case study on impact of climate change on the sundarbans...
TRANSCRIPT
Case study on impact of climate change on the Sundarbans ecosystems to
demonstrate usefulness of the high resolution RCM information
Mohammad Asad Hussain, A. K. M. Saiful Islam and Mohammad Alfi Hassan
Institute of Water and Flood Management (IWFM), BUET, Bangladesh
and
Balakrishnan Bhaskaran, Met Office Hadley Centre, UK
Final Workshop in BUET, Dhaka on 27 January 2013
Outline of Presentation
Background
Objectives
Why salinity is so important for the Sundarbans?
Major factors influencing salinity fronts
Hydrological and Meteorological Data
Bathymetry Data
River system and discharge
SLR Data
Meteorological projections
Sample Results
Background
"Case Studies on Climate Change and World Heritage" by UNESCO 2007 states that 45-cm rise in sea level, combined with other forms of anthropogenic stress on the Sundarbans, could lead to the destruction of 75% of the Sundarbans mangroves.
The Sundarbans mangrove forest is the largest single block of tidal halophytic mangrove forest in the world. Total forest area is 10,000 sq.km. of which 6000 sq.km. is in Bangladesh and about 1,700 sq.km. is occupied by water-bodies. Tidal fluctuation is 1.5~2.5m and approx. 70% goes under water in regular tidal flooding twice a day.
Objectives
Objective of this case study is illustrate the usefulness of high resolution RCM information to study the impact of climate change on the Sundarbans ecosystem.
Salinity plays vital role in the distribution of species in the Sundarbans as germination is linked with salinity.
The soil of Sundarbans is moderately saline to saline in the east and highly saline in the west. Soil Salinity of Sundarbans is low compared to other mangrove areas of the world where soil salinity exceeds that of sea water (MoEF 2005).
Why salinity is important for Sundarbans?
Fresh water discharge through the river system
Sea level rise
Seasonal variation of near-shore circulation pattern which is governed by tidal currents and seasonal wind
Major factors influencing salinity front
Four seasons have been considered, namely pre-monsoon (March - May), monsoon (June - September), post-monsoon (October - November) and winter (December - February).
For the future projections of salinity distributions four time-slices: base (1991~2010), 2020s (2011~2030), 2050s (2031~2070) and 2080s (2071~2100) have been considered.
The hydrodynamic model (Delft3D) was run for 272 cases (4 time slices, 4 seasons and 17 ensembles) to capture future possible scenarios.
Those which produce maximum and minimum salinity at points of interest have been extracted.
Salinity calculation
Projected Meteorological Data from PRECIS
Sea-
son
Air Temp. (0C) Relative
Humidity (%)
Solar Rad.
(Cal/cm2/min)
Cloud Cover
(%)
Wind
Speed.(m/s)
Max Min Max Min Max Min Max Min Max Min
Bas
e
DJF 23.5 21.3 71.0 63.5 292.9 278.9 29.0 18.0 1.6 0.4
MAM 30.2 28.8 74.6 71.0 295.6 278.5 40.0 29.0 6.1 5.1
JJAS 29.8 28.2 86.2 81.7 216.9 195.0 86.0 71.0 4.1 3.2
ON 26.9 25.3 79.1 74.9 270.2 254.1 50.0 32.0 1.4 0.5
20
20
s
DJF 28.7 26.2 74.3 68.5 292.5 276.2 42.9 41.9 6.2 5.0
MAM 31.6 29.5 74.4 71.3 294.6 279.9 38.0 29.0 6.2 5.0
JJAS 30.9 28.9 84.5 81.7 218.9 195.4 84.0 71.0 4.1 3.2
ON 28.2 26.2 77.9 73.9 271.0 253.1 43.0 32.0 1.1 0.4
20
50
s
DJF 30.3 27.4 73.0 67.5 291.4 275.8 42.9 41.9 6.4 5.1
MAM 32.7 30.4 74.0 71.1 294.8 278.6 39.0 30.0 6.3 5.4
JJAS 32.0 29.8 84.4 79.9 220.3 196.1 83.0 69.0 4.2 3.2
ON 29.9 27.4 75.8 71.5 273.8 260.8 39.0 29.0 0.7 0.1
20
80
s
DJF 27.6 24.0 68.4 63.7 297.1 273.5 32.0 18.0 6.4 5.1
MAM 33.5 30.8 73.8 71.5 293.5 279.1 37.0 29.0 6.4 5.5
JJAS 32.7 30.1 84.3 80.8 218.6 194.0 82.0 71.0 4.3 3.3
ON 30.9 28.2 75.0 69.2 275.7 257.2 41.0 29.0 0.7 0.1
Bathymetry Data
The bathymetry data being used for the present study is GEBCO, which is (shown in following figure) a 30 second global relief model of Earth's surface that integrates land topography and ocean bathymetry.
