1) In situ biophysical sensors were installed in lakes in the Sangha Tri-National Park of the Central African Republic to analyze the draining processes of the lakes over 15 months. 2) Data from the sensors showed synchronous responses by the lakes to major rainfall events, but different flood start times, indicating the lakes have isolated watersheds draining through underground aquifers rather than one common watershed. 3) The study demonstrates the complementarity of local sensor data and remote sensing in understanding the environment of this remote, dense rainforest region with poorly understood lake origins.

1. 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 – www.geo212.fr
contact@geo212.fr

2. Location:
Northwest of Congo Basin
Rainforest
Tri-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

3. 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

4. 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

5. Context Sensors Method Results Hypothesis Conclusion
Lake 1
Lake 2
Lake 4
Lake 5
Lake 6
Lake 7
Lake 3
15 months of data !
Prosensor®

6. 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

7. Mean daily pressure inside the lakes
95
97
99
101
103
105
107
109
29/09/10 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 01/04/12
Pressure(kPa)
Date
Synchroneous
responses of the
lakes regardless of
the water level
Context Sensors Method Results Hypothesis Conclusion

8. 15
17
19
21
23
25
27
29
95
97
99
101
103
105
107
109
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 01/04/12
Temperature(°C)
Pessure(kPa)
Date
Mean data for lakes 2 4 5 7
Mean pressure
Mean temperature
Hypothesis :
response of
the lake to a
rainy event
0
10000
20000
30000
40000
50000
60000
70000
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 01/04/12
Brightness(lux)
Confirmation
by brightness
data
Context Sensors Method Results Hypothesis ConclusionContext Sensors Method Results Hypothesis Conclusion

9. Supposed rainy events occur in high rainfall estimate periods
Context Sensors Method Results Hypothesis Conclusion

10. The beginning of flood moves back or forward depending on the lakes
Context Sensors Method Results Hypothesis Conclusion

11. Context Sensors Method Results Hypothesis Conclusion
2
4
5
7
Isolated
watersheds

12. 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

13. Conclusions
Context 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.

14. Geo212 – 25, rue Jean Dolent – 75014 Paris – www.geo212.fr
benedicte.navaro@geo212.fr
Thank you for your attention