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

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

Location and background

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

Conceptual System setup

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- Drought: Impacts agriculture- Upstream-Downstream conflicts- Community flooding- Reduced domestic water supply

Biophysical IWRM Challenges

Tools and Methods

Impacts on Communities

RecommendResearch and

action-oriented interventions

Contribute to IWRM

as VBD

C Research Output

Stakeholder Engagement

Sedimentation/Siltation

Water Yields Constraints

- Siltation of reservoirs- Reduced storage volume - Soil erosion: Shallow soils- Disrupts aquatic life- Impact on water quality

Stakeholder feedback on challenges

Outputs

Water budget estimates and

allocation

Sedimentation quantification

and control

Modeling and Scenarios analysis

Flood vulnerability and land use

planning

Modeling Water/Sediment Yields- Study catchment in Ghana (Zebilla: 1,695 km2) - - Modeling conducted to include upstream inputs to- and downstream sinks from study area

Model Calibration and Validation

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Calibration results for Yakala

R2=0.72NSE=0.68PBIAS= 12.6%

R2=0.84NSE=0.77PBIAS= 6.3%

Conditions for successful calib.R2 > 0.6NSE > 0.50 PBIAS is + 25%(Santhi et al., 2001; Moriasi et al., 2007)

Model Calibration and Validation

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R2=0.82NSE=0.78PBIAS= 15.5%

R2=0.83NSE=0.82PBIAS= 4.4%

Calibration and validation results for Nawuni

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.

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

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1996 1997 1998 1999 2000 2001 2002An

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MeasuredCalibration and validation Results for Nawuni

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

GEO-SFM ModelFlood Hazard Assessment

HydrographSmall reservoirs

Characterizing Flood Risk

Generate DailyHistorical Rainfall

(1961-2003) by reanalysis

Produce a synthetic

streamflow record

Compute Bankfull storage

Determine locations where bankfull storage

Is exceeded

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Next Steps

o Conduct data processing module, water balance routines and flow routing modules

o Generate flood hazard map

Flood modeling

Sediment modelingo Model scenarios of interventions e.g., introducing grass

strips to ascertain impacts on erosion and sedimentation

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 upstream-downstream 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

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 multi-stakeholder platform for policy and IWRM interventions