86 87 88 89 90 91 92 93
20
21
22
23
0
100
200
400
600
800
1000
1200
1400
Depth (m)
Adopted from Islam et al. (2002)
Pre-Monsoon Monsoon Post-Monsoon Winter
Discharge
(m3/s)12,300 69,250 31,000 9,300
River System and Discharge
Table: Seasonal variation of discharge through Ganges-Brahmaputra-Meghna river system, following Islam et al., (2002)
Significant discharge comes through the Meghna Estuary
Prediction of future River Discharge
The discharge from the rivers in the Asian monsoonregion (Changjiang, Ganges, and Mekong) is sensitiveto the seasonal cycle in precipitation. The amounts of the discharge from the Changjiang, Ganges, and Mekong increase (7.8%, 18.0%, and 9.9%) in the future. (Nohara et.al., 2006)
SLR Data
Year IPCC(upper range)
SMRC NAPA
2030 14 18 14
2050 32 30 32
2100 88 60 88
Table: Climate change scenarios considered in NAPA Project
SLR data by NAPA has been applied for this study
Results: Monsoon Salinity
(a) (b)
(c) (d)
Maximum salinity distribution at the northern Bay of Bengal during the monsoon at different time slices: (a) base, (b) 2020s, (c) 2050s and (d) 2080s
Results: Post-monsoon Salinity
Maximum salinity distribution at the northern Bay of Bengal during the post-monsoonat different time slices: (a) base, (b) 2020s, (c) 2050s and (d) 2080s
(a) (b)
(c) (d)
Results: Winter Salinity
Maximum salinity distribution at the northern Bay of Bengal during the winter at different time slices: (a) base, (b) 2020s, (c) 2050s and (d) 2080s
(a) (b)
(c) (d)
Results: Pre-monsoon Salinity
Maximum salinity distribution at the northern Bay of Bengal during the pre-monsoon at different time slices: (a) base, (b) 2020s, (c) 2050s and (d) 2080s
(a) (b)
(c) (d)
Salinity Distributions
• During the monsoon along the southern coast of the Sundarbans, salinity increases from the east to the west and it is true from base to all the future time slices. The competing processes of sea level rise and increased discharge from the GBM basin can also be observed from the four figures. The smaller fresh water plume during 2080s compared to base conditions suggests that eventually the rise in sea water level dominates over the increased discharge.
• During post-monsoon season, some of the areas in the Patuakhali and Chittagong coast will become more saline in future.
Salinity Distributions
• During winter fresh water plume is the smallest and it will get even smaller in future due to changing climates. It appears that salinity level may increase along the Urir Char, Sandwip, Chittagong and Sitakunda coasts. It also appears that salinity may reduce along the southern coast of Bhola island which might be due to alteration in residual circulation caused by the stronger north wind in this season.
• During pre-monsoon season, fresh water plume seems to get smaller in future in the Meghna Estuary area. It appears that salinity level may increase along the Urir Char, Sandwip, Hatiya, Chittagong and Sitakunda coasts. In this season, salinity may also reduce slightly along the southern coast of Bhola island.
Monsoon Salinity along Sundarbans Coastline
Post-Monsoon Salinity along Sundarbans Coastline
Winter Salinity along Sundarbans Coastline
Pre-Monsoon Salinity along Sundarbans Coastline
Salinity along Sundarbans Coastline
• Except during the monsoon season, all the figures show similar tendency with a drop of salinity level at the location 5 which is located at the Rupsha-Passur river confluence.
• During the monsoon season, overall salinity level is lower and it reduces from the west to the east which is primarily due to the huge fresh water plume created by the large river discharge from the Meghna River, being carried by the anticlockwise residual circulation (Hussain et al. ).
