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Piotr Wolski. Climatic and hydrological changes in the Okavango Delta. The Okavango. Mohembo. Maun. The Okavango. it is Delta, not delta it is the 5th largest Ramsar site in the world it is the 3rd largest inland “delta” in Africa. The Okavango. Regional tectonic setting

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Piotr Wolski

Climatic and hydrological changes in the Okavango Delta

the okavango
The Okavango



the okavango1
The Okavango
  • it is Delta, not delta
  • it is the 5th largest Ramsar site in the world
  • it is the 3rd largest inland “delta” in Africa
the okavango2
The Okavango

Regional tectonic setting

(after Kampunzu, 1998)

Frequency of tectonic events

(after McCarthy, 1993)

the okavango3
The Okavango
  • Depth of the Kalahari form aeromagnetic survey (after Brunner, 2007)

Seasonal flood pulse



exogenous variability change
Exogenous variability? change?

Interannual and multidecadal variability. Or is it change?



endogenous change
Endogenous change
  • Channel aggradation and abandonment
  • Animal action (hippo paths)
  • Water flow-vegetation-fire feedbacks
  • Tectonic activity

After McCarthy et al.

multidecadal variability
Multidecadal variability


CRU dataset and observations

Air temperature

is it rainfall or temperature that affects the okavango
Is it rainfall or temperature that affects the Okavango?

Multi-Taper Method (MTM) of spectral analysis




Only P

Only T

Discharges respond stronger to multidecadal variability in rainfall than in temperature!

Wolski et al. submitted

what causes mdv in rainfall
What causes MDV in rainfall?

Pacific Decadal Oscillation ???

how will future look like
How will future look like?
  • 18 CMIP3 GCMs
  • 42 runs for 20th and 21st century

20th century rainfall:

    • Multidecadal variability present
    • Timing is off!

21st century rainfall:

    • Similar magnitude of MDV to this observed in 20st century
    • No consistent trend
how will future look like1
How will future look like?

Multidecadal variability present, but superimposed on downward trend

Simulations of Okavango River discharges based on ensemble of GCM runs

climate change attribution
Climate change attribution
  • Adjustment of:
  • Atmospheric greenhouse gas concentrations
  • Sea surface temperatures
  • Plans to include:
  • Tropospheric sulphate aerosols
  • Stratospheric volcanic sulphate aerosols
  • Sea ice coverage

Stone, Lennard, Tadross, Allen, Stott & Pall (2011)

attribution results
Attribution - results



attribution results1
Attribution - results

Okavango discharge at Mohembo

Probability of high floods

decreases in warmer climate

FAR: -0.03 for 2009

-0.19 for 2010

skill of models
Skill of models

How good is the GCM-hydro model combination in simulating Okavango flows?





self organizing maps
Self Organizing Maps

Classes of synoptic variables in a 6x4 SOM based on NCEP reanalysis data

Air temperature @ 2 m

Relative Humidity @ 850 mbar

self organizing maps1
Self Organizing Maps

Frequency of synoptic states for each year

Significance of difference in frequency between “good” and “bad” years




There is potential for distinguishing “good” years from “bad” ones based on synoptic conditions

data deficiency and flood modelling
Data deficiency and flood modelling

Simulations of Okavango flows based on TRMM rainfall

“Tweaked” MIRA + obs

TRMM data



Particularly significant in the context of seasonal forecasting

endogenous change1
Endogenous change

Sat image

Inundation map

Time series

Distributary inundation

analysis of change in inundation
Analysis of change in inundation

Pixel inundation status

as a function of system’s inundation area

Deviation of probability

as a function of time

inundation trends independent on inputs
Inundation trends independent on inputs

Temporal trend in deviation of probabilities of inundation status

Causes not fully understood, yet have implications on planning, management, cc adaptation etc.


No change


  • Okavango – a complex hydrological system, dynamics of which is influenced by several endogenous and exogenous factors:
    • Interannual climate variability
    • Multidecadal climate variability
    • Climate change
    • Endogeneous system transformation
      • Tectonics
      • Geomorphological development of the wetland
  • Strong international and local appeal – thus need for forecasting at seasonal, decadal and climate-change time-scales
  • In the context of expansion of AfricaArray into hydrology/climate research:
    • understanding of hydrological role of isostatic adjustment in the Delta, both on seasonal and long-term time-scales
    • understanding of role of tectonic events on distribution of water in the Delta and monitoring of tectonic displacement during events
    • Verification/improvement of hydrological models using GRACE, in the context of poor rainfall data
    • Tectonic studies of incipient rifting etc…