Appendix A-3
DSF Demonstration Scenario No.3:
Catchment Cover Change
Mekong River Commission - Water Utilisation Project Component A:
Development of Basin Modelling Package and Knowledge Base (WUP-A)
Technical Reference Report:: DSF 650 DSF Testing and Evaluation
Contents
3
Scenario 3: Impact of Catchment Cover Change
3.1 Scenario Objective
3.2 Specification of Scenarios in the DSF
3.2.1 Climate Component
3.2.2 System Demand Component
3.2.3 Assumed Interventions Component
3.3 Model Setup
3.3.1 SWAT Models
3.3.2 IQQM Model
3.3.3 iSIS Model
3.4 Demonstration Scenario 3: Impact of Catchment Cover Change – Test
Results: Primary Level
3.5 Scenario 2: Catchment Cover Change– Test Results: Secondary Level
3.5.1 Test 1: Acceptable Minimum Monthly Dry Season
Flow
3.5.2 Test 2: Acceptable Reverse flow of Tonle Sap in Wet
Season
3.5.3 Test 3: Average Daily Peak Main Stream Flow in the
Flood Season
3.5.4 Test 4: Extent and Duration of Flooding
3.5.5 Test 5: Extent and Duration of Saline Intrusion
3.5.6 Test 7: Degree of Connection for Fisheries Purposes
3.6 Conclusions and Recommendations
106751643
1
1
1
1
1
1
1
1
4
4
4
7
7
15
16
19
20
22
25
A.3- ii
Mekong River Commission - Water Utilisation Project Component A:
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
Figures
Figure A 3-1:
Land use Change verses Baseline Scenario flows at Vientiane
(1997 – 2000)........................................................................................... 7
Figure A 3-2:
Land use Change verses Baseline Scenario flows at Kratie (1997 –
2000) ......................................................................................................... 8
Figure A 3-3:
Dry season flow duration curves for Vientiane and Kratie (dry
seaon; 1 Dec – 31 May data period; 1 Dec 1985 – 31 Dec 2000) 11
Figure A 3-4:
Percentile flow distribution of 90 %, 50% and 10% at Vientiane
with comparisons to Baseline conditions for 50% and 10%
(directly over-laid as no change).........................................................12
Figure A 3-5:
Percentile flow distribution of 90 %, 50% and 10% at Kratie with
comparisons to Baseline conditions (directly over-laid as no
change) ...................................................................................................12
Figure A 3-6:
Percentile flow distribution of 90 %, 50% and 10% at Kratie with
comparisons to Baseline conditions (directly over-laid as no
change) – truncated vertical axis ........................................................13
Figure A 3-7:
Annual flood frequency at Kratie ......................................................16
Figure A 3-8:
Daily time-series showing high flows for Vientiane and Kratie
(Feb 1985 – Dec 2000) ........................................................................17
Figure A 3-9
Map showing the difference in inundation depths downstream
Kratie for peak flood conditions in year 2000 (Baseline Conditions
verses Catchment Cover Change) ......................................................19
Figure A 3-10
Map showing the extent of Catchment Cover Change saline
intrusion for maximum dry-season salinity intrusion in year 2000
.................................................................................................................21
Figure A 3-11
Comparison of flows at Chau Doc for year 2000 (Baseline
Conditions versus Catchment Cover Change).................................23
106751643
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
Figure A 3-12
Comparison of flows at Tan Chau for year 2000 (Baseline
Conditions versus Catchment Cover Change).................................24
Figure A 3-13
Comparison of flows at Tan Chau for year 2000 (Baseline
Conditions versus Catchment Cover Change).................................24
Tables
Table A 3-1:
Conversion table for CN2 curve numbers ......................................... 3
Table A 3-2:
Between scenario comparison of annual minimum daily flows
(cumec) ..................................................................................................... 9
Table A 3-3:
Mean daily dry season flow for each month of the dry season ....... 9
Table A 3-4:
Mean start and end of the dry season based on the Baseline
Scenario’s median annual flow (Oct 1985 – Dec 2000) .................13
Table A 3-5:
Duration of Stable Flow Conditions for the Baseline Scenario
(days).......................................................................................................15
Table A 3-6:
Change in mean annual peak daily flow at Kratie ...........................15
Table A 3-7:
Comparison of Highest and Average Annual Peak Daily Flows ..17
Table A 3-8:
Comparison of flood return periods for mainstream monitoring
sites .........................................................................................................18
Table A 3-9:
Maximum inundation areas by depth class in the year 2000 .........20
Table A 3-10:
Inundation duration > 0.5 m depth for the year 2000 ...................20
Table A 3-11:
Maximum dry-season salinity intrusion areas ..................................21
Table A 3-12:
Impacts of catchment cover change on sustainable irrigation area
.................................................................................................................22
106751643
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Table A 3-12:
106751643
Impacts of catchment change on flows and water levels at Chau
Doc, Tan Chau and Phnom Penh (Mekong) for year 2000 ..........25
A.3- v
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Mekong River Commission - Water Utilisation Project Component A:
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
3
Scenario 3: Impact of Catchment Cover
Change
3.1
Scenario Objective
The objective is to assess the effect of reducing catchment forest cover by half. It is
expected that such a change would have impacts in both the wet and dry seasons,
making both more extreme.
3.2
Specification of Scenarios in the DSF
3.2.1
Climate Component
There are no differences compared with the Baseline Scenario.
3.2.2
System Demand Component
There are no differences compared with the Baseline Scenario.
