Journal of Japan Society of Civil Engineers, Ser.
B1 (Hydraulic Engineering), Vol. 71, No. 4, I_61-I_66, 2015.

ANALYSIS OF HYDROLOGIC VARIABLE
CHANGES RELATED TO LARGE SCALERESERVOIR OPERATION IN THAILAND
Donpapob MANEE1, Yasuto TACHIKAWA2 and Kazuaki YOROZU3
1

Student member of JSCE, Dept. of Civil and Earth Resources Eng., Kyoto University, (Kyoto 615-8540, Japan)
E-mail:manee.donpapob.85v@st.kyoto-u.ac.jp
2
Member of JSCE, Prof., Dept. of Civil and Earth Resources Eng., Kyoto University, (Kyoto 615-8540, Japan)
E-mail: tachikawa@ hywr.kuciv.kyoto-u.ac.jp
3
Member of JSCE, Assist. Prof., Dept. of Civil and Earth Resources Eng., Kyoto University, (Kyoto 615-8540, Japan)
E-mail: yorozu@hywr.kuciv.kyoto-u.ac.jp

The objective of this study is to investigate the trend of hydrologic variable changes by applying theMann-Kendall statistical trend test to daily historical record of reservoir operations data such as inflow to
dam reservoirs, release from dam reservoirs, dam storage as well as precipitation and temperature. The fivelarge scale dam reservoirs located in the northern, central and western parts of Thailand are selected toanalyze the trend of the Ping River basin having the Bhumibol dam, the Nan River basin having the Sirikit
dam, the Pasak River basin having the Pasak Jolasid dam and the Mae Klong River basin having the Srinagarind and Vajiralongkorn dams. Through the analysis, we found that the temperature and precipitation
show increasing trends except the Ping River basin; water resources availability in terms of inflow to the
Bhumibol dam and Pasak Jolasid dam shows decreasing trend during dry season; the inflow of all reservoirs in rainy season had increasing trends; and dam release of the all reservoirs shows significant increasing trends in the dry season. Furthermore, water storage for the Bhumibol dam were found decreasing
trends throughout the year while the Srinagarind dam was detected increasing trends. Also, the increasing
trends of water shortage in the Bhumibol and Pasak Jolasid dam, and raising trend of excess storage volume
in the Sirikit dam and both dams in the Mae Klong River Basin were observed.
Key Words: Trend Analysis, Mann-Kendall Trend Test, Reservoir Operation, Thailand

well as the utilization based on uncertainty in climatechange impact is very necessary. Tebakari et al.4)
found the impact of two large-scale reservoir operation on flow regimes before and after reservoir development, which reveals the great change of natural
flow to stabilized flow through the year. Now water
demands are highly interacted among various sectors, so unless the water resources are managed witha balance approach of supply and demand, its sustainability will become at risk.
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 temperature using long historical record are examinedwith the Mann-Kendall test. The trend analysis results provide information to propose adaptive dam
release operation rules under changing climate,which will be combined with GCM data analysis forchallenges and responses to extreme climatic events.

1. INTRODUCTION
Climate change causes clear temperature increasefor decades or even longer periods (IPCC, 2013)1),
which significantly affects various hydrologic processes in time and space distribution patterns andquantity of precipitation and evapotranspiration.
There are numerous earlier studies which focus ontrend analysis of atmospheric and hydrologic variables in quantity and quality using historical data and
GCM data. For example, Tao et al.2) analyzed thetrends of streamflow in the Tarim River basin during
the past 50 years using historical data, and Duong etal.3) examined the impact of river discharge changes
by using a distributed river flow routing 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 and it
ensures the well-being of the people. Therefore
proper planning of water resource development as1

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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

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

(km2)

2. MATERIALS AND METHODSRiver basins in Thailand are studied, which arelocated in the Southeast Asia with an area 513,115
km2. The basins experience tropical climate withclear dry and wet seasons defined as follows: the dry
season starts from November until April and therainy season starts from May until October in thecentral and northern part of Thailand. In the westernriver basins dry season starts from January until Juneand rainy season begins July until December.

Fig.1 Location map of large scale reservoirs and meteorological stations in Thailand.

