THA 2015 International Conference on“Climate Change and Water & Environmental Management in Monsoon Asia”
28-30 January 2015, Bangkok, Thailand.

Analysis of Hydrologic Variables Changes related to Large Scale
Reservoir Operation by Using Mann-Kendall Statistical Tests in
Thailand
D.Manee1, a *, Y.Tachikawa2,b and K.Yorozu3,cAbstract Recently, global warming has beensignificantly affected various hydrologic processes. The
large scale multiple purposes reservoir is one of thecountermeasures to manage and address both flood anddrought problems. Therefore, an appropriate operation
dam reservoir based on proper understanding of a changeof hydrologic variable characteristics is important. This
study applies the Mann-Kendall (MK) statistical trend
test to analyze increasing, decreasing or trendlesscharacteristics of precipitation, temperature, inflow to
dam reservoirs, release from dam reservoirs, and storage
volume in dam reservoir in Thailand from historicaloperation recorded data. The five large scale dam
reservoirs located in the northern and central parts ofThailand are selected to analyze the trend of the Ping
River basin (Bhumibol Dam), the Nan River basin(Sirikit Dam), the Pasak River basin (Pasak Jolasid Dam)and the Mae Klong River basin (Srinagarind and
Vajiralongkorn dams). Those reservoir operation timeseries which consist of daily inflow, release, rainfall andtemperature were analyzed and examined in association
with various aspects of dam reservoir operations.
Keywords Trend Analysis, Mann-Kendall trend test,
Reservoir Operation, Thailand

D.ManeeDepartment of Civil and Earth Resources Engineering
Kyoto University
Kyoto, Japan
Manee.donpapob.85v@st.kyoto-u.ac.jp
Y.Tachikawa
Department of Civil and Earth Resources Engineering
Kyoto University
Kyoto, Japantachikawa@ hywr.kuciv.kyoto-u.ac.jp
K.Yorozu
Department of Civil and Earth Resources Engineering
Kyoto University
Kyoto, Japanyorozu@hywr.kuciv.kyoto-u.ac.jp

Introduction
Climate changes cause clear temperature increase for
decades or even longer periods (IPCC, 2013)1), whichsignificantly affects various hydrologic processes of timeand space distribution pattern and quantity in such as
precipitation and evapotranspiration. There are numerous
earlier studies which focus on trend analysis of
atmospheric and hydrologic variables in quantity andquality using historical data and GCM data. For example,
Tao et al.2) analyzed the trends of streamflow in the
Tarim River basin during the past 50 years using
historical data, and Duong et al.3) examined the impact ofriver discharge changes by using a distributed river flowrouting model and GCM datasets in the Indochina
Peninsula region.
In the Southeast Asian countries such as
Thailand, agriculture is the main source of the economy
as well as ensures the well-being of the people. Thus, tosustain the water resource into the future is quite
essential. Unless the water resources are utilized with abalance approach of supply and demand, its sustainability
will become at risk. Therefore proper planning of water
resource development as well as the utilization based on
uncertainty in climate change impact is very necessary.
The large scale multiple purposes dam reservoirs are oneof countermeasures to manage and address water
resources problems. Therefore, the appropriate operationof dam reservoir based on a proper understanding of achange of hydrologic variables is very important.
In this study, the trend of various hydrologic
variables related to dam reservoir operations such as
inflow to dam reservoirs, release from dam reservoirs,
dam storage as well as precipitation and temperatureusing long historical record are examined with the MannKendall test. The trend analysis is one of the methods tosupport and confirm the change of hydrologic variables topropose an adaptive dam release operation rules andconstraint in advance combining with GCM data analysis
for the future.

THA 2015 International Conference on“Climate Change and Water & Environmental Management in Monsoon Asia”
28-30 January 2015, Bangkok, Thailand.