3.2.3
Assumed Interventions Component
The reduction in forest cover is achieved through changes in those SWAT sub-basin
models in which there is forest, i.e. it is assumed that all existing forest areas are reduced
by 50%.
Normally this would be achieved by changing both the CN2 curve number variable and
the crop management type variable. However, given the contrary results in some subbasins if the latter is changed, it was decided to change on the CN2 factor.
3.3
Model Setup
3.3.1
SWAT Models
Model Configuration
No changes required.
106751643
A.3- 1
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Model Input Variables
For each sub-basin covered with forest (five types of forest have been considered in this
tests, i.e. DECD, EHCD, EMLD, MEDM and WSEV), the variable "CN2" within the
associated input file (.mgt) for land and water management has been changed to the
value representing for the sub-basin covered with half-forest and agriculture. The
values of CN2 for each sub-basin are shown in Table A 3-1.
106751643
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Table A 3-1:
106751643
Conversion table for CN2 curve numbers
Subbasin
Hydrologic
Soil Group
CN2
Existing Mgt
Landuse
101
201
203
204
205
206
207
209
212
214
301
305
306
402
405
406
411
412
413
420
421
423
424
426
427
431
509
510
511
513
514
515
519
601
602
603
604
606
607
608
609
610
613
614
616
617
618
619
621
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
D
B
B
B
B
B
B
B
B
B
B
B
B
C
B
B
B
B
B
B
B
B
B
B
B
B
B
55
61
61
61
61
70
61
61
61
61
85
85
85
74
74
55
66
61
55
61
61
55
80
74
61
66
65
65
65
66
75
66
65
77
77
77
77
55
77
61
61
61
66
75
77
77
77
77
77
EHCD
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
WSEV
EHCD
MEDM
WSEV
EHCD
WSEV
WSEV
EHCD
WSEV
WSEV
WSEV
MEDM
DECD
WSEV
DECD
WSEV
WSEV
WSEV
DECD
DECD
DECD
DECD
DECD
EHCD
EMLD
WSEV
WSEV
WSEV
DECD
WSEV
EMLD
EMLD
EMLD
EMLD
EMLD
50% Conversion to AGRF
CN2 rev
Factor
1.18
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.03
1.18
1.07
1.03
1.18
1.03
1.03
1.18
1.05
1.03
1.03
1.07
1.00
1.03
1.00
1.03
1.03
1.03
1.00
1.00
1.00
1.06
1.00
1.18
1.04
1.03
1.03
1.03
1.00
1.03
1.04
1.04
1.04
1.04
1.04
65.1
62.8
62.8
62.8
62.8
72.0
62.8
62.8
62.8
62.8
87.5
87.5
87.5
76.1
76.1
65.1
70.6
62.8
65.1
62.8
62.8
65.1
84.1
76.1
62.8
70.6
65.2
66.9
65.2
67.9
77.2
67.9
65.2
77.3
77.3
81.4
77.3
65.1
80.0
62.8
62.8
62.8
66.2
77.2
80.0
80.0
80.0
80.0
80.0
A.3- 3
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3.3.2
IQQM Model
Model Configuration
No changes required.
Model Input Variables
No changes required.
3.3.3
iSIS Model
Model Configuration
No changes required.
Model Input Variables
No changes required.
3.4
Demonstration Scenario 3: Impact of Catchment Cover Change – Test Results:
Primary Level
Tests 1 to 3 : MRC Agreement, Article 6 requirements
Tests against Article 6
Scenario
conforms?
Most Critical
Location?
By how much?
Test 1:
Acceptable Minimum Monthly Dry
Season Flow
Yes
N/A
Essentially no change
Test 2:
Acceptable Reverse flow of Tonle
Sap in Wet Season
Yes
Kratie
Essentially no change
Test 3:
Average Daily Peak Main Stream
Flow in the Flood Season
Yes
N/A
Essentially no change
106751643
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Test 4: Extent and Duration of Flooding (km2) – downstream of Kratie
Flooded Area >0.5 m by Duration (km2)
Year 2000 (Wet Year)
Scenario
> 1 day
> 1 mth
> 2 mths
> 4 mths
> 6 mths
Baseline Conditions
59,300
55,444
51,687
43,679
29,650
Catchment Cover Change
59,411
55,477
51,776
43,634
29,706
Difference
Area (km2)
111
33
89
-44
56
Difference
Percent (%)
0.2
0.1
0.2
-0.1
0.2
Test 5: Extent and Duration of Saline Intrusion (km2) – in the delta
Maximum Saline Intrusion by Salinity Class (km 2)
Jan 2000 – June 2000
Scenario
> 1 g/l
> 4 g/l
> 8 g/l
> 15 g/l
Baseline Conditions
21,603
19,287
17,279
14,195
Catchment Cover Change
21,671
19,324
17,286
14,183
Difference
Area (km2)
69
37
7
-13
Difference
Percent (%)
0.3
0.2
0.0
-0.1
Test 6: Irrigated agriculture performance
Total Deficit Upstream of Kratie
(million hectare days)
Baseline Conditions
42.77
Scenario 3: Catchment Cover Change
37.87
Difference (million hectare days)
Difference (%)
4.90
11.8%
Test 7: Degree of Connection for Fisheries Purposes
Maximum Connected River Length (km)
106751643
Maximum Flooded Area (km2)
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Baseline Conditions
45,571
43,328
Scenario 3: Catchment Cover Change
45,571
43,333
Difference (Km or Km2)
0
5
Difference (%)
0
0.0
106751643
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3.5
Scenario 2: Catchment Cover Change– Test Results: Secondary Level
3.5.1
Test 1: Acceptable Minimum Monthly Dry Season Flow
Figure A 3-1 and Figure A 3-2 display daily flows for the last three years for Vientiane
and Kratie. The difference between them is shown by the residual series (lowest black
line). At both sites only one hydrograph is visible as there is minimal difference
between the Baseline Scenario and the Land use Change Scenario, as indicated by the
‘residual’ line oscillating around zero.
