Evaluation of Impacts and Adaptation

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The analysis focuses on two sub-basins: (i) the Mekong 3, which receives water from the entire upstream part of the MRB; and (ii) the Delta, which collects water from the whole basin, including the Tonle Sap Lake in Cambodia.

Business as usual (No CC, No adaptation)

Food

In this scenario, food demand in the basin will increase in proportion with population growth as in Table 8.3. Crop intensification is applied to satisfy future food demand as described above.

Impacts on water resources (Table 8.7)

In this scenario, maximum daily flow as well as maximum monthly flow does not change much (about 1%) compared to 1961-1990. However, the expansion of crop season into

Table 8.6. Scenarios for climate change and adaptation strategies.

No.

Scenario

Adaptation strategy

Objective

Agricultural adaptation measures

Environmental adaptation measures

1

BAU

No adaptation

Current trend without CC

Crop intensification is needed to satisfy food demand for growing population

No adaptation

2

2010-2039 for A2 and B2 2070-2099 for A2 and B2

No adaptation

Effects of CC

No change in agriculture

No adaptation

3

2010-2039 for A2 and B2 2070-2099 for for A2 and B2

CC-Agri

Effects of CC and enhancing food security

Crop intensification is needed to satisfy food demand growing population

No adaptation

4

2010-2039 for A2 and B2 2070-2099 for A2 and B2

CC-Agri-For

Effects of CC and enhancing environmental quality

Crop intensification is needed to satisfy food demand for growing population

Reforestation of all shrub land to mixed forest

Table 8.7. Impacts on water resources in scenario Baseline (BAU) for sub-basins Mekong 3 and Delta.

1961-1990

2010-

-2039

2070

-2099

Scenario /

%

%

sub-basin

Indicator

m3/s

m3/s

change

m3/s

change

BAU/Mekong 3 Hydrology

Min. monthly

560

463

-17

0

-100

flow

Max. monthly

40,995

41,214

+ 1

41,031

+0

flow

Ave monthly

12,894

11,976

-7

11,694

-9

flow

BAU/Delta Hydrology

Min. monthly

1,857

1,588

-15

151

-92

flow

Max. monthly

45,681

46,047

+ 1

45,861

+0

flow

Ave monthly

13,495

13,777

-11

13,193

-15

flow

BAU/Mekong 3 Hydrology

Max. daily

54,829

55,111

+1

54,932

+0

flow

BAU/Delta Hydrology

Max. daily

51,161

51,559

+1

51,359

+0

flow

Underlined values in 2010-2039 and 2070-2099 are higher than values in baseline 1961-1990. % change is compared to 1961-1990.

low flow season has significant impacts on the minimum flow by a reduction of about 15-17% in 2010-2039 and 90-100% in 2070-2099. This reduction will cause serious water shortage at upstream sub-basins and salinity intrusion in the Delta. Therefore measures at a different scale will be needed, such as water-saving irrigation techniques at field scale or reserving more water for low-flow season at sub-basin level. At present, the MRCS is beginning a study on integrated basin flow management (MRC, 2003b) with the objectives indicated in the 1995 Agreement: maintenance of flows on the mainstream of the Mekong River: (i) of not less than the acceptable minimum monthly natural flow during each month of the dry season; (ii) to enable the acceptable natural reverse flow of the Tonle Sap during the wet season; and (iii) to prevent average daily peak flows greater than those that naturally occur on average during the flood season. Extracting more water for irrigation in the dry season may cause a decrease of about 10-15% in the average stream flows in 2010-2039 and 2070-2099.

Industry

A decrease in the average stream flows in 2010-2039 and 2070-2099 implies less water for hydropower. However, the reduction is only about 10-15% in these periods.

Effect of climate change without adaptation

Although this scenario will not happen in reality without making suitable adaptations, it is calculated in order to analyse the effect of CC alone on the hydrology of the MRB, with the assumption that all other factors such as land use are the same as at present.

