EVALUATION OF TRIBUTARIES CONTRIBUTION IN THE
MEKONG RIVER BASIN DURING RAINY AND DRY SEASON
OSAMU TODA
Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi,
Bunkyo-ku, Tokyo 113-8657, Japan
HAJIME TANJI
National Institute for Rural Engineering, River and Coast Laboratory, 2-1-6 Kan-non dai,
Tsukuba, Ibaraki 305-8609, Japan
HIROAKI SOMURA
National Institute for Rural Engineering, River and Coast Laboratory, 2-1-6 Kan-non dai,
Tsukuba, Ibaraki 305-8609, Japan
KASTUHIRO HIGUCHI
National Institute for Rural Engineering, River and Coast Laboratory, 2-1-6 Kan-non dai,
Tsukuba, Ibaraki 305-8609, Japan
KOSHI YOSHIDA
Japan Science and Technology Agency, River and Coast Laboratory, 2-1-6 Kan-non dai, Tsukuba,
Ibaraki 305-8609, Japan
The Mekong River is one of the most important rivers in South-East Asia. The Mekong
flows through 6 countries, and has many tributaries in each country. It is important for a
water resource management to evaluate the tributaries contributions of the Mekong. in
this study, we evaluated the contributions of tributaries to the main stream, using MRC’s
hydrologic data, water depth and discharge date from January 1991 to March 2001.
Secondly the relationship between minimum and maximum flow were evaluated to grasp
the characteristics of the Mekong Tributaries. As the results, it became obviously that the
contributions of Laos’s tributaries were high in rainy season, and that the relationship
between minimum and maximum flow of Laos’s tributaries were parallel to Japanese
rivers which had high coefficients of river regime.
INTRODUCTION
The Mekong River is one of the largest international rivers in the world. The Mekong
River flows through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. The area
of the basin itself amounts to 795,500 km2 with a total length of 4,620 km. Both figures
place the Mekong among the world’s largest rivers (Hori [1]). It is very important to
1
2
consider the water utilizations of these
international rivers. Because some
changes of the water use in one country
would bring great influences to lower
countries. In the case of the Mekong, any
development of the water resource on the
main stream have been regulated by
Mekong River Commission (MRC) that
has been established by lower four
countries excepting China and Myanmar.
Tributaries of the Mekong, however have
been developed by each country. To
comprehend the influences of the water
resources
developments
at
these
tributaries to lower streams, it is
important to evaluate contributions of
each tributary to the main stream. As the
lower Mekong belongs to Asian
Figure 1. A Cover of YEARBOOK
Monsoon region, a year is divided into
published by MRC (1987)
two seasons, rainy and dry seasons. In
dry season it is important to consider
maximum discharge for flood control, and in rainy season minimum discharge is critical
factor for harvesting, estimating irrigated water and constructing some new facilities for
irrigation like a pump station.
However we don’t have enough information of them. Although MRC gives a map of
the lower Mekong and some tributaries, this map shows only the annual flows (MRC [2];
Figure 1). Dry season flow, it’s important for water use, and rainy season one, it’s
important for defense against flooding, are not given from the MRC’s map. In addition,
this map has not been renewed for more than 10 years.
In this study, the contribution of tributaries in each season, dry and rainy season, was
evaluated, using latest statistical hydrology data from MRC. Then sound utilizations of
water resources in dry season and flood control in the rainy season whole the Mekong
basin are estimated.
METHODOROGY
Season
Seasons were defined that rainy season was from 1 April to 31 October and Dry season
was from 1 November to 31 March. The rainy season of one year was the average from 1
April to 31 October of this year, and the dry season was the average from 1 November of
this year to 31 March of next year. Although each country had their own period of season,
this definition was taken from the Laos season cited from the government tourist office of
3
Laos. Because Laos locates on the middle Mekong basin where the rain fall heavily
affects the lower Mekong, and has many tributaries whose discharge are not too small.
Hydrological data
Daily data of the water depth and discharge from 1 January 1991 to 31 March 2001 were
used. The lack of discharge data at some tributaries was complemented from each
discharge-curve that has been calculated from data of the past.