Time Series Analysis Tool
18,000
17,000
16,000
15,000
14,000
13,000
12,000
Flow(cumecs)
11,000
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
26/11/1997 26/03/1998 24/07/1998 21/11/1998 21/03/1999 19/07/1999 16/11/1999 15/03/2000 13/07/2000 10/11/2000
01/Feb/1985 - 31/Dec/2000
Figure A 3-1:
106751643
Land use Change verses Baseline Scenario flows at Vientiane (1997 – 2000)
A.3- 7
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Time Series Analysis Tool
60,000
55,000
50,000
45,000
40,000
Flow(cumecs)
35,000
30,000
25,000
20,000
15,000
10,000
5,000
0
26/11/1997 26/03/1998 24/07/1998 21/11/1998 21/03/1999 19/07/1999 16/11/1999 15/03/2000 13/07/2000 10/11/2000
01/Feb/1985 - 31/Dec/2000
Figure A 3-2: Land use Change verses Baseline Scenario flows at Kratie (1997 – 2000)
Differences between the Baseline and Catchment Cover Change Scenarios’ lowest
minimum flows, on an annual basis are provided in Table A 3-2. The table compares
both the extreme lowest daily flow over the entire 16 years, as well as the mean annual
lowest flow.
It can be seen that there is essentially no difference in the lowest annual flows, be they
the mean or extreme annual minimum values. The greatest difference is at Stung Treng
where mean minimums are reduced by just 2% in each case. This is due to the fact that
flow changes in the dry season from this type of intervention, are a function of changes
in wet season flows, and these are also minimal as shown in Table A 3-7.
106751643
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Table A 3-2:
Between scenario comparison of annual minimum daily flows (cumec)
Chiang
Saen
Luang
Prabang
Vientiane
Baseline Lowest Annual
Minimum Flow
338
582
680
Land use Change Lowest
Annual Minimum Flow
337
578
Difference (cumec)
-1
Nakhom
Phanom
Mukdahan
Pakse
Stung
Treng
Kratie
909
975
1,014
1,328
1,324
676
899
980
1,006
1,303
1,334
-5
5
1
-8
1
-26
2
10
1
-10
1
0
-3
1
Baseline Mean Annual
Minimum Flow
713
803
844
1,220
1,235
1,298
1,814
1,819
Land use Change Mean
Annual Minimum Flow
712
799
840
1,210
1,234
1,285
1,775
1,807
Difference (cumec)
-1
-4
1
-4
-10
1
-1
-14
1
-39
2
-12
1
Difference (%)
Difference (%)
-
-
0
-
-
0
-
0
1
Using Test 1.1, the mean daily dry season flows for each month of the dry season are
tabulated below in Table A 3-3. There is essentially no change in mean monthly flows
at all sites and for all months of the dry season, i.e. land use change has not resulted in
any change in dry season flow volume.
Table A 3-3:
Station Name
Chiang Saen
Luang Prabang
Vientiane
106751643
Mean daily dry season flow for each month of the dry season
Scenario
Base Line mean
B-L Std Dev
Land use Change
Difference (cumec)
Dec
-
Difference (%)
Base Line mean
B-L Std Dev
Land use Change
Difference (cumec)
Difference (%)
Base Line mean
B-L Std Dev
Land use Change
-
Jan
Feb
1580
224
1,557
23
1150
149
1,155
5
-1
2182
344
2,142
40
-2
2170
351
2,128
Mar
April
May
944
137
944
0
876
137
879
3
939
169
942
3
1421
302
1,423
2
0
0
0
0
0
1436
239
1,462
26
2
1383
241
1,413
1105
174
1,058
47
-4
1065
132
1,058
980
169
975
5
0
975
158
975
1126
197
1,144
18
2
1145
165
1,144
1778
416
1,910
132
7
1897
467
1,910
-
-
-
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Station Name
Nakhon
Phanom
Mukdahan
Pakse
Stung Treng
Kratie
Scenario
Difference (cumec)
Difference (%)
Base Line mean
Standard Deviation
Land use Change
Difference (cumec)
Difference (%)
Base Line mean
B-L Std Dev
Land use Change
Difference (cumec)
Difference (%)
Base Line mean
B-L Std Dev
Land use Change
Difference (cumec)
Difference (%)
Base Line mean
B-L Std Dev
Land use Change
Difference (cumec)
Difference (%)
Base Line mean
B-L Std Dev
Land use Change
Difference (cumec)
Difference (%)
Dec
-
-
-
-
-
-
Jan
Feb
42
-2
30
2
3053
464
2,978
75
-2
3111
474
3,065
46
-1
3600
530
3,512
88
-2
5482
688
5,327
155
-3
5618
760
5,563
55
-1
1888
400
1,938
50
3
1907
413
1,981
74
4
2046
471
2,102
56
3
3077
753
3,150
73
2
3130
794
3,288
158
5
-
-
-
-
-
Mar
April
7
-1
0
0
1462
189
1,445
17
-1
1473
192
1,474
1
0
1537
212
1,517
20
-1
2259
384
2,203
56
-2
2283
406
2,282
1
0
1362
228
1,356
6
0
1389
232
1,396
7
1
1474
264
1,465
9
-1
2025
359
1,989
36
-2
2044
370
2,049
5
0
-
-
-
-
-
-
-
-
May
1
0
13
1
1614
243
1,608
6
0
1676
256
1,675
1
0
1969
367
1,962
7
0
2512
501
2,493
19
-1
2552
532
2,567
15
1
2908
772
2,925
17
1
3063
785
3,085
22
1
3795
1233
3,818
23
1
4724
1747
4,767
43
1
4851
1830
4,918
67
1
Test 1.2 compares the dry season flow duration curves (daily flows between 1 Dec and
31 May) for two representative mainstream primary reporting stations, Vientiane and
Kratie (Figure A 3-3). The curves present the cumulative duration (frequency) of flows
equal to or greater than each flow level. As for the mean monthly flows, it can be seen
there is little difference in flows between the two scenarios at either site.
