sangha 2012 biodiversity in pigmy land - presentation at geomorphology 2013
TRANSCRIPT
In situ biophysical data analysis establishing the draining process of Central African lakes whose origins are hypothetical:
complementarities with an original small scale satellite based study
Bénédicte Navaro, Gilles Lerouge, Camille Netter, Thierry Rousselin
Geo212 – 25, rue Jean Dolent – 75014 Paris – [email protected]
Location:
Northwest of Congo BasinRainforestTri-National Park of Sangha (UNESCO – World Heritage of Humanity)Central African Republic South of Bayanga
A data analysis conducted by Geo212 within the scientific program « Sangha : Biodiversity on Pygmy land - Insects of the World in Central African Republic » (2010 -2012)
Context Sensors Method Results Hypothesis Conclusion
Issues :
Dense rainforest, hardly accessible
No mapping
Isolated area covered with lakes with hypothetical origins
Necessity :
Understanding the environmental distribution of fauna
Understanding the river system organisation (location / distribution / communication)
An old hypothesis :
Through photointerpretation, some lakes were interpreted as sinkholes of a paleo cryptokarst above continental sands.
Context Sensors Method Results Hypothesis Conclusion
Image Spot-5, résolution spatiale 5m
1891 : 1st exploration by Alfred Fourneau
2012 : Sangha 2012 team
2009 : Beginning of Sangha 2012 project
2010 : Preliminary field mission : installation of 17 sensors in strategic locations
2012 : Principal field mission : collect and analysis of the sensors data
Context Sensors Method Results Hypothesis Conclusion
Context Sensors Method Results Hypothesis Conclusion
Lake 1
Lake 2
Lake 4
Lake 5Lake 6
Lake 7
Lake 3
15 months of data !
Prosensor®
Comparison with ancillary data
Pressure and temperature : recording every 30 minutes
Brightness and temperature : recording every 2 hours
Lakes area
GDEM ASTER 2 Rainfall Estimate per dekade on a 10x10 km grid
Context Sensors Method Results Hypothesis Conclusion
Mean daily pressure inside the lakes
10/08/10 18/11/10 26/02/11 06/06/11 14/09/11 23/12/11 01/04/1295
97
99
101
103
105
107
109
Date
Pres
sure
(kPa
)
Synchroneous responses of the lakes regardless of the water level
Context Sensors Method Results Hypothesis Conclusion
18/11/10 07/01/11 26/02/11 17/04/11 06/06/11 26/07/11 14/09/11 03/11/11 23/12/11 11/02/12 01/04/1295
97
99
101
103
105
107
109
15
17
19
21
23
25
27
29Mean data for lakes 2 4 5 7
Mean pressureMean temperature
Date
Pess
ure
(kPa
)
Tem
pera
ture
(°C)
Hypothesis : response of the lake to a rainy event
18/11/10 07/01/11 26/02/11 17/04/11 06/06/11 26/07/11 14/09/11 03/11/11 23/12/11 11/02/12 01/04/120
10000
20000
30000
40000
50000
60000
70000
Brig
htne
ss (l
ux)
Confirmation by brightness data
Context Sensors Method Results Hypothesis ConclusionContext Sensors Method Results Hypothesis Conclusion
Supposed rainy events occur in high rainfall estimate periods
Context Sensors Method Results Hypothesis Conclusion
The beginning of flood moves back or forward depending on the lakes
Context Sensors Method Results Hypothesis Conclusion
Context Sensors Method Results Hypothesis Conclusion
2
45
7
Isolated watersheds
Context Sensors Results Hypothesis Conclusion
Simultaneous responses to major rainy events regardless the period of the year
BUT
Not a hydromorphic soil :
- No streaming observed above the ground : Lakes drained by underground flow, surfaces
are free piezometrical surfaces.
- During swelling or flood, simultaneity caused by saturation
- During low water period simultaneity caused by direct rain contribution
Deferred starts of respective floods
Not just one common watershed :
Aquifere loading time different from one lake to the other
ConclusionsContext Sensors Results Hypothesis Conclusion
This analysis over the Sangha rainforest demonstrates the complementarity between local in situ measures interpretation and a small scale geomorphological analysis.
Proof of a non hydromorphic soil
Draining process explained by independent aquifers
Link between lakes and deep karst not confirmed and unlikely …
The protocol of the mission and the use of Aster GDEM v2 explore the prospects to highlight river patterns with fairly simple logistics.