Tributaries
23 tributaries and main stream flowing from Chiang Sean to Kratie were estimated
(Table1). The total catchment area of the Mekong is 795,500km2. Catchment areas of
each tributary ware quoted from MRC. From these data, proportions of each tributary
were calculated.
Contributions of tributaries were represented as percentage of the tributary-discharge
to main stream’s discharge. Discharge of Kratie in Cambodia was applied as 100% of the
main stream’s discharge. At the downstream from Kratie, hydrology condition is
exceedingly complicate. In rainy season of every year, the floods inundate the
downstream because this area is flat and there is extensive lateral flow of flood waters
from Kratie to the border with Viet Nam (MRC [3]). In addition, hydrological data of
recent years is too scarce to analyze at the present time.
Coefficient of river regime
To evaluate hydrological characteristics of tributaries, coefficients of river regime were
estimated, using maximum-minimum discharge data of Nam Ngum in 1991, Se Done in
1996 and other tributaries in 2000.
Table 1. Percentage of Catchments of 23 Tributaries and Chiang Sean
R iver
M ekong
N am
N am
at
M ae Kok M ae Ing
C hiang
C atchm ent
189000 10701
2
(km )
(%)
23.8
1.3
C ountry
Thai
Thai
N am
H eung
N am
Loei
N am
M un-C hi Songkra
m
N am
Tha
N am O u
N am
Soung
N am
Khan
N am
N gum
7267
4901
4012
119707 13123
8918
26033
6578
7490
16906
0.9
0.6
0.5
15.0
1.6
1.1
3.3
0.8
0.9
2.1
Thai
Thai
Thai
Thai
Thai
Laos
Laos
Laos
Laos
Laos
Kratie
P rek
C hhlong
Tonle
Sap
P rek
Thnot
6124
N am
N hiep
N am
Sane
N am
Se B ang Se B ang
Se D one Se Kong Se San P rek Te
C ading
Fai
H ieng
4577
2230
14822
10407
19958
7229
28815
49830
4364
5957
85851
0.6
0.3
1.9
1.3
2.5
0.9
3.6
6.3
0.5
0.7
10.8
0.8
Laos
Laos
Laos
Laos
Laos
Laos
C am b
C am b
C am b
C am b
C am b
C am b
C am b
4
RESULT AND DISCUSSION
The annual flows of each tributary and contributions of each tributary to the main stream
at Kratie were calculated (Table 2a, 2b). Contributions of most tributaries, except
Mun-Chi, Nam Khan, Nam Ngum and Se San River, to the main stream in rainy season
are higher than contributions in dry season. On the other hand, the contributions of
discharges at Chiang Sean, Mun-Chi, Nam Ngum and Se San in rainy season are lower
than contributions in dry season. Nam Khan has almost same contributions in rainy and
dry season.
