Piotr WolskiClimatic and hydrological changes in the Okavango Delta

Mohembo

Maun

The Okavango

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

Frequency of tectonic events (after McCarthy, 1993)

Regional tectonic setting (after Kampunzu, 1998)

The Okavango

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

February August

Seasonal flood pulse

Seasonal flood pulse

Human factor

Exogenous variability? change?

• Interannual and multidecadal variability. Or is it change?

Mohembo

Maun

Endogenous change

After McCarthy et al.

• Channel aggradation and abandonment

• Animal action (hippo paths)

• Water flow-vegetation-fire feedbacks

• Tectonic activity

Multidecadal variability

Air temperatureRainfall

CRU dataset and observations

Is it rainfall or temperature that affects the Okavango?

Both Neither Only P Only TObserved

Multi-Taper Method (MTM) of spectral analysis

Wolski et al. submittedDischarges respond stronger to multidecadal

variability in rainfall than in temperature!

What causes MDV in rainfall?

Pacific Decadal Oscillation ???

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 like?

Multidecadal variability present, but superimposed on downward trend

Simulations of Okavango River discharges based on ensemble of GCM runs

Is climate change happening already?

Climate change attribution

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

Adjustment of:

•Atmospheric greenhouse gas concentrations

•Sea surface temperatures

Plans to include:

•Tropospheric sulphate aerosols

•Stratospheric volcanic sulphate aerosols

•Sea ice coverage

Attribution - results

temperaturerainfall

Attribution - results

Okavango discharge at Mohembo

FAR: -0.03 for 2009-0.19 for 2010

Probability of high floods decreases in warmer climate

Skill of models

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

“Good”years

“Bad”years

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 Maps

Frequency of synoptic states for each year

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

p-value

0

1

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

Data deficiency and flood modelling

Simulations of Okavango flows based on TRMM rainfall

TRMM data“Tweaked” MIRA + obs

Particularly significant in the context of seasonal forecasting

observed

TRMM-based

Endogenous change

Sat image Inundation map Time series

Distributary 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

Temporal trend in deviation of probabilities of inundation status

drier wetterNo change

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

Summary

• 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

Summary

• 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…