Figure A 3-4 to Figure A 3-6 provide a between scenario comparison for the 90, 50 and
10 percentile flow volumes by month (in million cubic metres, MCM) over the whole
year for Vientiane and Kratie respectively. Whilst these curves are generated directly in
the DSF, the data they draw on is effectively obtained by ‘reading off’ values from the
106751643
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types of flow duration curves shown in Figure A 3-3, but calculated for each month not
the entire dry season. The vertical axis in Figure A 3-6 is truncated to highlight the
differences in dry season flows. Lack of difference in the mean daily flows by month
are paralleled across the range of flow percentiles, i.e. there are no changes at any time
or at any flow level at either Vientiane or Kratie.
cumecs
Flow: Exceedence-Probability
10,000
9,500
9,000
8,500
8,000
7,500
7,000
6,500
6,000
5,500
5,000
4,500
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0
5
10
15
20
25
g
b
c
d
e
f
b
c
d
e
f
g
b
c
d
e
f
g
b
c
d
e
f
g
Figure A 3-3:
106751643
30
35
40
45
50
55
60
65
Cumulative Exceedence Probability %
70
75
80
85
90
95
100
[S9] Kratie ([01/Dec-31/May], 85-00)
[S9] Vientiane ([01/Dec-31/May], 85-00)
[S7] Vientiane ([01/Dec-31/May], 85-00)
[S7] Kratie ([01/Dec-31/May], 85-00)
Dry season flow duration curves for Vientiane and Kratie (dry seaon; 1 Dec – 31 May
data period; 1 Dec 1985 – 31 Dec 2000)
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Flow: Percentile Distribution
36,000
34,000
32,000
30,000
28,000
Monthly Total [MCM]
26,000
24,000
22,000
20,000
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
Jan
Feb
g
b
c
d
e
f
b
c
d
e
f
g
b
c
d
e
f
g
Figure A 3-4:
Mar
Apr
May
Jun
Jul
b
c
d
e
f
[S9] Vientiane 90%ile (85-00) g
b
c
d
e
f
[S9] Vientiane 50%ile (85-00) g
b
c
d
e
f
[S9] Vientiane 10%ile (85-00) g
Aug
Sep
Oct
Nov
Dec
[S7] Vientiane 90%ile (85-00)
[S7] Vientiane 50%ile (85-00)
[S7] Vientiane 10%ile (85-00)
Percentile flow distribution of 90 %, 50% and 10% at Vientiane with comparisons to
Baseline conditions for 50% and 10% (directly over-laid as no change)
Flow: Percentile Distribution
120,000
110,000
100,000
Monthly Total [MCM]
90,000
80,000
70,000
60,000
50,000
40,000
30,000
20,000
10,000
Jan
Feb
g
b
c
d
e
f
b
c
d
e
f
g
b
c
d
e
f
g
Figure A 3-5:
106751643
Mar
Apr
May
Jun
b
c
d
e
f
[S9] Kratie 50%ile (85-00) g
b
c
d
e
f
[S9] Kratie 10%ile (85-00) g
b
c
d
e
f
[S9] Kratie 90%ile (85-00) g
Jul
Aug
Sep
Oct
Nov
Dec
[S7] Kratie 50%ile (85-00)
[S7] Kratie 10%ile (85-00)
[S7] Kratie 90%ile (85-00)
Percentile flow distribution of 90 %, 50% and 10% at Kratie with comparisons to
Baseline conditions (directly over-laid as no change)
A.3- 12
Mekong River Commission - Water Utilisation Project Component A:
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
All assessments are made for
DEMONSTRATION purposes only
Flow: Percentile Distribution
20,000
19,000
18,000
17,000
16,000
15,000
Monthly Total [MCM]
14,000
13,000
12,000
11,000
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
Jan
Feb
Mar
g
b
c
d
e
f
b
c
d
e
f
g
b
c
d
e
f
g
Figure A 3-6:
Apr
May
Jun
b
c
d
e
f
[S9] Kratie 50%ile (85-00) g
b
c
d
e
f
[S9] Kratie 10%ile (85-00) g
b
c
d
e
f
[S9] Kratie 90%ile (85-00) g
Jul
Aug
Sep
Oct
Nov
Dec
[S7] Kratie 50%ile (85-00)
[S7] Kratie 10%ile (85-00)
[S7] Kratie 90%ile (85-00)
Percentile flow distribution of 90 %, 50% and 10% at Kratie with comparisons to
Baseline conditions (directly over-laid as no change) – truncated vertical axis
Low flow variability is quantified by Test 1.3. Table A 3-4 provides values for the start,
end and duration of the dry season (where the definition of the dry season is based on
the flows below the median flow under Baseline Conditions. There is no more than
one day difference in any of the dates or durations, i.e. effectively no change.
