Impact of Large Reservoir Development on Water Resource Regime*Mahanakorn University of Technology, Saisunee Budhakooncharoen1(1: Assistant Professor, D.Eng.)* Mahanakorn University of Technology (MUT), 51 Cheum Samphan rd., Nong Chok, Bangkok 10530, Thailande-mail: saisunee@mut.ac.thAbstract
In this paper, the impact of large reservoir development to the water resource regime was preliminary investigated. Sirikit
dam in the Nan river basin, north of Thailand was a case of study. Analysis of pre- and post-dam hydrologic records
indicates that the impoundment and the amount of rainfall are one of the major causes of change in the flow regime
downstream of the reservoir. But, the post-flow annual hydrograph change, especially the reduction of mean monthly peakrunoff magnitude and the higher flow during the low flow period mainly results from the impact of large reservoirdevelopment. It is seen that the large strategic scheme play a significant role in capable of storing surplus volume of wet
season runoff for the later dry season. However, these overall results are based on a single large storage. For better
understanding the response of flow to the large reservoir, further study should emphasize across a range of dam types andcatchment characteristics. Some other factors affected, such as the frequency of flood and drought, shift in low flow, timingof high and low flow or change across different flow duration should also be further investigated. It is therefore revealed
the necessity to establish the integrated database of the regional hydrological and meteorological properties from intensive
field observation. This precious data set should be accessible for all researchers.
Keyword: impact of large reservoir development, Sirikit dam, Nan river, change in the flow regime

1. Introduction
Since the past decades, many urban areas around theworld have been experiencing a dramatic increase innatural disasters due to water-related extreme events. The
lack of precipitation as well as heavy downpour causes
disasters ranging from extreme drought to unprecedented
flood. In addition, climate change, environmental
degradation, population growth, urbanization andanthropogenic effect cause human society more vulnerable
to flood and drought disasters. (Budhakooncharoen, S,2003)
Various efforts have been set out for establishment of theproper prevention and mitigation measures to cope withthis problem. Storage reservoirs have been one of thecommon developments worldwide over the past decades
for the proactive preparedness systems to relief the humanvulnerability against water-related extreme events, floodand drought disasters.

2. State of the Problem
Dams have provided various benefits for society
including generation of electricity and the reduction ofdevastating floods as well as a controlled supply of waterthat can be used for alleviating drought and a variety ofmunicipal and industrial demands. However, due to thecomplexity of interaction, building a dam has changed
forever the flow regime in the river. It generates significant
hydro-geomorphic alterations with impacts occur both
upstream and downstream of the reservoir. (Magilligan,F.J. (2001). The major visual impacts of the changed flow
regime can be found in numerous manners, such assedimentation, channel / river bank erosion, overgrown ofvegetation, salinity intrusion, ecology degradation or affectto the timing, magnitude, duration and frequency of flowin the river.
To avoid the past mistakes and satisfy a wide range ofneeds are major challenge in water resource planning andmanagement. The situation prompted the necessity toinvestigate the impact of impoundment and large-scalewater resource development scheme on long-term change
in the water resource regime. The result obtained may be
used as the essential input information for later planningand development.

3. Study Area
Nan river basin in north of Thailand is presented as thecase study to determine the impact in the past and the ongoing flow regime of pre- and post-impoundment. Thebasin is located in the tropical monsoon zone between
latitude 150 42’ 12” to 190 37’ 48” N and longitude 990 51’30” to 1010 21’ 48” E as shown in Fig.1. It covers an areaof some 34,140 sq.km.
The Nan river originates in the mountainous terrain andextends from Nan province in the northern part of Thailand
then flows southwards to Nakorn Sawan province to meetwith the other tributaries, Ping, Wang and Yom then form
the Chao Phraya river.
The multi-purpose Sirikit earthfill dam was constructedalmost at the middle reach of the river in 1971. The live
storage is approximately 6.0 bcm. The main purposes ofthis reservoir are for water supply, hydropower generation,irrigation and flood control.

4. Hydrological Data
In this study, the meteorological range of data wasconducted from 1952 to 2002. These include the climate
data that were collected and analyzed from 5 stations in thebasin area. Monthly rainfall data from 76 synoptic stationsin and around the study area were compiled and analyzed.
Flow data from 92 gauging stations were used to assess thewater resource within the basin.
It is revealed from the result of study that the mainvariation in climatic parameters throughout the basin
especially the temperature is due to altitude. The meanannual temperature in the basin area varies from 28.2 0C at
an elevation of some 20 m in southern part of the basin to
25.57 0C at the elevation of 150 m in northern part of thewatershed area.
The arena distribution of rainfall within the Nan river
basin is varied between 1,100 – 1,400 mm per annum witha mean annual rainfall of 1,260 mm over the basin as awhole. Temporal distribution of rainfall in the Nan river
basin is considerable significant. The amount of rainfall
during the rainy season (May to October) is 89% byaverage.
The average annual runoff from the basin outlet isapproximately 12,200 mcm per annum. The specific yield
based on the mean annual runoff is 11.33 l/s/sq.km.
(Budhakooncharoen, S, 2004)

5. Study Objective
Since the reservoirs vary in size and function, therefore
generalizations are difficult (Power, M.E. et. al., 1996).
The main goal of this preliminary research is therefore
merely aimed to determine the rainfall or flow regulation
has most affected to the long-term change of average flow
regime in the river after the impoundment of a single large
reservoir.