Materials and Methods
The study area is Thailand which is located in
the Southeast Asia with an area 513,115 km2. Its climate
is tropical climate with clear dry and wet seasons. The
seasons are defined as follow: the dry season starts from
November until April and the wet season starts from May
until October in the central and northern part of Thailand.
In the western river basins dry season starts from January
until June and wet season begins July until December.
Study areas and data collection
The five large scale dam reservoir basins located in thenorthern, western and central parts of Thailand as shownin Fig. 1 are selected to analyze the historical trend of
precipitation, temperature, dam inflow, dam release, andstorage. The five basins are the Ping River basin with the
Bhumibol Dam (BB), the Nan River basin with the Sirikit
Dam (SK), the Pasak River basin with the Pasak JolasidDam (PS), and the Mae Klong River basin with the
Srinagarind dam (SR) and Vajiralongkorn dam (VRK).
Daily temperature and precipitation data werecollected from 25 meteorological stations for the period
1980-2011 and time series of monthly inflow, release andstorage data were obtained from the Electricity
Generating Authority of Thailand (EGAT) and Thai
Royal Irrigation Department (RID) for the period ofstarting reservoir operation. The reservoir characteristics
of reservoir description are given in Table 1. The spatial
distribution of the selected meteorological stations inThailand is shown in Fig. 1.
Table 1 Characteristic features of selected reservoirs
Name
Location

BB
SK
SR
VRK
PS
17°14′33″N 17°45′50″N 14°24′31″N 14°47′58″N 14°51′41″N
98°58′20″E 100°33′48″E 99°07′42″E 98°35′49″E 101°03′58″E

Fig. 1 Location map of large
meteorological stations in Thailand

scale

reservoirs

and

The Mann-Kendall trend test4), 5) is one of the widely used
non-parametric tests to detect significant trends in timeseries. The Mann-Kendall trend test is based on thecorrelation between the ranks of a time series and theirtime order. For the statistics S is calculated as equation(1). This statistic represents the number of positive
differences minus the number of negative differences for
all the differences considered as

CatchmentArea
26,38613,13010,8803,720

12,292

Max.Storage (MCM)

13,462

9,510

17,745

8,860

785

Mean Inflow (MCM)

5,783

5,780

4,790

5,585

2,725

Opening Year

1964

1974

1980

1984

1999

Period (years)

50

40

34

30

15

n 1

S    sgn x j  xi 

(km2)

Trend analysis methods
Detecting trends in hydrologic, climatic, water quality
and other natural time series has been an active topic formore than three decades now. Statistical tests for thedetection of significant trends in hydrologic andclimatologic time series can be classified as parametricand non-parametric methods. The parametric trend tests
require sample data to be independent and normally
distributed, while the non-parametric tests need only thatthe data be independant4).

n(1)

i 1 j i 1

where n is the number of total data points, xi and xj are thedata values in time series i and j (j>i), respectively, andsgn (xj-xi) is the sign function as: 1, if x j  xi  0
sgn x j  xi    0 if x j  xi  0 1 if x j  xi  0(2)

The variance of Mann-Kendall statistic is calculated byequation (3) asm

Var ( S ) n(n  1)(2n  5)   t i (t i  1)(2t i  5)
i 1

18

(3)

THA 2015 International Conference on“Climate Change and Water & Environmental Management in Monsoon Asia”
28-30 January 2015, Bangkok, Thailand.

where n is the number of total data points, m is thenumber of tied groups. The tied group mean a simple data
having a same value. The ti indicates the number of ties
of extent i. In case of the sample size n > 10, the standard
normal test statistic ZS is estimated by equation (4) as S 1, if S  0

Var
(
S
)


Z S   0 if S  0
 S 1
, if S  0 Var ( S )(4)

The positive values of ZS show increasing trends while
negative ZS values present decreasing trends. In this study
1% and 5 % significance level (α) were used. WhenZ S  Z1 / 2, the null hypothesis is rejected and significant

trend exists in the time series. Z1 / 2 is obtained from thestandard normal distribution table. Therefore, the null
hypothesis of no trend is rejected if Z S > 1.96 and Z S >2.576 at the 5% and 1 % significance level respectively.

Table 2, the ZS values in equation (4) are presented. The
positive values represent the increasing trend and the boldvalues shows statistically significant at the 5% or 1%
significant level.
All of the entire stations show the little significant
increasing trends except some stations near to the BB
dam stations while the other stations trends present
increasing trend in term of temperature and rainfall. For
the results of temperature trend, the Wichianburi and
Maejo stations in the Pasak River basin and the Ping
River basin showed dramatically insignificant increasing
trends at the 1% significance level of temperature in allseason. The other stations have slightly increasing anddecreasing trends in dry and wet seasons. The
temperature in the Pasak, Nan and Mae Klong Riverbasins shows the increasing trends, which means theevapotranspiration trend in those river basins would
increase because the temperature is a main factor of
evapotranspiration components.
Table 2 Results of the statistical tests for seasonal andannual temperature and precipitation over the period
1980-2011
Station