Table A 3-4:
Mean start and end of the dry season based on the Baseline Scenario’s median annual flow
(Oct 1985 – Dec 2000)
Reporting Site
Development
Scenario
Baseline
Difference
Chiang Saen
Start Date
9-Dec
9-Dec
(Sc1 median = 1,768)
End Date
27-May
26-May
Duration (days)
168
168
Luang Prabang
Start Date
8-Dec
8-Dec
(Sc1 median = 2,355)
End Date
4-Jun
4-Jun
Duration (days)
178
178
Start Date
5-Dec
4-Dec
Vientiane
106751643
0
-1
0
0
0
0
-1
A.3- 13
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(Sc1 median = 2,416)
End Date
3-Jun
3-Jun
Duration (days)
180
181
Nakhon Phanom
Start Date
30-Nov
28-Nov
(Sc1 median = 3,869)
End Date
1-Jun
1-Jun
Duration (days)
183
185
Mukdahan
Start Date
29-Nov
28-Nov
(Sc1 median = 4,028)
End Date
31-May
31-May
Duration (days)
184
184
Pakse
Start Date
28-Nov
27-Nov
(Sc1 median = 4,972)
End Date
29-May
29-May
Duration (days)
182
183
Stung Treng
Start Date
4-Dec
3-Dec
(Sc1 median =7, 189)
End Date
1-Jun
1-Jun
Duration (days)
179
180
Kratie
Start Date
4-Dec
3-Dec
(Sc1 median = 7,369)
End Date
1-Jun
1-Jun
Duration (days)
180
180
0
1
-2
0
2
-1
0
0
-1
0
1
-1
0
1
-1
0
0
Relative flow height changes are analysed by the low flow variability tool. The median
duration of events where the flow does not change by more than 10% of the median
annual flow for at least 30 days are shown in Table A 3-5.
This analysis reveals more change than in the above low flow analyses, indicating that
there is a slight decrease in dry season flow stability. However, only at Pakse does the
reduction exceed 10%.
106751643
A.3- 14
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Table A 3-5: Duration of Stable Flow Conditions for the Baseline Scenario (days)
(Where flows change by < 10% of the Baseline Condition’s mean annual flow for a period of at least 30 days)
Baseline Annual
Median Flow
(cumec)
Reporting Site
Chiang Saen
Luang Prabang
Vientiane
Nakhom Phanom
Mukdahan
Pakse
Stung Treng
Kratie
Mean Number of Days of Stable Flow
Baseline
Conditions
Land use Change
1,768
2,355
2,416
3,869
4,028
4,972
7,189
7,369
3.5.2
46
45
55
59
61
56
56
55
50
44
56
57
62
62
57
56
Difference
(days)
Difference
(%)
-4
1
-1
2
-2
-6
0
-1
-7
3
-2
4
-3
-10
-1
-1
Test 2: Acceptable Reverse flow of Tonle Sap in Wet Season
Test 2.1 compares the mean peak annual wet season (daily) flows and standard
deviation for Baseline Conditions with the development scenario at Kratie. The 1995
Mekong Agreement defines wet season flows at Kratie as the basis for determining if
acceptable inundation level in Tonle Sap lake is achieved. Consequently, the difference
between flows at Kratie are compared for Baseline and Land use Change conditions in
Table A 3-6. The table shows that there is only a 1% difference in flows, i.e. there is
essentially no change in peak annual flows at Kratie.
Figure A 3-7 displays the relationship of peak annual discharge with the average annual
return period under the Land use Change scenario. Tabular values are presented in
Table A 3-8.
Table A 3-6:
Change in mean annual peak daily flow at Kratie
Reporting Site
Kratie
106751643
Baseline Conditions
Mean Annual Peak
Flow (cumec)
44,651
Standard Deviation
(cumec)
9,785
Land use Change
Mean Annual Peak
Flow (cumec)
45,218
Difference
cumec
567
%
1
A.3- 15
Mekong River Commission - Water Utilisation Project Component A:
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Annual Flood Frequency Analysis
Flow (Simulated flow) (cumecs)
Kratie K = 0.18:
105,000
100,000
95,000
90,000
85,000
80,000
75,000
70,000
65,000
60,000
55,000
50,000
45,000
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
0
1
2


Figure A 3-7:
3.5.3
4
5
10
20 25
Return Period (years)
[S9] Kratie - Gringorten (1985-2000)

50
100
200
500
[S9] EV1 Kratie - Gringorten (1985-2000)
[S9] GEV Kratie - Gringorten (1985-2000)
Annual flood frequency at Kratie
Test 3: Average Daily Peak Main Stream Flow in the Flood Season
A portion of the daily flow time-series data are plotted in Figure A 3-8 to provide a
visual reference as the scale of changes between the Baseline and Land use Change
Scenarios. Only two stations have been plotted to retain clarity. It is evident that there
is no discernable difference at any point in the hydrographs.
As can be seen from Table A 3-7 neither the highest nor mean annual flood over the 16
year period is increased under the land use at any site by more than one or two percent.