6. Methodology
The lengthy record of river discharges at station N1
upstream of Sirikit dam which is free from significant
diversion or regulation were used in comparison with thosefrom station N7 downstream of the river regulated
structures to assess the hydrological impact of thereservoir. The rainfall observations during the past 50
years at stations 28013 (near to the non-regulated gauging
station N1) and 38012 (near to the regulated gauging
station N7) were ultimately used to determine if theycaused the tendency change in the flow regime.
The streamflow and rainfall data during 1952 to 2002
were divided into pre- and post-dam hydrologic series. The
pre-dam period is during 1952 to 1971, while the post-dam
one is during 1972 to 2002. The runoff records both up anddownstream of the dam were statistically analyzed tocompare the pre- and the post-dam hydrograph withrespect to the mean monthly values. The mean annual flow
volume was also taken into consideration. The annual
rainfall observations were fitted to test for the presence ofan increasing or decreasing trend using the whole timeseries in comparison between the station up anddownstream of the impoundment. The average annual
rainfall during the pre- and the post-dam periods wereinvestigated.

7. Discussion of Results
In this study, the changes in both shape of streamflowhydrograph and the magnitude of long-term mean monthlyhydrological data were investigated. The results obtained

can be summarized as follows:
The change in average annual hydrological values toinvestigate the effect of Sirikit dam on the flow regime in
the Nan river is shown in Table 1. The annual rainfall timeseries were fitted with a least square trend line throughout
the period of 1952 to 2002 as shown in Fig.2.
Table 1: Change in average annual hydrological values
to investigate the effect of Sirikit dam on flow regime in
the Nan river

StationAverage annual value(unit per annum)
1952-1971(Pre-dam)

1972-2002(Post-dam)

Changeunit perannum+91.5 mm
-237.3 mm+23.1 mcm

%following
Post-dam
28013
1,167.0
1,258.4+7.8
38012
1,460.4
1,223.1-16.2
N12,775.7
2,798.8+0.8-706.9 mcm
N710,227.1
9,520.2-6.9
Remarks: 28013 is the code of rainfall station near to thenon-regulated gauging station N1 upstream of the Sirikit
dam while 38012 is that near to the regulated gauging
station N7downstream of the reservoir
It is revealed that the long-term statistical analysis ofannual rainfall at station 28013 upstream of the reservoir
has no discernible trend. In comparison between the preand the post-dam periods, the magnitude of average annual
rainfall observed at station 28013 has insignificantly
increased from 1,167.0 mm per annum during the pre-damperiod (during 1952 to 1971) to 1,258.4 mm per annum
during the post-dam period (during 1972 to 2002) or has
increased by 91.5 mm per annum (or 7.8%) following theimpoundment.
The average annual runoff upstream of the dam at the
non-regulated gauging station N1 which is located near to
rainfall station 28013 has consequently shown nosignificant difference in comparison between the pre- andthe post-dam periods. The average annual runoff during thepost-dam period is higher than that during the pre-dam
period only 23.1 mcm or increased only 0.8% following
the impoundment.
A slight decreasing trend in annual rainfall at station
38012 downstream of the reservoir has been observed
from Fig.2. There is no supporting evidence presented inthis study concerning the cause of downward trend of therainfall observed at this station. The decreasing trend of
rainfall observed from this station directly affects the flow
regime in the river downstream of the dam.
As shown in Table 1, in comparison between the timeseries before and after the dam, the magnitude of averageannual rainfall at station 38012 which is located near to theregulated streamflow station N7 has decreased from1,460.4 mm per annum during the pre-dam period (during1952 to 1971) to 1,223.1 mm per annum during the postdam period (during 1972 to 2002) or decreased by 237.3
mm per annum (or 16.2%) following the impoundment.