Data analysis
The observation data used in this study consists of a daily
observation of precipitation, temperature, daily inflow,
release and volume storage. The daily precipitation, dam
inflow and dam release flow data were summed up formonthly, seasonally, and yearly and developed monthly,
seasonally and annual time series data for the trend
analysis. The daily temperature data were averaged
monthly, seasonally, and yearly and developed for eachtime series data. Similarly the dam storage volume timeseries were averaged on each month. After compiling alltime series data on each parameter, the monthly timeseries in the designed month was selected to generate newtime series for the Mann-Kendall trend analysis.
In addition, the seasonal analysis also calculate
by dividing on dry season (November to April) and wet
season (May to October) for the northern and central of
study area. The climate for western of study area isdelayed, so the calculation was chosen the dry season(January to June) and the wet season (July to December)based on each climate characteristics.
Results and discussion

Temperature

Precipitation

Dry Season Wet Season Annual Dry Season Wet Season Annual

Bhumibol Dam

0.214-0.714-0.0180.712-0.188-0.021Chiang Mai

0.464-2.194*-1.053-0.335-0.314-0.649Chiang Rai

1.963*

0.839

1.731

0.251

-0.565-0.523

Lampang

2.569**

0.036

1.855

0.126

-0.628-0.607Lamphun0.788-1.632-0.263-0.068

1.564

1.19

Mae Hong Son-0.244-0.732-0.619

0.555

0.795

0.795

Mae Jo2.963**

0.691 2.904**

0.34-1.216-1.112

Phayao-0.338

0.319

0.638

-0.1872.108*

1.836

Mae Sariang

0.143

-1.07

-0.749

1.13

0

0.251

Nan

1.035

0.125

0.517

0.272

0.188

0.293

Phrae

-0.393-0.624-0.232

1.193

0.649

0.733

Tha Wang Pha-0.892

1.035

0.91

0.054

0.715

0.412

Uttaradit

1.427-0.1250.5

0-1.047-1.2140.9

1.465

Phetchabun

1.285

0.178

1.285

2.156*

Pak Chong2.623**-0.963

1.267

0.921

Lom Sak

2.177*-0.125

1.409

0.076

0.195

0.433

Lop buri

0.178

0.071

0.107

0.23

-1.088-0.481

Loei

1.249

-0.624

0.892

0.586

1.005

1.235

Bua Chum

0.393

0.225

0.161

-0.3030.195-0.206

Wichian Buri

2.814**

0.966

2.176* 2.701**

1.741

0.108

Tak Fa0.696-0.731

0.232

0.954

1.047

1.17

Kanchanaburi-0.892

1.873

0.232

-0.147-0.586-0.356

Thong Pha Phum

0.731

2.301*

1.855

0.52

0.011

0.412

Uthong

-1.82

0.464

-0.892-3.181**2.972**-0.544

Umphang

0.535-1.356-0.393

1.363

0.824

1.761

Bold Character represent trends identified with

Analysis of temperature and precipitation trend*Statistically significant trends at 5% significance level (1.960)**Statistically significant trends at 1% significance level (2.576)

Results of applying statistical tests for seasonal andannual temperature and precipitation trend over theperiod 1980-2011 are presented in Table 2 and Fig. 2. In

2.219* 2.623**

THA 2015 International Conference on“Climate Change and Water & Environmental Management in Monsoon Asia”
28-30 January 2015, Bangkok, Thailand.

Fig.2 Spatial distribution of meteorological stations with increasing, decreasing and no trends for seasonal and annual data during theperiod 1980-2011.
Table 3 Results of the statistical tests for monthly, seasonal
According to the precipitation trend results, the precipitation
and annual inflow over the period from the operation to 2013
trends of the Pasak, Nan and Mae Klong River basins alsoslightly increase as similar trends with the temperature.
Test statistic (Zs)
Month/Season

Analysis of inflow trend
The output of the analyzed inflow series was
summarized in Table3. The inflow series of the SK dam
at the Nan River basin and the SR dam at the Mae
Klong River basin were found significant increasing
trends throughout the year, which are corresponding
with the increasing precipitation trends. Both increasing
and decreasing trends were detected at the BB, PS andVRK dams. The VRK and BB dam have a strong
significant decreasing trend at 1% significance levels inearly dry season. The Pasak reservoir was found only
three months of slightly increasing trends comparing
with other nine month significant decreasing trends,
even though, the precipitation trends in the Pasak Riverbasin have been detected increasing trends in all stations
that cover the basin. This suggests the higher
evapotranspiration rate occurred, which made thereducing the amount of water resources in the basin.