This unexpectedly small impact of a reduction of by 50% of the remaining LMB forest
cover has been investigated by the DSF development team. In part it is due to the
limitations of the SWAT model to humid tropical regions compared to its area of
development in the mid-west of the USA. These limitations forced land use change to
be simulated only by changes in CN2 values, as indicated in the above sections, rather
than by also changing the landcover parameters. The limitations of SWAT are discussed
in the DSF Modeller’s User Guide.
However, apart from those limitations, it has been found that the Potential EvapoTranspiration (PET) is limited by the prevailing humidity in the LMB, such that clearing
forest does not necessarily reduce these values and in some cases, in areas with shallow
soils, they can actually increase. Hence the net minimal change in runoff and stream
flow rates observed.
106751643
A.3- 16
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Time Series Analysis Tool
60,000
55,000
50,000
45,000
Flow(cumecs)
40,000
35,000
30,000
25,000
20,000
15,000
10,000
5,000
26/11/1997 26/03/1998 24/07/1998 21/11/1998 21/03/1999 19/07/1999 16/11/1999 15/03/2000 13/07/2000 10/11/2000
g
b
c
d
e
f
b
c
d
e
f
g
b
c
d
e
f
g
b
c
d
e
f
g
Kratie: Flow (Simulated flow ) [WUPA Scenario 3 - Impact of Catchment Cover Change]
Vientiane: Flow (Simulated flow ) [WUPA Scenario 3 - Impact of Catchment Cover Change]
Kratie: Flow (Simulated flow ) [WUPA Scenario 1 - Baseline]
Vientiane: Flow (Simulated flow ) [WUPA Scenario 1 - Baseline]
01/Feb/1985 - 31/Dec/2000
Figure A 3-8:
Table A 3-7:
Daily time-series showing high flows for Vientiane and Kratie (Feb 1985 – Dec 2000)
Comparison of Highest and Average Annual Peak Daily Flows
Baseline
Highest Annual
Peak Flow
Land use
Change
Highest Annual
Peak Flow
Difference - highest
(cumec)
Difference - highest
(%)
Mean Annual Peak
Flow
Mean Annual Peak
Flow
Difference in means
(cumec)
Difference - means
(%)
Baseline
Land use
Change
106751643
Chiang
Saen
Luang
Prabang
Vientiane
Nakhom
Phanom
Mukdahan
Pakse
Stung
Treng
Kratie
13,300
7,997
17,830
31,654
35,195
47,212
66,067
67,268
13,389
18,396
18,156
31,903
35,474
47,537
66,731
67,925
89
399
326
249
280
325
664
657
1
2
2
1
1
1
1
1
9,754
14,058
14,733
23,828
25,827
34,035
43,502
44,651
9,935
14,333
14,940
24,119
26,209
34,396
44,036
45,218
182
275
208
291
382
361
533
567
2
2
1
1
1
1
1
1
A.3- 17
Mekong River Commission - Water Utilisation Project Component A:
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In addition to the comparison of average annual flood peaks, Table A 3-8 shows the
changes in peak flows for different return periods for each mainstream monitoring site.
As would be expected, the changes in peak flows (cumec) for each return period
confirm the lack of change in mean and extreme peak values shown above in Table A
3-7; after all, they are merely alternative indicators of the same change. It can also be
noted that floods having an average annual return period of two years are equivalent to
the median annual flood, i.e. on average half the annual flood peaks are greater and half
smaller.
Figure A 3-7 above provides a graphical representation of the Annual Flood Frequency
Curve at Kratie, this being a critical location for determining changes in flooding in
both the Tonle Sap Lake and the Cambodian and Vietnamese floodplain areas.
Table A 3-8:
Comparison of flood return periods for mainstream monitoring sites
(Changes >10% highlighted in bold)
Main Stations
Chiang Saen
Scenarios
Baseline
Landuse Change
% change
Luang Prabang Baseline
Landuse Change
% change
Vientiane
Baseline
Landuse Change
% change
Nakhon Phanom Baseline
Landuse Change
% change
Mukdahan
Baseline
Landuse Change
% change
Pakse
Baseline
Landuse Change
% change
Stung Treng
Baseline
Landuse Change
% change
Kratie
Baseline
Landuse Change
% change
106751643
2
10,126
10,352
2
15,112
15,396
2
15,914
16,113
1
24,500
24,787
1
26,092
26,494
2
33,803
34,166
1
42,795
43,362
1
43,989
44,589
1
5
11,815
12,008
2
16,922
17,150
1
17,265
17,489
1
27,817
28,065
1
30,321
30,675
1
40,261
40,695
1
51,347
52,019
1
52,622
53,314
1
Return Periods
10
20
12,449
12,844
12,593
12,955
1
1
17,402
17,631
17,606
17,821
1
1
17,556
17,676
17,790
17,915
1
1
29,091
29,896
29,323
30,117
1
1
32,265
33,672
32,584
33,958
1
1
43,678
46,435
44,148
46,929
1
1
56,199
60,327
56,895
61,020
1
1
57,476
61,578
58,186
62,281
1
1
50
13,159
13,259
1 17,769
17,948
1
17,737
17,979
1
30,551
30,762
1
35,003
35,249
1
49,373
49,892
1
64,998
65,659
1
66,184
66,852
1
A.3- 18
100
13,305
13,259
0
17,816
17,991
1
17,754
17,997
1
30,858
31,065
1
35,731
35,952
1
51,189
51,721
1
68,062
68,682
1
69,182
69,809
1
All assessments are made for
DEMONSTRATION purposes only
3.5.4
Mekong River Commission - Water Utilisation Project Component A:
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
Test 4: Extent and Duration of Flooding
Figure A 3-9 shows the difference in inundated area for peak flood conditions in the
year 2000. As would be expected from the above time-series analysis results, there are
no observable differences between the two scenarios. Flooded levels are either the
same or reduced by less than 0.01 m.
