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Likely Effects Of Climate Change on Runoff from Limpopo Basin - Botswana

Likely Effects Of Climate Change on Runoff from Limpopo Basin - Botswana. By Prof. B. P. Parida Dept. of Environmental Science University of Botswana. Background. Much of Southern Africa below Latitude 20 o S is basically Semi-Arid This includes BOTSWANA where

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Likely Effects Of Climate Change on Runoff from Limpopo Basin - Botswana

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  1. Likely Effects Of Climate Change on Runoff from Limpopo Basin - Botswana By Prof. B. P. Parida Dept. of Environmental Science University of Botswana

  2. Background • Much of Southern Africa below Latitude 20o S is basically Semi-Arid • This includes BOTSWANA where Annual Rainfall ~ 250 – 500 mm Evaporation ~ 2000 mm Topography ~ Flat and Deep Sandy Soil (These together limit the effectiveness of Rainfall)

  3. Sources of Water • Rivers and Streams : Average runoff is very low~0.6 mm for the whole country > All endogenous rivers originating in Botswana are ephemeral with an average of 10–70 days of flow in a year. • Other Sources: Desalination, Water carriers to tap international rivers – Very Expensive • Groundwater :

  4. Location of Boreholes in Botswana

  5. Sources..(Groundwater..contd)

  6. Water Supply.. • Clearly Botswana is one amongst the water stressed countries of the world (where water use is more than 20% of their renewable water supply) • Botswana is highly dependent on its natural resource base : mining, livestock, wild-life based tourism, textiles, soda ash and agriculture

  7. In Summary: Water is the single most important constraint to the development of Botswana

  8. The Limpopo Basin • The major source of water in the eastern Botswana and it is important because of its strategic value to Botswana, South Africa Zimbabwe and Mozambique. • Drains from a catchment area of about 80 000 km2 (~1/8 area of Botswana) • Approximately carry about 10 mm of surface water annually.

  9. The Limpopo Basin • The catchment serves as the main source for Gaborone & rapid and ongoing expansion of southeastern Botswana. • In Botswana alone.. 4 dams viz: Gaborone, Bokaa, Letsibogo and Shashe have been built with a total capacity of 350 M m3 (against projected demand of about 325 M m3 by 2020)

  10. Indicative Studies Reveal that: • A rise of 0.02 to 0.05oC per decade in Africa in the present century(IPCC) (greatest over the interior of semi-arid margins of Saharaand Central Southern Africa.) • Limpopo basin is likely to experience between 5 to 15% decrease in rainfall, 5 to 20% increase in Evaporation, and 25 to 35 % decrease in streamflow. (Desanker and Magadza, 2001).

  11. Main objective To quantify the effect of climate change on the flow regime of Limpopo basin – hence develop flow scenarios. • to find by what percentage the runoff will either increase or decrease in the coming years? • what will be the trend in flow over the coming decades in this century?

  12. Philosophy: • Rainfall ~ Runoff is a complex paradigm as it represents the catchment’s response which is a function of land use • So, any adaptation strategy that needs to be developed to overcome the effects of climate change, would primarily centre around a well structured catchment management strategy along with strategies for water use management.

  13. PROPOSED METHODOLOGY • Use past climatic and flow information (monthly rainfall, PE, and flow data over the past 20-30 years including data on landuse) to carry out water balance, hence compute the annual yield. • Use the past data as above to develop an ANN model which can be used for flow predictions. • Use the generated climatic scenario to develop flow scenario for the Limpopo.

  14. CONCEPT OF MULTI-CELL WATER BALANCE :P = AET + OF + ΔSM + ΔGWS +GWR + Interception

  15. Data Collection: • Rainfall Data: Monthly data at 37 stations (1971 – 1998) • Evaporation Data: Monthly data at 4 stations (1971 – 1998) • Flow Data : Monthly data at 1 station (1978 – 1998) • Soil Data : Field Capacity of major soil types in the basin as per the soil atlas.

  16. Procedure for Multi-cell Water Balance: • Long term average rainfall values and evaporation values are superimposed on the soil map and the catchment is then divided into 75 cells – each cell basically representing uniform rainfall, evaporation and soil type. • For each cell and for a given year runoff is computed. (Thornthwaite and Mather, 1957) • For each year total runoff from the catchment is computed at the outlet by adding up flows from the 75 cells. • Total runoff / Total rainfall = Runoff coeff.

  17. A TYPICAL WATER BALANCE COMPUTATION TABLE ( Field Capacity = 125 mm )

  18. CONCEPTUALISATION OF ARTIFICIAL NEURAL NETWORK BASED ON A BIOLOGICAL NEURON

  19. HOW DOES THE ANN WORK ??

  20. A MULTI-LAYERED FEED FORWARD NEURAL NETWORK WITH ONE HIDDEN LAYER

  21. Conclusion Using scenario of monthly rainfall and evaporation values from the GCM / RCM models (AF 07), scenarios on likely runoff from the Limpopo basin by the above two techniques will be created.

  22. Co-operation needed: • Monthly rainfall and evaporation data scenarios for the Limpopo basin in the next 30 – 50 years will be necessary (GCM /RCM / Empirical / Other climate Model). • Formulation of appropriate catchment management strategies as possible adaptation strategy.

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