As a result of this decreasing rainfall, it becomes one of
the reasons the average annual runoff downstream of thedam at the regulated gauging station N7 has decreased
from 10,227 mcm per annum to 9,520 mcm per annum ordecreased by 6.91% following the impoundment.
However, it cannot be completely concluded that thedecline of annual runoff at this observed station issingularly due to the river regulation and the amount ofrainfall. It might be partially expected in view of either theamount of water that is used for irrigation upstream of thegauging station N7, climate or land coverage change.
Besides the annual runoff volume, comparison of themean monthly streamflow hydrograph was also taken intoinvestigation. Fig.3 illustrates the hydrologic change on thebasis of comparison of mean monthly streamflowhydrograph for the periods 1952 – 1971 (pre-dam period)and 1972 – 2002 (post-dam period).
Before the creation of the dam, the annual streamflowhydrograph of the Nan river at the non-obstructed station
N1 had a relatively consistent annual hydrograph with thepost-dam hydrograph of the same observed station. But,
the magnitude of peak flow regime downstream of thereservoir is evident post-disturbance according to theannual hydrograph at the regulated gauging station N7
downstream of the dam. High runoff seems essentially tobe eliminated by dam. In September, the mean monthly
peak discharge at gauging station N7 downstream of thedam has decreased by 39%. In addition, it is clearly seenfrom the graph that low flow in the river downstream of
the water resource development scheme during the dryseason (during November to April) has increased following
the impoundment.
It is seen that the Sirikit dam plays a significant role incapable of storing surplus volume of wet season runoff(during May to October) for the later dry season (duringNovember to April). The substantial surplus storage seems
to influence in providing flood mitigation to downstream
reach.

8. Conclusions
This preliminary research effort has reported theadjustment of the flow regime following the impoundmentof a single large reservoir. It can be concluded that theimpoundment and the amount of rainfall are one of themajor causes of the flow regime change. But, the post-flowhydrograph change, especially the reduction of peak runoff
magnitude and the higher flow during the low flow period
downstream of the dam mainly results from the impact ofimpoundment.
The impoundment effect under the condition in thisstudy is likely to preclude the effects of reservoir functions
and appropriate spatial controls. Without these controls,
long-term change in the flow regime caused by change ofclimate or land use may be incorrectly ignored. To copewith the misleading problem due to lack of spatial control,
focus should be made also on the un-impounded river
basin.

Thus, to observe, understand and model thisphenomenon on a macro scale, further study shouldemphasize on the advance development of precioushydrometeorological data observation and management.
The frequency of flood and drought events resulting fromflow regulation across a range of dam types and catchment
characteristics should be further studied. In addition, shift
in low flow, timing of high and low flow or change acrossdifferent flow duration might be significant. It should betaken also into investigation.
It revealed the necessity to establish the integrated
database of the regional long-term hydrological andmeteorological properties from intensive observation. Thisis due to the effect of management on the annual cycle ofwater flow has been the subject of considerable researchon historical flows in the main stem. The study on
variability entails uncertainty requires long-term historicalrecords. This precious data set may serve as the robust predam data in the future and these data should be accessible
for all researchers.

References
Budhakooncharoen, S. Floods and droughts : sustainableprevention and management, Proceeding of the 4thInternational Conference on Ecosystems and Sustainable
Development, Siena, Italy, 4 – 6 June, 2003
Budhakooncharoen, S. Hydrometeorological data collection andwater resources assessment in Nan river basin, Proceedingof the Regional Seminar on Hydro-informatics Membrane
System in 21st Century through Utilization, Delineationand Implementation (Crystallization II), Kasetsart
University, Bangkok, Thailand September 10, 2004
Magilligan, F.J. and Nislow, K., Long-term changes in regionalhydrologic regime following impoundment in a humidclimate watershed. J. American Water Resources
Association, 37, 1551 – 1569, 2001
Power, M.E., W.E. Dietrich and J.C.Finalay, Dam anddownstream aquatic biodiversity :potential food web
consequences of hydrologic and geomorphic change.
Environmental Management, 20(6), 887-895, 1996.

Annual Rainfall(mm per annum)

Annual rainfall at station 28013 (near to non-regulated gauging station N1)
2500
2000
1500
1000
5000
1940

1950

1960

1970

1980

1990

2000

2010

2000

2010

Annual Rainfall(mm per annum)

Annual rainfall at station 38012 (near to regulated gauging station N7)
2500
2000
1500
1000
5000
1940

Fig.

1950

1960

1970

1980

1990

2: Trends of annual

rainfallMean Monthly
Discharge (mcm)

Non-regulated annual hydrograph at gauging station N1(Upstream of Sirikit Dam)
( ____ pre-impoundment - - - - post-impoundment)
1000
750
500
250
0AprMayJunJulAug

SepOctNovDecJanFebMarJanFeb

Mar

Mean Monthly
Discharge (mcm)

Regulated annual hydrograph at gauging station N7(Downstream of Sirikit Dam)
( _____ Pre-impoundment - - - - Post-impoundment)
4000
3000
2000
1000
0AprMay

JunJulAug

Fig. 3: Annual streamflow hydrograph

SepOctNov

Dec