Figure A 3-9
106751643
Map showing the difference in inundation depths downstream Kratie for peak flood
conditions in year 2000 (Baseline Conditions verses Catchment Cover Change)
A.3- 19
Mekong River Commission - Water Utilisation Project Component A:
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Table A 3-9:
Maximum inundation areas by depth class in the year 2000
Flooded Area by depth class (km 2)
Year 2000 (Wet Year)
Scenario
Baseline Conditions
>0m
Catchment Cover Change
Table A 3-10:
(km2)
Difference
Area
Difference
Percent (%)
>0.5 m
> 1.0 m
> 2.0 m
> 4.0 m
43,328
42,003
38,612
29,664
16,409
43,333
5
42,006
3
38,596
-16
29,654
-10
16,418
9
0.0
0.0
0.0
0.0
0.1
Inundation duration > 0.5 m depth for the year 2000
Flooded Area >0.5 m by Duration (km2)
Year 2000 (Wet Year)
Scenario
Baseline Conditions
Catchment Cover Change
3.5.5
(km2)
Difference
Area
Difference
Percent (%)
> 1 day
59,300
> 1 mth
55,444
> 2 mths
51,687
> 4 mths
43,679
> 6 mths
29,650
59,411
55,477
51,776
43,634
29,706
111
33
89
-44
56
0.2
0.1
0.2
-0.1
0.2
Test 5: Extent and Duration of Saline Intrusion
The lack of any differences between the two scenarios is clear from the lack of
differences apparent in Table A 3-11 which shows the area of each salinity exceedance
class and the differences with the Baseline Scenario. There is no more than a 0.3%
increase in the area of any salinity class. Figure A 3-10 shows the area affected.
106751643
A.3- 20
Mekong River Commission - Water Utilisation Project Component A:
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Technical Reference Report:: DSF 650 DSF Testing and Evaluation
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Figure A 3-10 Map showing the extent of Catchment Cover Change saline intrusion for maximum
dry-season salinity intrusion in year 2000
Table A 3-11:
Maximum dry-season salinity intrusion areas
Maximum Saline Intrusion by Salinity Class (km 2)
Jan 2000 – June 2000
Scenario
> 1 g/l
Baseline Conditions
Catchment Cover Change
106751643
(km2)
Difference
Area
Difference
Percent (%)
> 4 g/l
> 8 g/l
> 15 g/l
21,603
19,287
17,279
14,195
21,671
19,324
17,286
14,183
69
37
7
-13
0.3
0.2
0.0
-0.1
A.3- 21
Mekong River Commission - Water Utilisation Project Component A:
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3.5.6
Test 6: Irrigated agriculture performance
Compared to the baseline conditions the shortfall in theoretical water requirement (as
measured in million hectare days of irrigation equivalent per year1) is marginally reduced
by the changes in water availability caused by catchment cover change. Table 3-12
illustrates the impact in each year.
Table A 3-12:
Impacts of catchment cover change on sustainable irrigation area
Sustainable area (Mha-days)
Year
Baseline
Catchment
change
Difference
% change
1986
162.12
165.13
3.00
1.9%
1987
158.52
161.55
3.03
1.9%
1988
173.42
176.63
3.21
1.9%
1989
171.01
174.24
3.23
1.9%
1990
161.79
164.99
3.20
2.0%
1991
150.43
153.43
3.01
2.0%
1992
160.45
163.51
3.06
1.9%
1993
141.04
143.87
2.83
2.0%
1994
164.32
167.28
2.96
1.8%
1995
168.36
171.38
3.02
1.8%
1996
167.89
170.98
3.10
1.8%
1997
152.36
155.36
3.00
2.0%
1998
154.15
157.19
3.04
2.0%
1999
168.92
172.02
3.10
1.8%
2000
166.55
169.78
3.23
1.9%
Average
161.42
164.49
3.07
1.9%
1
Sustainable area represents the aggregate area in a year (expressed in million hectare-days, ie the sum of the areas
under irrigation on each day of the year) equivalent to that which could be theoretically irrigated with the water
abstracted. The sustainable area is found to be commonly less than the recorded irrigated area, reflecting the
likelihood that not all crops are irrigated to the full extent determined from theoretical calculations. It may be noted
that theoretical calculations include an estimated efficiency factor, and part of the apparent deficit may be attributed to
uncertainties in the estimated efficiencies. Deficit area is the difference between the target area and the sustained area,
again expressed in Mha-days.
106751643
A.3- 22
Mekong River Commission - Water Utilisation Project Component A:
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3.5.7
Test 7: Degree of Connection for Fisheries Purposes
There is no change in the maximum longitudinal fish migration network extent as there
were no physical barriers introduced that would reduce it.
It is evident from the inundation and inundation-duration maps and tables above that
there are no changes in flooded area and therefore no changes to the lateral connectivity
of the river to the floodplain.
3.5.8
Impacts on flows in the tidal areas
Impacts of climate change on flows below Kratie are illustrated by reference to
representative flows in the year 2000 at Chau Doc, Tan Chau and Phnom Penh
(Mekong) in the figures and table below.