Table 2a. Average Flows of Rainy and Dry Season (m3/s)
R
A
I
N
Y
D
R
Y
R iver
/ Year
M ekong
at
C hiang
91
92
93
94
95
96
97
98
99
00
91
92
93
94
95
96
97
98
99
00
4171
2274
3383
3210
4117
3936
3124
3669
3389
4476
1780
1220
1395
1293
1639
1375
1068
1026
1631
1982
N am
N am
M un-C hi
H eung
Soung
66
46
49
100
80
66
46
77
73
8
19
15
30
6
15
9
6
16
20
1230
994
489
934
721
882
847
375
737
2381
462
189
102
399
401
748
185
118
373
697
178
41
175
141
173
195
155
74
132
122
28
18
10
44
35
48
33
21
34
36
N am
Khan
N am
N gum
N am
N hiep
N am
Sane
106
45
124
233
133
139
129
67
117
139
31
31
21
61
35
40
35
19
38
41
655
230
146
272
401
327
237
268
167
443
316
56
44
55
78
111
70
76
32
57
69
186
128
290
242
295
237
235
173
234
238
23
22
32
32
27
36
18
20
30
34
320
N am
Se B ang Se B ang
M ekong
Se D one Se Kong Se San
C ading
Fai
H ieng
at Kratie
392
826
1499
1114
108
115
117
73
165
216
684
523
633
577
171
474
689
30
76
22
34
48
52
35
32
47
36
894
796
366
898
761
985
780
260
898
1418
109
144
89
82
139
200
68
134
318
120
713
285
126
311
144
276
116
15
10
13
8
73
15
390
1753
1445
1745
1827
269
1818
2632
228
188
334
316
376
445
301
235
574
514
3599
2816
2881
2786
902
3076
3847
408
1028
817
728
1169
249
1288
1016
928
22026
15260
16330
24563
20235
21794
22408
11919
21099
29522
5735
5402
5170
5358
6046
8192
5372
5367
7393
6593
Table 2b. Contributions of Each Tributary to Main Stream at Kratie (%)
R
A
I
N
Y
D
R
Y
R iver
/ Year
M ekong
at
C hiang
91
92
93
94
95
96
97
98
99
00
91
92
93
94
95
96
97
98
99
00
18.9
14.9
20.7
13.1
20.3
18.1
13.9
30.8
16.1
15.2
31.0
22.6
27.0
24.1
27.1
16.8
19.9
19.1
22.1
30.1
N am
N am
M un-C hi
H eung
Soung
0.3
0.3
0.3
0.4
0.4
0.3
0.4
0.4
0.2
0.1
0.4
0.3
0.6
0.1
0.2
0.2
0.1
0.2
0.3
5.6
6.5
3.0
3.8
3.6
4.0
3.8
3.1
3.5
8.1
8.1
3.5
2.0
7.4
6.6
9.1
3.4
2.2
5.0
10.6
0.8
0.3
1.1
0.6
0.9
0.9
0.7
0.6
0.6
0.4
0.5
0.3
0.2
0.8
0.6
0.6
0.6
0.4
0.5
0.5
N am
Khan
N am
N gum
N am
N hiep
N am
Sane
0.5
0.3
0.8
1.0
0.7
0.6
0.6
0.6
0.6
0.5
0.5
0.6
0.4
1.1
0.6
0.5
0.7
0.4
0.5
0.6
3.0
1.0
1.0
1.7
1.6
1.6
1.1
1.2
1.4
2.1
1.1
1.0
0.8
1.1
1.5
1.8
0.9
1.4
0.6
0.8
1.0
0.8
0.8
1.8
1.0
1.5
1.1
1.0
1.5
1.1
0.8
0.4
0.4
0.6
0.6
0.5
0.4
0.3
0.4
0.4
0.5
5.6
N am
Se B ang Se B ang
M ekong
Se D one Se Kong Se San
C ading
Fai
H ieng
at Kratie
2.6
5.1
6.1
3.8
1.9
2.1
2.3
1.0
2.5
1.4
2.8
2.6
2.9
2.6
1.4
2.2
2.3
0.5
1.4
0.4
0.6
0.8
0.6
0.7
0.6
0.6
0.5
4.1
5.2
2.2
3.7
3.8
4.5
3.5
2.2
4.3
4.8
1.9
2.7
1.7
1.5
2.3
2.4
1.3
2.5
4.3
1.8
3.2
1.9
0.8
1.3
0.7
1.3
0.0
2.0
0.3
0.2
0.2
0.1
0.9
0.0
2.6
7.1
7.1
8.0
8.2
2.3
8.6
8.9
4.0
3.5
6.5
5.9
6.2
5.4
5.6
4.4
7.8
7.8
14.7
13.9
13.2
12.4
7.6
14.6
13.0
7.6
19.9
15.2
12.0
14.3
4.6
24.0
13.7
14.1
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
5
Table 2c. Contributions of Each Tributary to Main Stream at Kratie (10 years average)
R iver
/ Year
C ountry
R ainy (m 3/s)