Monsoon Salinity along Sundarbans Coastline
Locations: 1 2 3 4 5 6 7 8
BaseMax 17.5 15.4 11.6 12.7 7.3 7.3 4.6 3.3
Min 17.0 14.6 9.8 10.4 6.0 6.3 2.7 2.5
2020Max 18.8 16.4 11.5 11.6 7.5 7.0 3.5 3.5
Min 18.0 15.5 10.8 11.1 7.0 6.5 3.1 3.0
2050Max 19.8 17.1 12.0 11.8 8.1 6.9 3.6 3.5
Min 19.0 16.4 11.5 11.4 7.6 6.5 3.1 3.1
2080Max 23.2 21.2 16.1 15.3 11.7 10.5 6.6 4.9
Min 21.9 19.9 15.1 15.2 11.3 10.3 5.7 4.3
Post-Monsoon Salinity along Sundarbans Coastline
Locations: 1 2 3 4 5 6 7 8
BaseMax 23.3 23.0 19.4 20.8 15.2 22.4 22.1 20.0
Min 22.7 22.3 18.9 20.4 14.7 22.0 21.6 19.5
2020Max 23.8 23.3 19.9 21.2 15.8 23.0 22.2 20.2
Min 23.4 23.0 19.5 20.5 15.1 22.4 21.9 20.0
2050Max 24.1 23.6 20.3 21.5 16.6 23.7 22.4 20.5
Min 23.4 23.0 19.5 20.5 15.5 22.7 21.7 19.9
2080Max 24.7 24.4 22.0 22.6 19.1 23.9 22.8 21.3
Min 24.3 23.9 21.5 22.2 18.6 23.0 22.5 21.0
Winter Salinity along Sundarbans Coastline
Locations: 1 2 3 4 5 6 7 8
BaseMax 27.4 27.1 23.5 24.9 19.2 27.7 26.5 24.4
Min 27.3 26.9 23.5 25.0 19.2 27.8 26.6 24.4
2020Max 27.9 27.3 24.7 26.3 20.9 28.3 27.0 25.3
Min 27.5 26.9 23.9 25.4 20.0 27.7 26.4 24.4
2050Max 28.8 28.0 25.1 26.6 21.7 28.5 27.6 25.9
Min 28.6 27.9 24.7 25.9 20.9 28.0 27.1 25.3
2080Max 29.5 29.0 26.9 27.8 24.5 28.5 28.1 26.6
Min 29.3 28.9 26.7 27.3 23.7 28.1 27.9 26.4
Pre-Monsoon Salinity along Sundarbans Coastline
Locations: 1 2 3 4 5 6 7 8
BaseMax 25.5 25.5 21.7 22.6 16.8 25.1 24.0 21.9
Min 25.2 25.0 21.2 22.3 16.5 24.9 23.8 21.7
2020Max 26.1 26.0 22.3 23.2 17.7 25.0 24.4 22.4
Min 25.7 25.8 22.3 22.8 17.3 25.0 24.3 22.5
2050Max 26.3 26.4 23.3 23.2 18.0 24.9 24.5 22.9
Min 26.2 26.2 23.1 23.1 18.0 25.0 24.5 22.9
2080Max 26.9 26.8 24.6 24.6 20.8 25.2 25.0 23.6
Min 26.7 26.4 24.0 24.0 20.3 25.0 24.5 23.0
Comment:
• The researchers are aware that for future scenario generation, salinity level along the Sundarbans coast may be highly influenced by human interventions which control the flow through the Gorai river. Considering flow variation through this river some important scenarios may be generated. But to reduce the number of cases for numerical experiments and also considering the objectives of the project, such scenarios have been avoided.
Conclusions:
• Maximum increase of salinity has been found during the monsoon season (average 4 psu), followed by the winter (average 2.4 psu), the post-monsoon (average 1.8 psu) and the pre-monsoon (average 1.7 psu) for 2080s time slice compared to base condition along the Sundarbans coast of Bangladesh.
Thank You
River System and Discharge
River System and Discharge
(MoEF 2005)
At least 100 cumec water flow is required through Gorai channel to protect the biodiversity in Sunderban, which at present gets less than 30 cumec during the dry season (CEGIS).
Diversion of about 200 cumec water flow through Gorai river system, particularly during critical dry period of April, will push the 1ppt saline line by 45km seaward (IWM).
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Observed Hydrological and Meteorological Data
Figure 3. Model calibration (water levels at RangadiaChittagong)
Model validation: simulated tidal zones
River System and Discharge
The discharge from the rivers in the Asian monsoonregion (Changjiang, Ganges, and Mekong) is sensitiveto the seasonal cycle in precipitation. The amounts of the discharge from the Changjiang, Ganges, and Mekong increase (7.8%, 18.0%, and 9.9%) in the future. (Nohara et.al., 2006)
River Dry Season Flow (m3/s) Wet Season Flow (m3/s)
Benta - 500
Rupsha 400 10,500
Pasur 6,000 22,500
Madhumati - 11,630
Sibsha 1500 60,000
Bhairab - 139
BWDB, 2005
SLR Data
SMRC study with 22 years of historical tidal data at three coastal stations revealed that rate of SLR is many fold higher than the meanglobal rate due to regional tectonic subsidence.
Tidal Station Region Latitude (N) Longitude (E) Datum (m) Trend (mm/year)
Hiron Point Western 21°48’ 89°28’ 3.784 4
Char Changa Central 22°08’ 91°06’ 4.996 6
Cox’s Bazar Eastern 21°26’ 91°59’ 4.836 7.8
Table: Trend of tidal level in three coastal stations (MoEF, 2005)