Figure A 3-11
Comparison of flows at Chau Doc for year 2000 (Baseline
Conditions versus Catchment Cover Change)
Time Series Analysis Tool
9,000
(S1 is baseline, S3 is
catchment cover change
scenario)
8,000
Flow(cumecs)
7,000
6,000
5,000
4,000
3,000
2,000
1,000
15/01/2000
15/03/2000
14/05/2000
13/07/2000
[S1] Chau Doc: Flow
11/09/2000
10/11/2000
[S3] Chau Doc: Flow
01/Jan/2000 - 30/De c/2000
106751643
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Mekong River Commission - Water Utilisation Project Component A:
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Figure A 3-12
Comparison of flows at Tan Chau for year 2000 (Baseline
Conditions versus Catchment Cover Change)
Time Series Analysis Tool
28,000
26,000
24,000
22,000
Flow(cumecs)
20,000
(S1 is baseline, S3 is
catchment cover change
scenario)
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
15/01/2000
15/03/2000
14/05/2000
13/07/2000
[S1] Tan Chau: Flow
11/09/2000
10/11/2000
[S3] Tan Chau: Flow
01/Jan/2000 - 30/De c/2000
Figure A 3-13 Comparison of flows at Tan Chau for year 2000 (Baseline Conditions versus
Catchment Cover Change)
Time Series Analysis Tool
40,000
38,000
36,000
34,000
(S1 is baseline, S3 is
catchment cover change
scenario)
32,000
30,000
28,000
Flow(cumecs)
26,000
24,000
22,000
20,000
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
15/01/2000
15/03/2000
14/05/2000
13/07/2000
[S1] Phnom Penh (Mekong): Flow
11/09/2000
10/11/2000
[S3] Phnom Penh (Mekong): Flow
01/Jan/2000 - 30/De c/2000
106751643
A.3- 24
Mekong River Commission - Water Utilisation Project Component A:
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Table A 3-13: Impacts of catchment change on flows and water levels at Chau Doc,
Tan Chau and Phnom Penh (Mekong) for year 2000
Flow (m3/s)
3.6
Stage (mAD)
Max
Value
Max
Date
Min
Value
Min
Max
Date
Min
Value
Min
Date
Max
Value
Phnom Penh Mekong
upstream – Catchment
cover change
40,877
18/07
1,615
03/04
9.98
16/09
0.77
02/04
Phnom Penh Mekong
upstream - Baseline
40,815
18/07
1,632
03/04
9.98
16/09
0.79
02/04
Phnom Penh Mekong
upstream - Difference
62
0 days
-17
0 days
0.00
0 days
-0.02
0 days
Tan Chau – Catchment
cover change
28,078
20/9
1,803
04/04
5.61
19/09
0.36
01/04
Tan Chau - Baseline
28,063
20/9
1,826
03/04
5.61
19/09
0.37
01/04
Tan Chau - Difference
15
0 days
-23
1 day
0.00
0 days
-0.01
0 days
Chau Doc - Catchment
cover change
9,831
23/09
69
06/04
4.92
22/09
0.18
29/04
Chau Doc - Baseline
9,813
23/09
73
06/04
4.92
22/09
0.18
29/04
Chau Doc - Difference
18
0 days
-3
0 days
0.00
0 days
0.00
0 days
Date
Conclusions and Recommendations
The objective is to assess the effect of reducing catchment forest cover by half in the
Lower Mekong Basin by conversion of all forest types to a type of non-irrigated
agriculture (SWAT land use code AGRF). It was expected that such a change would
have impacts in both the wet and dry seasons, making both more extreme. Flows
entering the LMB from China were not altered because these flows are not modelled,
but treated as an inflow based on the same historical flows as were used in the Baseline
Scenario.
The changes in land use were represented by altering values of CN2 (‘Curve Number’)
within each of the SWAT sub-basins that contained any of the five forest types. No
change was made to any other parameters, in part because of the hard coded limitations
reported on in the calibration report.
106751643
A.3- 25
All assessments are made for
DEMONSTRATION purposes only
Mekong River Commission - Water Utilisation Project Component A:
Development of Basin Modelling Package and Knowledge Base (WUP-A)
Technical Reference Report:: DSF 650 DSF Testing and Evaluation
As the SWAT sub-basins in fact cover a range of land use types, the CN2 values are
representative of not just the predominant land use, but also the other minor land uses.
This in part explains the variation in CN2 values between sub-basins despite them
having the same nominal ‘land use’ and hydrologic soil type. Because only half the
forest cover was removed, the CN2 values for each forested sub-basin were altered by
50% of what would be the case if all the forest were to be cleared. The new value for
each sub-basin is provided in a look-up table (Table A 3-1) that was derived by linear
optimisation of the range of CN2 values within each land use – hydrologic soil type
such that departures from the mean were on average minimised.
The testing demonstrated that the Catchment Cover Change scenario has minimal
impact on either dry or wet season flows. This was unexpected but when investigated
proved to be correct. Fundamentally, the maximum potential evapo-transpiration
(PET) is highly constrained under the generally prevailing humid conditions in the
Lower Mekong Basin. Coupled with higher potential leaf-transpiration rates for nonforest vegetation and the majority of soils in the hilly forested areas having soil depths
less than 2 m, means that grass and crops have an equal, or sometimes even greater
potential to use the available soil moisture than does forest. Hence actual evapotranspiration rates can be close to, equal or higher than those for forest.
Consequently, the results of this scenario are quite profound if confirmed by actual data
collected in the region. They show that there would be essentially no change in the
volume of runoff and only small changes in the timing. Trans-boundary impacts,
therefore, are not generated by even a 50% reduction in remaining forest cover.
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