(%)
D ry (m 3/s)
(%)
A vg.(m 3/s)
(%)
M ekong
at
C hiang
Thai
3575
17.4
1441
23.8
1264
18.9
N am
N am
M un-C hi
H eung
Soung
N am
Khan
N am
N gum
N am
N hiep
N am
Sane
N am
Se B ang Se B ang
M ekong
Se D one Se Kong Se San
C ading
Fai
H ieng
at Kratie
Thai
Thai
Laos
Laos
Laos
Laos
Laos
Laos
Laos
Laos
Laos
C am b
C am b
C am b
67
0.3
15
0.2
21
0.3
959
4.7
367
6.1
334
5.0
139
0.7
31
0.5
43
0.6
123
0.6
35
0.6
40
0.6
655
3.2
320
5.3
246
3.7
281
1.4
65
1.1
87
1.3
226
1.1
27
0.5
64
1.0
958
4.7
115
1.9
270
4.0
496
2.4
41
0.7
135
2.0
806
3.9
140
2.3
238
3.6
309
1.5
36
0.6
87
1.3
1485
7.2
351
5.8
462
6.9
2844
13.9
848
14.0
930
13.9
20516
100
6063
100
6695
100.0
At the Chiang Sean, the contribution of dry season discharge to the main stream is
23.8 % and the contribution of rainy season one is 17.4 %. Thus the flow from China
during dry season is more dominant to the Mekong than in rainy season. It indicates that
dams constructed by China, on the upper Mekong, contribute to the flow extending in dry
season. Mun-Chi, a tributary flowing in North-Eastern Thailand, has 15.0% of the
catchments, and the contribution is 4.7% in rainy season and 6.1% in dry season.
According to Association for International Cooperation of Agriculture & Forestry
(AICAF) (AICAF [4]), the mean annual rainfall in this area is about 1000mm. It can be
considered that the reason why Mun-Chi has relatively small discharge, in spite of own
large catchments, is its short rainfall.
At
Chiang
Sean
At
Chiang
Sean
23.8
23.8
17.4
23.8
1.1 Nam Tha
1.1 Nam Tha
3.3 Nam Ou
0.7
0.8 Nam Suang
0.6
Nam
Mae Kok
1.3
3.0
0.9
Nam Mae Ing
2.1 Nam Ngum
1.4
1.1
Nam
0.3
Heung 0.6
Nam Loei 0.6
0.9 Nam Khan
3.3 Nam Ou
0.5
0.8 Nam Suang
0.6
Nam
Mae Kok
1.3
5.3
0.9
Nam Mae Ing
0.6 Nam Nhiep
0.5
0.3 Nam Sane
4.7
1.9 Nam Cading
2.4
1.6
Nam
Songkram
Nam
0.2
Heung 0.6
Nam Loei 0.6
1.3 Se Bang Fai
1.6
Nam
Songkram
0.6 Nam Nhiep
0.3 Nam Sane
1.9
1.9 Nam Cading
0.7
1.3 Se Bang Fai
2.3
3.9
2.5 Se Bang Hieng
Mun-Chi
1.5
15.0
15.0
Percentage of
Total Catchment Aria
6.1
Percentage of
Average Annual Flow
13.9
Kratie
2.5 Se Bang Hieng
0.6
15.0
7.2 3.6 Se Kong
10.8
Mun-Chi
0.9 Se Done
4.7
Tonle Sap
6.3 Se San
15.0
Percentage of
Total Catchment Aria
6.1
Percentage of
Average Annual Flow
5.8 3.6 Se Kong
14.0
Tonle Sap
10.8
0.5 Prek Te
Kratie
Rainy Season
6.3 Se San
0.5 Prek Te
0.7 Prek Chlong
0.8
Prek
Thnot
0.9 Se Done
6.1
0.7 Prek Chlong
0.8
0.9 Nam Khan
2.1 Nam Ngum
1.1
Prek
Thnot
Dry Season
Figure 2. Tributaries Contributions (April 1991－March 2001)
6
In Laos, the total catchments of 9 tributaries, having discharge data, are 11.3%, and their
contributions are 13.8% in dry season, 19.4% in rainy season. Thus the tributaries of Laos
have large discharges, though their small catchments. It means that the tributaries of Laos
influence to the main stream strongly.
