Arias, M.E., Cochrane, T.A., Piman, T. and Kummu M. (2012) Impacts of hydrology and habitat changes on the primary production of Southeast Asia's largest lake. IWA (International Water Association) World Congress on Water, Climate and Energy. Dublin, Ireland, 13-18 May 2012.

1. Impacts of Hydrology and Habitat
Changes on the Primary
Production of the Tonle Sap,
Southeast Asia’s Largest Lake
Mauricio E. Arias*, Tom A. Cochrane, Thanapon
Piman, and Matti Kummu

2. The Mekong: context & challenges
• River length: 4,800 km
• Basin area: 795,000 km2
• Relevance
– livelihoods/ecosystems
adapted to hydrological cycle
– biodiversity
• Mekong challenges
– Hydropower
– Irrigation
– Climate change

3. Tonle Sap: the Great Lake of Cambodia
• 2,600-15,000 km2
• Largest natural flow reversal
system in the world
• Core of one of the largest
inland fisheries in the world
• Fish is the main protein source
in Cambodia
• Important habitat for migratory
fish
• Tonle Sap flood pulse
changing with hydropower and
climate in the Mekong

4. Research Overview
• Broader project: My contribution:
www.mekongflows.org Community
patterns
• Goal: Modelling and monitoring
impacts from large-scale
disruptions to the Mekong’s
How do changes
Landscape in hydrology Production
patterns impact the Tonle
hydrology Sap habitats?
1. hydrology/hydropower
operation modelling
2. Downstream hydrological Water quality
and ecological impacts

5. Modelling Overview
Main question: how do changes in hydrology impact the Tonle
Sap habitats?
Landscape patterns
Primary production
Link aquatic primary production to
Link habitat cover to flooding
Approach hydrodynamics and habitat cover and
and project future changes
project future changes
Tools GIS modelling Dynamic modelling
Hydrological models and Hydrological models, habitat cover,
Data sources
monitoring, elevation surveys field surveys

6. Production and landscape modelling procedures

7. Floodplain Habitats
Habitat code
Gallery Forest (GF)
Open Water (OW)
Rainfed habitats (RF)
0 15 30 60
Seasonally flooded habitats (SF) Kms
Transitional habitats (T)

8. Water level changes : shifting the balance?
Average year
10
•Large seasonal fluctuation
•Large inter-annual range
9
•Levels during dry season getting higher
8
Dry year
10
•Levels during wet season getting lower
7 observed driest (1998)
9 Development
6 Climate change
masl
8 Climate change + hydropower
5
7
observed average (1997)
4
Hydropower 6
masl
3 Climate change
5
Climate change + hydropower
2 4
1 3
May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr
2
1
May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr

9. Habitat Cover Changes
ClimateClimate change
change +Hydropower
Hydropower
shift2030a1rv
SHIFT CODE
shift2030a1b
Habitat code
SHIFT CODE
GF to OW
SHIFT CODE
Habitat code to GF
Gallery Forest (GF) SF to OW
GF
GF to OW
Open Water (OW) Gallery Forest (GF)
SF to T GF
SF to
RF to OW
Rainfed habitats (RF) Open Water (OW)
TS to RF
SF to GF
SF to T
Seasonally flooded habitats (SF) 0 3 6 12 0 2.5 5
0 2.5 5
0 2.5 5 10
10
10
Rainfed habitats (RF) Kms
Kms 0 15 30 Kms
60 Kms
Transitional habitats (T) T to SF T
T to to
SF RF
Seasonally flooded habitats (SF) Kms
Transitional habitats (T)
Baseline map Changes from baseline

10. Changes in sedimentation
1.2 Total annual change
Climate change Hydropower Climate change
+ hydropower
Average of monthly net sedimentation,
0.8
-13% to -35% -38% - 36% to -51%
Baseline (1987-2005)
million tons
0.4 Climate change
g/m2
% change
900
800
700
600
Hydropower 80 60
500 40
20
400
300
200
Climate change + hydropower
0
-20
0.0 100
0 -40
-60
-80
-100
Baseline average year Changes by hydropower
-0.4
AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN

11. Changes in Aquatic Primary Production
Total annual change
Climate Hydropower Climate
1.3 change change +
hydropower
Average of monthly primary production,
1.1
-13% to - - 25% to -
-27 %
0.9
21% 36%
million tonsC/day
0.7
0.5
Baseline (1987-2005)
Climate change
0.3
Hydropower
Climate change + hydropower
0.1
MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR

12. Summary of Results
Primary
Water levels Habitat cover Sedimentation
Production
Open water (+35%)
Dry season (+0.8m)
Hydropower Gallery forest (-23%) -38% -27%
Wet season (-0.5m)
Rainfed (+15%)
Open water (+21%)
Climate Dry season (≈0m)
Gallery forest (-80%) -13 to -35% -13 to -21%
change Wet season (-0.2m)
Climate Open water (+38%)
Dry season (+0.8m)
change + Gallery forest (-31) -36 to -51% -25 to -36%
Wet season (-0.7m)
hydropower Rainfed (+11%)

13. Implications of Results
• Habitat cover and ecosystem function will be
impacted by changes in the Mekong
hydrology
• Ecosystem and livelihoods will have to adapt
to new conditions
• Strong pressures on natural resources and
lack of livelihood diversification are
challenges to adaptation.
• primary production ≈ food

14. Acknowledgments