BB

SK

PS

SR

VRK

0.415

-2.12*

Jan-2.141*

1.27

0

Feb

-1.8242.552*-1.2040.978 -2.345*

Mar-0.3510.804-0.9851.038-0.769

Apr

-0.97

0.781

0.109

0.326 -2.157*

May

1.029

0.874

-0.495

0.83

Jun

0.151

0.408

-0.594

0.86

0.071

Jul

0.46

0.92

-1.1881.986*

2.212*

Aug-0.0670.804-0.891

2.194*

1.249

Sep-0.033

1.014

0.495

1.393

1.855

Oct

0.184

0.478

0.891

1.127

0.928

Nov

-1.4220.384-1.188

0.296

0.036

-2.894** 2.598** -2.078*

0.385

0

Dec

0.732

Dry Season-1.9391.766-1.642

1.334

0.319

Rainy Season

0.151

0.851

0.396

1.186

2.07*

Annual-0.233

0.702

0.495

1.512

1.97*

Bold Character represent trends identified with
*Statistically significant trends at 5% significance level (1.960)**Statistically significant trends at 1% significance level (2.576)

THA 2015 International Conference on“Climate Change and Water & Environmental Management in Monsoon Asia”
28-30 January 2015, Bangkok, Thailand.

Analysis of release flow trend
The monthly, seasonal and annual trends of release flow
obtained by the Mann-Kendall test are given in Table 4.
According to these results, the significant increasing
trend at 1% significance level of release flow during dry
season were found except the Pasak dam, however the
release flow of the Pasak dam has a little significanceincreasing trends especially on December and January at
the 1% significance level. The inflow trends of the BB
and SK dams during early wet season (May to July)found increasing trends as shown in Table 3. Therefore,
the release of the BB, SK and PS dams in wet season
have significant decreasing trend to store the flood flows
in the Ping, Nan and Pasak Rivers. In brief, the results
found generally increasing significant trends in dry
season of release flow in all reservoirs for high water
demand, and decrease trends in the BB, SK, and PS
dams to store water in the rainy season.
Table 4 Results of the statistical tests for monthly,
seasonal and annual release flow over the period startingfrom the operation to 2013
Month/Season

Test statistic (Zs)
BB

SK

PS

SR

VRK

Jan

3.543** 4.928** 3.266** 3.579**

Feb

4.215** 3.944**

2.72**

Mar2.595**

2.179*

1.584 3.765** 3.395**

Apr

1.457

0.478

-0.792 2.846** 4.221**May-1.405-0.711

Jun

-1.69

Jul

-2.476*Aug-2.108*1.683 4.092** 4.858**

0 3.172**

2.532*-0.1510

2.046*-0.469-1.456-0.6932.075*

0.206

-1.34

-0.693

1.66

1.069

Sep

-3.279** -2.039*-0.7920.049-0.019Oct-2.509*-1.2

0.891 3.143**

1.427

Nov

-0.569-0.198Dec

1.941 3.111** 2.771**-0.8912.787* 2.676**
3.41**

2.034*Dry Season

2.957**

3.29**

0.766

3** 3.358**

Rainy Season-2.225*

-1.13

0.099

1.832

1.356

Annual

0.353

1.379

0.396

3.13**

2.72**

Bold Character represent trends identified with
*Statistically significant trends at 5% significance level (1.960)**Statistically significant trends at 1% significance level (2.576)

Analysis of storage trend
Results of the Mann-Kendall test to the monthly storage
volume at the end of each month are presented in Table
5. As shown, the majority of the monthly trends in the
BB and PS dam have a significant decreasing trend due
to reservoir operation based on existing rule curve while

