such as J=35 in 1969, J=32.8 in 1991 in B3; and J=35.7 in 1968, J=39.1 in 1969 in B4 Figure 4a also shows an insignificant decreasing trend of J in B3 and an increasing trend in B4 This means that dry conditions increase in B1, B2, and B3 while they decrease in B4 In the N1 and N2 subregions, the de Martonne index significantly increases while there is almost no change in N3 In N1, the J index is rather small in some strong El Niño years such as J=19.68 in 1965, J=24 in 1972 and high in the strong La Niña year such as J=90.93 in 1999 In N2, J is also small in the El Niño years such as in 1963 (J=26.11) and in 1976 (J=19.76) The minimum value of J is equal to 28.83 in the weak El Niño year of a J index b PED index c SPI index Evolution of meteorological drought characteristics in Vietnam Fig Sen's trends of J (left), PED (middle), and SPI (right) at each station in the seven sub-regions in Vietnam during 1961–2007 Circles with thickblack contours indicate that the trends are significant at a 10 % level Warm/cold colors indicate more/less severe drought conditions 1977 and was at a maximum of 68.09 in the strong La Niña year of 1999 in N3 In general, the southern sub-regions are more clearly affected by the ENSO events, in which lower/ higher values of J occur more frequently in the El Niño/La Niña years, corresponding to the drier/wetter conditions over the regions, respectively The inter-annual changes of the PED index and the linkages with the ENSO years are shown in Fig 4b It is noted that there are homogeneous increasing trends of the PED values during 1961–2007 in all sub-regions, indicating an increase of drought conditions over the whole country The effects of the ENSO years on the PED index not show a systematic behavior In B1, some relatively high values of PED occur in El Niño years such as PED=2.62 in 1987, PED=2.35 in 1991, and PED=2.16 in 2006 However, the maximum of PED (PED=3.19) is found in the moderate La Niña year of 1998 The same situations as in B1 are also realized in B2, B3, Fig Number of stations listed in Table affected by drought and its linear trend, estimated by using the J index (black line), the PED index (red line), and the SPI index (blue) and B4 In the southern sub-regions, drought conditions mainly occur in the El Niño years and wet conditions are frequently observed in the La Niña years Nevertheless, some exceptions exist here and there, for example, the significant negative values of PED found in the non-ENSO year of 1996 and the high positive PED values found in the La Niña year of 1998 Similar to the impacts on the J index, the ENSO years also have significant influences on SPI in the sub-regions from B4 to N3 (Fig 4c) SPI is rather small (i.e., drought conditions may occur) in the El Niño years and rather high in the La Niña years In the B1, B2, and B3 sub-regions, such ENSO influences are not clearly observed 3.4 Long-term drought trends The trends of the three indices for the 1961–2007 period are computed using Sen's method (Sen 1968) and the non- H Vu-Thanh et al parametric Mann–Kendall test (Kendall 1975) is used to examine the trend-significant level (Fig 5) Briefly, Sen's trend of a data series (x1, x2, …, xn), where xi represents the value at the time i, is the median of the series composed of n(n−1)/2 elements {, k=1,2,…,n−1; j>k} Figure shows that all the three indices indicate a significant increase of drought conditions in the north of Vietnam with the highest rate occurring in the B3 sub-regions In the southern sub-regions, PED shows increasing conditions while J and SPI show decreasing drought trends for almost all the stations This difference can be explained by the fact that the increase of temperature over the regions can well compensate the increase of precipitation (Endo et al 2009) in the PED formula, leading to an increase of PED during 1961–2007 One should note that using the daily precipitation data of the second half of the twentieth century, Endo et al (2009) showed that precipitation increased in the south while it decreased in the north of Vietnam Therefore, the opposite drought trends between the north and south of Vietnam indicated by the J and SPI indices will follow the precipitation trends The number of meteorological stations listed in Table affected by drought during 1961–2007 is represented in Fig According to the J and SPI indices, the number of affected stations has a slightly increasing linear trend while a decreasing linear trend is detected according to the PED index As an affected station represents a certain area that has drought conditions, an increase in the number of affected stations (estimated by using the J and SPI indices) represents an increase of the drought-affected area over time Conclusions This study has examined the drought conditions over the seven sub-regions of Vietnam using the J, PED, and SPI indices According to the de Martonne J index, droughts can occur with high probabilities from November to March, from April to August according to the PED index, and from May to October according to the SPI index in almost all sub-regions On annual timescales, the J index is not useful to determine drought conditions because of the high total annual precipitation in the tropics, which leads to high values in the J index Moderate drought (MD) occurrence probabilities estimated by the PED index are generally greater than those estimated by the SPI index and vice versa in the case of severe drought (SD) The inter-annual analysis of the three indices in the southern sub-regions of Vietnam revealed that drought conditions mainly occur in the El Niño years and wet conditions are frequently observed in the La Niña years However, such ENSO influences are not clearly observed in the northern sub-regions During the period of 1961–2007, drought conditions significantly increase in the northern part of Vietnam In the southern regions, PED shows an increasing trend while J and SPI show decreasing drought trends for almost all the stations An increase in the number of stations affected by drought estimated by using the J and SPI indices represents an increase of the drought-affected area over time Due to the lack of socioeconomic and agricultural drought records over the seven sub-regions, this study has not aimed to identify which meteorological drought indices would be the best to represent the drought conditions in Vietnam This question remains unanswered and is an important challenge for a further study in order to better understand the relationships between the different drought types in Vietnam Acknowledgments This research was supported by the Vietnam Ministry of Science and Technology Foundation (DT.NCCB-DHUD.2011-G/ 10), the 11-P04-VIE DANIDA project, and the Vietnam National Foundation for Science and Technology Development (NAFOSTED, code 105.06-2013.03) References Alley WM (1984) The Palmer Drought Severity Index: limitations and assumptions J Clim Appl Meteorol 23:1100–1109 de Martonne E (1926) Une nouvelle fonction climatologique: L’Indece d’aridite La Meteorol 2:449–458 Endo N, Matsumoto J, Lwin T (2009) Trends in precipitation extremes over Southeast Asia SOLA 5:168–171 Friedman D.G (1957) The prediction of long-continuing drought in 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Vietnam with the highest rate occurring in the B3 sub-regions In the southern sub-regions, PED shows increasing... vice versa in the case of severe drought (SD) The inter-annual analysis of the three indices in the southern sub-regions of Vietnam revealed that drought conditions mainly occur in the El Niño... in Fig 4b It is noted that there are homogeneous increasing trends of the PED values during 1961–2007 in all sub-regions, indicating an increase of drought conditions over the whole country The