Impacts on water resources

The trend and monthly variations in stream flow of sub-basin Mekong 3 due to climate change are shown in Fig. 8.7. Although the general trend shows only slight increases, higher extremes are expected for the future. In A2 (Table 8.8), maximum monthly flows in some sub-basins increase in 2010—2039 compared to the 1961-1990 period (14% in Mekong 3) and the Baseline scenario. In 2070-2099 the increments are even higher (41% in Mekong 3 and 19% in the Delta). In B2 the increments are smaller than in A2 (10% and 35% in Mekong 3 in 2010-2039 and 2070-2099, respectively). The monthly average flow is almost unchanged in 2010-2039, but increases in 2070-2099 with a lower level than the maximum (17% in Mekong 3). Minimum monthly flow will decrease slightly in 2010-2039 (7% in A2 and 15% in B2 in the Delta), but significantly in 2070-2099 (26% in A2 and 29% in B2 in the Delta). In this scenario, maximum daily flow also increased significantly in both Mekong 3 and the Delta in 2070-2099, about 30% in A2 and 15% in B2.

Food

Under both the A2 and B2 scenarios, a positive impact is the increase of average flow, i.e. more water will be available for agriculture. Moreover, outputs from the SWAP model at the field scale show an increase in rice yield of 10% (2010-2039) to 40% (2070-2099) in A2, but less in B2, at 10% to 26%, respectively.

Fig. 8.7. Trend and variation of stream flow in sub-basin Mekong 3 under A2 and B2.

Environment

There are two negative impacts on the environment. An increase of maximum monthly flow, and in particular daily flow, means that there will be more severe floods in the MRB due to climate changes. On the other hand, a decrease in minimum monthly flow implies deeper salinity intrusion in the low flow season under the climate change scenarios. Compared to B2, the flood level is slightly higher in A2 and the salinity intrusion level is slightly less than in B2.

Industry

An increase in average monthly flow implies positive impacts on hydropower.

Adaptation strategy for enhancing food security (CC-Agri)

Impacts on water resources

The combined impacts of climate changes and agricultural development for food demand in the future are shown in Table 8.9. Maximum monthly and daily flows are not much different than in the scenario CC-NoAgri because only in the dry season more irrigation water is extracted. Reduction of monthly minimum flows will be worse, in particular for the B2 scenario, because both more water will be extracted for irrigation purposes and climate change will lower extreme minimum flows. On the other hand, the reduction in average stream flows due to irrigation is compensated by the increase due to climate change, therefore the variations in both A2 and B2 are only 5-10% compared to 10-15% in the Baseline scenario.

Food

Compared to the Baseline scenario, positive impacts of climate change on food production are the increase in rice yield and the increase of average flow that makes more water available for agriculture. However, the decrease in minimum flow will be a constraint to the expansion of the crop season into low flow seasons. Adaptation

Table 8.8. Impacts on water resources in scenario CC-NoAgri.

1961-1990

2010-

-2039

2070

-2099

Scenario / sub-basin

Indicator

m3/s

m3/s

% change

m3/s

% change

A2/Mekong 3

Hydrology

Min. monthly flow

560

574

+2

424

-24

Max. monthly flow

40,995

46,635

+ 14

57,659

+ 41

Ave monthly flow

12,894

12,940

+0

15,147

+ 17

A2/Delta

Hydrology

Min. monthly flow

1,857

1,726

-7

1,370

-26

Max. monthly flow

45,681

45,091

-1

54,269

+19

Ave monthly flow

15,459

15,399

-0

17,698

+14

A2/Mekong 3

Hydrology

Max. daily flow

54,829

57,387

+5

74,055

+35

A2/Delta

Hydrology

Max. daily flow

51,161

50,294

-2

66,439

+30

B2/Mekong 3

Hydrology

Min. monthly flow

560

592

+6

466

-17

Max. monthly flow

40,995

45,163

+ 10

55,168

+35

Ave monthly flow

12,894

12,973

+1

13,672

+6

B2/Delta

Hydrology

Min. monthly flow

1,857

1,588

-15

1,311

-29

Max. monthly

45,681

45,271

-1

53,181

+ 16

flow

Ave monthly flow

15,459

15,397

-0

16,140

+4

B2/Mekong 3

Hydrology

Max. daily flow

54,829

54,016

-1

62,118

+13

B2/Delta

Hydrology

Max. daily flow

51,161

50,188

-2

58,696

+15

Underlined values in 2010-2039 and 2070-2099 are higher than values in baseline 1961-1990. % change is compared to 1961-1990.