the SR dam shows increasing trend throughout the year.
The SK and VRK dams remain the similar trends ofincreasing trends in wet season and of decreasing trends
in dry season as a natural condition. Moreover, thesignificant decreasing trend at the 1% significance levelfrom February to July of BB Dam and from January to
April and August of PS dam were detected.
Insufficient and excess storage volume
Insufficient and excess storage volume below orexceeding the rule curves of all reservoirs were also
evaluated from the beginning of each dam operation.
The insufficient storage volume means the amount ofvolume below the lower rule curve while the excess
volume represents the total of volume exceeding theupper rule curve. The explanation of insufficient andexcess storage volume is illustrated as Fig. 3. The
results of this analysis are summarized in Table 6. At the
BB dam, increasing of insufficient storage volume
trends was detected, which is corresponding on previous
analysis of reduced inflow and increasing outflow trends
results. The PS Dam also had a similar significant
inflow and outflow trends results with the BB dam
nevertheless the results of insufficient and excess
storage volume results of the PS dam have indistinct.
The SK, SR and VRK dams show gradually a rising
trends of excess storage volume same an inflow trends
of preceding section that represent the water resourcesin those areas would be increased.
Table 5 Results of the statistical tests for monthly,
seasonal and annual storage over the period startingfrom the operation to 2013
Month
Jan

Test statistic (Zs)
BB

SK

SR

VRK

-1.491-0.012 -1.971*

PS

1.186

1.294

Feb

-2.112*-0.827 -2.847**

1.127

0.807

Mar

-2.629**-1.456 -2.956**

0.889

0.619

Apr

-2.801**-1.619 -3.175**

0.741-0.169May-2.681**-1.503-1.5340.415-0.694Jun-2.422*-1.107-1.7820.682-0.178Jul-1.982*-0.641-1.881

0.652

0.107

Aug

-1.6640.128 -2.573**

1.275

1.534

Sep

-1.5770.454-1.089

1.571

1.249

Oct

-1.284

0.618

1.386

1.601

1.463

Nov

-0.957

0.757

1.089

1.423

1.427

Dec

-1.1120.734-0.693

1.156

1.463

Bold Character represent trends identified with
*Statistically significant trends at 5% significance level (1.960)**Statistically significant trends at 1% significance level (2.576)

THA 2015 International Conference on“Climate Change and Water & Environmental Management in Monsoon Asia”
28-30 January 2015, Bangkok, Thailand.

Fig. 3 Explanation of insufficient and excess storage volume

The number of insufficient events in the BB,
SK and SR reservoir were found biennially on average.
The PS dam has a small capacity comparing with otherdams, so many insufficient and excess events detected
within fifteen years. However, the VRK dam is also
found the excess events with fourteen times to cope withflood. According to the dam characteristics in Table.1,
the large scale reservoir such as the BB and SR dams
have an advantage to address the flood for four and two
events, respectively.
Table 6 Results of insufficient and excess storage volume.

inflow of all reservoirs in rainy season has
increasing trends.
 For dam release from the reservoirs,
generally increasing significant trends in dry
season of release flow in all reservoirs. OnlyBB and SK dam has decreasing trends during
rainy season.
 As a result, dam storage for BB dam founddramatically decreasing trends throughout theyear while the SR dam detected decreasing
trends.
 These insufficient and excess storage results
indicate the increasing trends of water
shortage in BB dam (Ping River basin) andPS dam (Pasak River basin) and also raising
the trend of flood in SK dam (Nan River
Basin) and both dams in Mae Klong River
Basin.
We showed the climate change impacts have been
already appeared in water resources in Thailand. The
further study will be combined with the analysis of thehistorical data and GCM outputs. The analysis using ahydrologic model and GCM outputs are ongoing topredict future dam inflow under a changing climate todevelop a new rule curve for adaptive dam reservoir
operation.
Acknowledgement

Dam
BB
SK
PS
SR
VRK

Number of Event
Insufficient24

Zs
1.15 Increasing

Excess4

Insufficient

28-0.01 Decreasing-

Excess140.99 Increasing

Insufficient

10

0.00 No Trend

Excess170.72 Increasing

Insufficient

17-0.03 Decreasing

Excess2---

Insufficient8-1.86 Decreasing

Excess140.00 No Trend

Conclusions
The trends of precipitation, temperature, dam inflow,
and dam release flow and volume storage were analyzed
statistically based on long term historical data by using
the Mann-Kendall trend test. Our findings are
summarized as follows:Through the analysis, we found that thetemperature and precipitation trends wereincreasing trends nevertheless Ping Riverbasin temperature and precipitation trends
were decreasing trends.
The water resources availability in term of
inflow to BB dam and PS Dam weredecreasing trend during dry season. The

This works is based on the data from Thai
Meteorological Department for temperature andprecipitation data, Thai Royal Irrigation Department andElectricity Generating Authority of Thailand forreservoir operation data. We gratefully acknowledge this
support.
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