(1) Study areas and data collection
The five large scale dam reservoir basins locatedin the northern, western and central parts of Thailand
as shown in Fig. 1 are selected to analyze the historical trend of precipitation, temperature, dam inflow, dam release, and storage. The five basins are
the Ping River basin with the Bhumibol dam (BB),the Nan River basin with the Sirikit dam (SK), thePasak River basin with the Pasak Jolasid dam (PS),and the Mae Klong River basin with the Srinagarinddam (SR) and Vajiralongkorn dam (VRK).
Daily temperature and precipitation data werecollected from 25 meteorological stations for theperiod of 1980-2011; and time series of daily dam
inflow, dam release and storage data were obtained
from the Electricity Generating Authority of Thailand (EGAT) and Thai Royal Irrigation Department(RID) for the period of starting reservoir operation.
The reservoir characteristics are given in Table 1.
The spatial distribution of the selected meteorological stations in Thailand is shown in Fig. 1.

distribution function, while the non-parametric tests
need only that the data to be independant 5).
The Mann-Kendall trend test6), 7) is one of thewidely used non-parametric tests to detect significant
trends in time series. The Mann-Kendall trend test isbased on the correlation between the ranks of timeseries and their time order. The statistics S calculated
as equation (1) represent the number of positive differences minus the number of negative differences
for all the differences considered asn 1

S    sgn x j  xi n(1)

i 1 j i 1

where n is the number of total data points, xi and x j
are the data values in time series i and j (j>i), respectively, and sgn (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) Trend analysis methods
Detecting trends in hydrologic, climatic, water
quality and other natural time series has been an
active topic for more than three decades now. Statistical tests for the detection of significant trends inhydrologic and climatologic time series can be classified as parametric and non-parametric methods.
The parametric trend tests require sample data to be
independent and distributed according to a certain(2)

The variance of the Mann-Kendall statistic is calculated by equation (3) asm

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

(3)

18

where n is the number of total data points, m is thenumber of tied groups. The tied group means a sim2

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ple data having the same value. The ti indicates thenumber of ties of extent i. In case of the sample size n> 10, the standard normal test statistic ZS is estimatedby equation (4) as S 1, if S  0

Var
(
S
)


Z S   0 if S  0
 S 1
, if S  0 Var ( S )Table 2 Results of the statistical tests for seasonal and annual
temperature and precipitation over the period 1980-2011.
Station(4)

The positive values of ZS show increasing trends
while negative ZS values present decreasing trends. In
this study the 1% and 5 % significance level α wereused. When Z S  Z1 / 2 , the null hypothesis is rejected
and a significant trend exists in the time series. Z1 / 2
is obtained from the standard normal distributiontable. Therefore, the null hypothesis of no trend isrejected if Z S > 1.96 and Z S > 2.576 at the 5% and
1 % significance level respectively.

(3) Data analysis
The observation data used in this study consists ofdaily precipitation, temperature, daily dam inflow,
dam release and storage volume. The daily precipitation, dam inflow and dam release flow data weresummed up for monthly, seasonally, and yearly anddeveloped monthly, seasonally and annual time series data for the trend analysis. The daily temperature
data were averaged monthly, seasonally, and yearlyand developed for each time series data. Similarly thedam storage volume time series were averaged at theend of each month. After compiling of all-time series
data on each parameter, the monthly time series inthe designed month was selected to generate newtime series for the Mann-Kendall trend analysis.
In addition, the seasonal analysis was also conducted by dividing the dry season (November toApril) and the rainy season (May to October) for thenorthern and central of study area. The season for thewestern of study area is delayed, so the calculation
was chosen for the dry season (January to June) andthe rainy season (July to December) for each hydrologic parameter.

