Modelling Water Yield, Sedimentation, and Flood Dynamics in Two Sub-basins of the Volta Basin

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Tool: Companion modeling Methodology: Stakeholders identify a collective challenge and use conceptual frameworks to identify their systems in a play fashion Collective identification of social and ecological dynamics Outcome: Identification of a shared representation of issues at stake(actors, resources, dynamics and relationships) through local stakeholder consultation
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  • 1. Modelling Water Yield, Sedimentation, and Flood Dynamics in 2 sub-basins of the Volta Basin Emmanuel Obuobie, Fred Kizito, Christophe Le Page and Jean Philippe Venot
  • 2. Location and background
  • 3. Social aspects of IWRM - Tool: Companion modeling - Methodology: Stakeholders identify a collective challenge and use conceptual frameworks to identify their systems in a play fashion - Collective identification of social and ecological dynamics - Outcome: Identification of a shared representation of issues at stake (actors, resources, dynamics and relationships) through local stakeholder consultation
  • 4. Conceptual System setup Biophysical IWRM Challenges - Drought: Impacts agriculture - Upstream-Downstream conflicts - Community flooding - Reduced domestic water supply Tools and Methods Outputs Water budget estimates and allocation t n e m e g a g n E r e d l o h e k a t S Water Yields Constraints Impacts on Communities s a M R W I o t e t u b i r t n o C t u p t u O h c r a e s e R C D B V d n a g n i l e d o M Sedimentation/ Siltation - Siltation of reservoirs - Reduced storage volume - Soil erosion: Shallow soils - Disrupts aquatic life - Impact on water quality s o i r a n e c S s i s y l a n a Stakeholder feedback on challenges Recommend Research and action-oriented interventions Sedimentation quantification and control Flood vulnerability and land use planning 4
  • 5. Modeling Water/Sediment Yields - Study catchment in Ghana (Zebilla: 1,695 km 2) - Modeling conducted to include upstream inputs to- and downstream sinks from study area
  • 6. Model Calibration and Validation Calibration results for Yakala R2=0.84 NSE=0.77 PBIAS= 6.3% Conditions for successful calib. R2 > 0.6 NSE > 0.50 PBIAS is + 25% (Santhi et al., 2001; Moriasi et al., 2007) R2=0.72 NSE=0.68 PBIAS= 12.6% 6
  • 7. Model Calibration and Validation R2=0.83 NSE=0.82 PBIAS= 4.4% Calibration and validation results for Nawuni R2=0.82 NSE=0.78 PBIAS= 15.5% 7
  • 8. Key water Yield Results  Methodology: Simulated discharge in ‘cms’ was converted to ‘cmy’  Outcome: Estimate of water fluxes that can be imported into WEAP for allocation to the different water users in the basin  Mean annual water yield: 1.4 Billion m3 of which 0.16 Billion m3 is generated within the basin. The remaining 90% is generated upstream of the basin.
  • 9. Sediment Yield Estimation - Methodology: Empirical relationship between water discharge and sediment concentration yields sediment discharge. - Sediment discharge is used to simulate and calibrate sediment transport (t/day) in the catchment; yield is computed as a function of study area t/ha - Outcome: Estimate of sediment yields permits scenarios for interventions to mitigate problem e.g. grass strips 0.6 0.50 0.45 Calibration and validation Results for Nawuni 0.40 Simulated Simulated 0.5 Measured 0.35 Measured 0.4 0.30 0.3 0.25 0.20 0.2 0.15 ) H / ( Y m i d e S t o T l a u n A ) H / ( Y m i d e S t o T l a u n A 0.10 0.05 0.1 0 0.00 1996 Calibration Years 1997 1998 1999 2000 Validation Years 2001 2002
  • 10. Sediment Yield Estimation - Average annual sediment yield for Zebila catchment: 3.4 t/ha/yr - Sediment yield by land use type: Cropland/woodland, Savanna Land use Cropland/woodland Savanna Sediment (t/ha/yr) 4.7 2.1 Contribution to sedimentation (%) 69 31 - Average sediment yield in reservoirs in Zebila catchment: 0.012 t/ha/yr (2035 t/yr) - Global average sediment yield: 15 t/ha/yr - Average for Africa: 9 t/ha/yr
  • 11. Flood Hazard Assessment GEO-SFM Model Hydrograph Small reservoirs
  • 12. Characterizing Flood Risk Generate Daily Historical Rainfall (1961-2003) by reanalysis Produce a synthetic streamflow record Determine locations where bankfull storage Is exceeded ? Compute Bankfull storage
  • 13. Next Steps Sediment modeling o Model scenarios of interventions e.g., introducing grass strips to ascertain impacts on erosion and sedimentation Flood modeling o Conduct data processing module, water balance routines and flow routing modules o Generate flood hazard map
  • 14. Concluding Remarks - Modeling tools are useful for studying sedimentation/erosion and flooding dynamics within the framework of Integrated Water Resources Management (IWRM) - Estimates indicate that 90% of the sub-basin water resources are from upstream sources which signifies implications for upstreamdownstream collaboration on IWRM issues - Sedimentation control through interventions ensures that: - Reservoirs are not subjected to uncontrollable siltation levels - Storage capacity of reservoirs is lengthened and they are used more productively which - Enhances community water provision and livelihoods in the Basin
  • 15. Concluding Remarks- 2 - Flood hazard modeling is far advanced. When completed the generated hazard maps will inform decision making regarding land use planning in the study catchment. This will help reduce vulnerability to flooding disasters. - Results from sedimentation and flood modeling feed into multistakeholder platform for policy and IWRM interventions
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