measures will be needed such as water-saving irrigation techniques or sub-basin flow management, as mentioned in the Baseline scenario. Furthermore, in this phase of the ADAPT study, impacts of climate change on yield of food crops are analysed based on the results of the SWAP field-scale model. For the Mekong Basin, two crops have been selected for analysis: rice and maize. Thailand and Vietnam, two of the riparian countries, are rice exporters number one and two, respectively, in the world market. Therefore, rice yields under different scenarios from the SWAP model are analysed in a location in the Mekong River Delta to reflect the impact of climate change on food production. For the A2 scenario, rice production will increase substantially in the future, about 10% in 2010-2039 to 40% in 2070-2099. The main reasons for the yield increase are changes in CO2 concentration and temperature. The B2 scenario shows a small increase, about 10% in 2070-2099.

Environment

There are two negative impacts on the environment: floods and salinity intrusion. The flood impacts will be at similar levels to the scenario CC-NoAgri. An adaptation strategy is the reforestation of shrub land (17.7% basin area) by mixed forest as shown in scenario CC-Agri-For. The water shortage and salinity intrusion in the dry season will be more serious than in the Baseline scenario.

Industry

Climate change has a positive impact on hydropower due to the increase in average monthly flow that partly can compensate for the reduction due to irrigation.

Adaptation strategy for enhancing environmental quality (CC-Agri-For)

Impacts on water resources (Table 8.10)

Climate change may cause more severe flooding in the MRB. An adaptation strategy is to afforest the shrub land (17.7% of the basin area) by mixed forest. Compared with scenario CC-Agri, the effect on maximum monthly flow is significant in scenario A2, with about 10% reduction in Mekong 3 in 2070-2099, but less under scenario B2. The effect is not as strong as expected, possibly due to: (i) the high slope of the basin; and (ii) the large agricultural area. The effect on maximum daily flow is not as significant as that on maximum monthly flow. However, compared with scenario CC-Agri, reforestation strategy will cause an approximately 5-8% lower monthly average flow due to higher evapotranspiration from the forest areas.

Food

Compared with scenario CC-Agri, there are no significant effects on food production due to this reforestation strategy.

Environment

Maximum monthly and daily flows (except daily flow in B2 during 2070-2099) are lower than in scenario CC-Agri, i.e. less severe flooding in the basin. The effects in flood reduction are higher in CC scenario A2 than B2. However, the decrease in maximum flow is not high enough to reduce flooding significantly. It implies that more

Table 8.9. Impacts on water resources in scenario CC-Agri.

1961-1990

2010-

-2039

2070

-2099

Scenario / sub-basin

Indicator

m3/s

m3/s

% change

m3/s

% change

A2/Mekong 3

Hydrology

Min. monthly flow

560

557

-1

31

-95

Max. monthly flow

40,995

46,646

+ 14

57,723

+ 41

Ave monthly flow

12,894

12,109

-6

13,977

+8

A2/Delta

Hydrology

Min. monthly flow

1,857

1,508

-19

346

-81

Max. monthly flow

45,681

45,446

-1

54,495

+19

Ave monthly flow

15,459

13,828

-11

15,447

-0

A2/Mekong 3

Hydrology

Max. daily flow

54,829

57,629

+5

74,178

+35

A2/Delta

Hydrology

Max. daily flow

51,161

50,533

-1

66,648

+30

B2/Mekong 3

Hydrology

Min. monthly flow

560

472

-16

0

-100

Max. monthly flow

40,995

45,504

+ 11

55,106

+34

Ave monthly flow

12,894

12,162

-6

12,560

-3

B2/Delta

Hydrology

Min. monthly flow

1,857

1,304

-30

27

-99

Max. monthly flow

45,681

45,262

-1

53,138

+ 16

Ave monthly flow

15,459

13,857

-10

13,947

-10

B2/Mekong 3

Hydrology

Max. daily flow

54,829

53,979

-2

62,070

+13

B2/Delta

Hydrology

Max. daily flow

51,161

50,165

-2

58,655

+15

Underlined values in 2010-2039 and 2070-2099 are higher than baseline values in 1961-1990. % change is compared to 1961-1990.

adaptation measures, such as protection for people, should be considered. On the other hand, minimum monthly flow also decreases, i.e. deeper salinity intrusion will occur.

Industry

Compared to scenario CC-Agri, a decrease in average monthly flow will cause lower potential for hydropower.

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