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

2.219* 2.623**

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
*Statistically significant trends at 5% significance level (1.960)**Statistically significant trends at 1% significance level (2.576)
Dashed line indicate the separation of gauging station on each basin

Fig. 2. In Table 2, the ZS values in equation (4) arepresented. The positive values represent the increasing trend and the bold values show statisticallysignificant at the 5% or 1% significant level.
All of metrological stations showed the slight significant increasing trends except some stations nearto the BB dam stations while the other stations trends
present increasing trend in terms of temperature andrainfall. For the results of the temperature trend, theWichianburi and Maejo stations in the Pasak Riverbasin and the Ping River basin showed insignificant
increasing trends at the 1% significance level of temperature in all seasons.
The other metrological stations have slightly increasing and decreasing trends in the dry and rainyseasons. The mean temperature in the Pasak, Nan and
Mae Klong River basins showed the increasing trend,
which means the evapotranspiration trend in thoseriver basins would increase because the temperature
is a main factor of evapotranspiration component.
The precipitation trends of the Pasak, Nan and Mae
Klong River basins also slightly increase as similar
trends with the temperature.

3. RESULTS AND DISCUSSION(1) Analysis of temperature and precipitation
trend
Results of applying the statistical tests for seasonal
and annual temperature and precipitation trend overthe period 1980-2011 are presented in Table 2 and

3

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Fig.2 Spatial distribution of meteorological stations with increasing, decreasing and no trends for seasonal and annual data during theperiod 1980-2011. (a) mean temperature in dry season (b) mean temperature in rainy season (c) annual mean temperature(d) precipitation in dry season (e) precipitation in rainy season (f) annual precipitation(2) 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 theMae Klong River basin were found significant increasing trends throughout the year, which werecorresponding with the increasing precipitation
trends. Both increasing and decreasing trends weredetected at the BB, PS and VRK dams. The VRK and
BB dam have a strong significant decreasing trend at
the 1% significance level in early dry season. The
Pasak reservoir was found only three months ofslightly increasing trends comparing with other ninemonth significant decreasing trends, even though theprecipitation trends in the Pasak River basin havebeen detected increasing trends in all stations thatcover the basin. This suggests the higher evapotranspiration rate occurred correspond to the resultsof the increasing mean temperature trend in the Pasak
River basin, which caused the decreasing amount ofwater resources in the basin.

(3) Analysis of release flow trend
The monthly, seasonal and annual trends of dam
release flow obtained by the Mann-Kendall test are
given in Table 4. According to these results, thesignificant increasing trend at the 1% significancelevel of dam release flow during the dry season were
found except the Pasak dam, however the dam release flow of the Pasak dam has a little significant
increasing trend especially on December and January
at the 1% significance level. The inflow trends of the
BB and SK dams during the early rainy season (Mayto July) found increasing trends as shown in Table 3.
Therefore, the release of the BB, SK and PS dams in
the rainy season have significant decreasing trend to
store the flood flows in the Ping, Nan and Pasak
Rivers. In brief, the results show generally increasing
significant trends in the dry season of release flow inall reservoirs for high water demand, and decrease
trends in the BB, SK, and PS dams to store water inthe rainy season.
4

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Table 4 Results of the statistical tests for monthly, seasonal andannual dam release flow over the period starting from the operation to 2013.

Table 3 Results of the statistical tests for monthly, seasonal andannual inflow over the period from the starting operation to 2013.
Month/Season