In Cambodia, only Se San and Se Kong have available discharge data. These two
tributaries flow together near the Mekong. The total catchments of them are 9.9% and the
total contribution in dry season is 19.8%, 21.1% in rainy season. As Laos, Se San and Se
Kong effect to the main stream largely.
In both of dry and rainy season, the tributary contributions of Laos and Cambodia
have big influences to the main stream, in spite of small catchments, comparing with
tributaries of Thailand. Though Mun-Chi is the largest tributaries having 15.0 % of the
catchments, the discharge contribution is only 6.1 % in dry season and 4.7 % in rainy
season. On the other hand, although the total catchments of tributaries in Laos; 11.3 % or
Cambodia; 9.9 % is smaller than Mun-Chi, the contribution of each is bigger than
Mun-Chi, about 3 to 4 times as big as Mun-Chi. In rainy season, the sum of their
contributions is double of Chiang Sean, and it indicates that the tributaries of Laos and
Cambodia would contribute toward the rainy flood at the lower Mekong largely. In dry
season, meanwhile, the contribution of discharge at Chiang Sean is about one-quarter of
the main stream’s flow. Then it is indicated that the discharge from the upper Mekong in
dry season contributes to the drought at the lower Mekong.
From the relationship between minimum and maximum flows, coefficients of river
regime of the Mekong Tributaries are evaluated (Mizumura [5]; Figure 3). Comparing the
results with some international or Japanese rivers, the characteristics of the Mekong
tributaries are parallel to the Japanese rivers, and the Mekong River is parallel to the
international rivers. On tributaries of the Mekong, consequently, water resources of them
from rainfall are apt to run off. In the case of Nam Ngun, the relationship between
minimum and maximum flow are relatively close to the international, continental rivers.
This is considered as an effect from having a dam in own stream.
Table 4. Coefficients of River Regime (2000)
R iver
M ekong
at
C hiang
M in.(m 3/s)
M ax.(m 3/s)
C oefficient of
R iver R egim e
808
10700
13
N am
N am
M un-C hi
H eung
Soung
4
504
120
57
5360
94
7
366
52
N am
Khan
N am
N gum
(1991)
N am
N hiep
N am
Sane
N am
Se B ang Se B ang Se D one
M ekong
Se Kong Se San
C ading
Fai
H ieng (1996)
at Kratie
11
894
84
211
1788
8
18
929
52
11
891
78
21
4493
210
21
3209
152
34
7834
230
1
2998
2216
167
7712
46
305
13054
43
4021
67132
17
7
105
M ississippi
M ekong
at K ratie
Maximum Flow (m3/s)
M ekong
Tone
104
Kiso
Yodo
S e B ang H ieng
Shi
nano
M ogam i
N am C ading M un-C hi
Tenryu
A rakaw a.
S e D one (1996)
N aka
N ile
S e S an
S e K ong
M ekong at C hiang S ean
R hine
S e B ang Fai
M endoza
Saskatchew an
N am N gum (1991)
N am K han
N am N hiep
N am S ane
103
YodoJapanese R iver
N ile InternationalR iver
N am H eung
N am S oung
M ekong
hiang Sean
TheatMCekong
SeTri
San
butaries of The M ekong
10
2
1
10
102
Minimum Flow (m3/s)
103
104
Figure 3. Relationship between Minimum and Maximum Flow
REFERENCES
[1] Mekong River Commission: MRC, “LOWER MEKONG HYDROLOGIC
YEARBOOK”, (1987).
[2] Hori H., “The Mekong – Environment and Development”, (2001).
[3] MRC, “State of the Basin Report 2003”
[4] Association for International Cooperation of Agriculture & Forestry, “Agriculture
and Forestry in Thailand, in Japanese”, (1995)
[5] Mizumura K., “Hydrology, in Japanese”, Sankaido, (2002)