Test statistic (Zs)
BB

SK

PS

SR
0.415

VRK

Month/Season-2.12*

Test statistic (Zs)
BB

SK

PS

SR

VRK

Jan

-2.141*

1.27

0

Feb

-1.8242.552*-1.2040.978 -2.345*

Jan

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

Mar-0.3510.804-0.9851.038-0.769Feb

4.215** 3.944**

2.72**

1.683 4.092** 4.858**

Apr

-0.97

0.781

0.109

0.326 -2.157*

Mar

2.595**

2.179*

1.584 3.765** 3.395**

May

1.029

0.874

-0.495

0.83

0.732

Apr

1.457

0.478

-0.792 2.846** 4.221**

Jun

0.151

0.408

-0.594

0.86

0.071

May

-1.405-0.711

0 3.172**

2.532*

Jul

0.46

0.92

-1.1881.986*

2.212*

Jun-1.69-0.1510

2.046*-0.469Aug-0.0670.804-0.891

2.194*

1.249

Jul-2.476*-1.456-0.6932.075*

0.206

Sep-0.033

1.014

0.495

1.393

1.855

Aug

-2.108*-1.34-0.693

1.66

1.069

Sep

-3.279** -2.039*-0.7920.049-0.0190.891 3.143**

1.427

Oct

0.184

0.478

0.891

1.127

0.928

Nov

-1.4220.384-1.188

0.296

0.036

Oct

-2.509*

-1.2

0

Nov

-0.569-0.198Dec

1.941 3.111** 2.771**

Dec-2.894** 2.598** -2.078*

0.385-0.8912.787* 2.676**
3.41**

2.034*Dry Season-1.9391.766-1.642

1.334

0.319

Rainy Season

0.151

0.851

0.396

1.186

2.07*

Dry Season

2.957**

3.29**

0.766

1.97*

Rainy Season-2.225*

-1.13

0.099

1.832

1.356

Annual

0.353

1.379

0.396

3.13**

2.72**

Annual-0.233

0.702

0.495

1.512

Bold character represent trends identified with

3** 3.358***Statistically significant trends at 5% significance level (1.960)

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

Dashed line indicate the separation between dry and wet seasons**Statistically significant trends at 1% significance level (2.576)
Dashed line indicate the separation between dry and wet seasons(4) 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 themonthly trends in the BB dam have a significant
decreasing trend due to reservoir operation based on
the existing rule curve while the SR dam shows anincreasing trend throughout the year. The SK and
VRK dams remain the similar trends of increasing
trends in rainy season and of decreasing trends in dry
season as a natural condition. The water storage trendof the Pasak dam found strongly decreasing trend at
the 1% significance level in late dry season due toincreased water consumption as indicated in Table 4.

Table 5 Results of the statistical tests for monthly, seasonal andannual storage over the period starting from the operation to 2013.
Test statistic (Zs)
Month
BB
SK
PS
SR
VRK

(5) Insufficient and excess storage volume
Insufficient and excess storage volumes 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 the insufficientand excess storage volume is illustrated in 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
to the results of section (2) and (3) for the analysis of
reduced inflow and increasing outflow trends results.

Jan-1.491-0.012 -1.971*

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)
Dashed line indicate the separation between dry and wet seasons

5

I_65

Table6 Results of insufficient and excess storage volume.
Dam
BB
SK
PS
SR
VRK

Fig. 3 Explanation of insufficient and excess storage volume.



The PS dam also had significant inflow and outflow trend results similar 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 rising trends ofexcess storage volume which are the same as inflow
trends of preceding section that represent the water
resources in those areas would be increased.
The number of insufficient events in the BB, SK
and SR reservoirs found biennial on average. The PS
dam has a small capacity comparing with other dams,
so many insufficient and excess events were detected
within fifteen years. However, the VRK dam wasalso found the excess events with fourteen times tocope with floods. According to the dam characteristics in Table.1, the large scale reservoirs such as theBB and SR dams have an advantage to address thefloods for four and two events, respectively.



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

-

-

-

As a result, dam storage for the BB dam wasfound dramatically decreasing trends throughout
the year while the SR dam was detected increasing trends.
These insufficient and excess storage results
indicate the increasing trends of water shortagein the BB and PS dam and also the rising trend offlood in the SK, SR and VRK reservoirs.

The further study will be combined with theanalysis of the historical data and GCM outputs fordevelopment a new rule curve for adaptive dam reservoir operation.
ACKNOWLEDGMENT: This study is based on
the data from Thai Meteorological Department for
temperature and precipitation data, Thai Royal Irrigation Department and Electricity Generating Authority of Thailand for reservoir operation data. We
gratefully acknowledge this support.

4. CONCLUSIONSREFERENCES
The trends of hydrologic variables were analyzed
statistically based on long term historical data byusing the Mann-Kendall trend test. Our findings are
summarized as follows:






1)

2)

Through the analysis, we found that the temperature and precipitation trends show increasing
trends nevertheless at the Ping River basin temperature and precipitation trends show decreasing trends.
The water resources availability in terms of inflow to the BB and PS dam shows decreasing
trend during the dry season. The inflow of allreservoirs in the rainy season has increasing
trends.
For dam release from the reservoirs, generally
increasing significant trends appear in the dry
season in all reservoirs. Only the BB and SK
dam have decreasing trends during the rainyseason.

3)

4)

5)

6)
7)

6

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(Received September 30, 2014)