Storing and sharing water in sand rivers: a water balance modelling approach EGU General Assembly 2009 David Love Pieter van der Zaag Stefan Uhlenbrook Richard Owen

Outline Alluvial aquifers Lower Mzingwane Valley study area Data collection Modelling: approach Modelling: results Conclusions

Alluvial aquifers

Lower Mzingwane Valley study area

Data collection

Modelling: approach

Modelling: results

Conclusions

Alluvial aquifers Water stored in sand bed of river Evaporation only in top 0.90 m Recharged annually / by dam releases

Water stored in sand bed of river

Evaporation only in top 0.90 m

Recharged annually / by dam releases

Alluvial aquifers 3 ha aquifer Blue: unutilised Red: 4 ha irrigation Yellow: irrigation plus managed releases

3 ha aquifer

Blue: unutilised

Red: 4 ha irrigation

Yellow: irrigation plus managed releases

Lower Mzingwane Valley High water stress PET 1800 mm/a, P 360 mm/a Infrastructure favours commercial farmers Catchment 5,955 km 2 Irrigation 2,597 ha Smallholder irrigation 854 ha Extensive alluvial aquifers

High water stress

PET 1800 mm/a, P 360 mm/a

Infrastructure favours commercial farmers

Catchment 5,955 km 2

Irrigation 2,597 ha

Smallholder irrigation 854 ha

Extensive alluvial aquifers

Data collection Water users identified, interviewed Dams identified from national database and satellite imagery Field investigations of alluvial aquifers Choice of assumptions and data sources giving a more conservative estimate of water resource availability Problems with discharge availability

Water users identified, interviewed

Dams identified from national database and satellite imagery

Field investigations of alluvial aquifers

Choice of assumptions and data sources giving a more conservative estimate of water resource availability

Problems with discharge availability

Modelling: approach WAFLEX Water balance model 10 days time step Spreadsheet model Supply Demand Operation of the reservoirs

WAFLEX

Water balance model

10 days time step

Spreadsheet model

Modelling: approach Challenges with WAFLEX Does not consider “losses” Groundwater is not considered New alluvial groundwater module – water balance using parameters from hydrogeological investigations

Challenges with WAFLEX

Does not consider “losses”

Groundwater is not considered

New alluvial groundwater module – water balance using parameters from hydrogeological investigations

Modelling: results S0: Current scenario: System is under-utilised S1: Planned 1,205 ha: demand met S2: Use alluvial aquifers, irrigate 4,207 ha extra along the river, demand met

S0: Current scenario: System is under-utilised

S1: Planned 1,205 ha: demand met

S2: Use alluvial aquifers, irrigate 4,207 ha extra along the river, demand met

Modelling: results S3: Without Zhovhe Dam: demand met: planned 1,205 ha & 1,256 ha extra S4: Oakley Block instead of Zhovhe Dam: demand met: planned 1,205 ha 2,008 ha extra S5: Oakley Block and Zhovhe Dam: no benefit over S2

S3: Without Zhovhe Dam: demand met: planned 1,205 ha & 1,256 ha extra

S4: Oakley Block instead of Zhovhe Dam: demand met: planned 1,205 ha 2,008 ha extra

S5: Oakley Block and Zhovhe Dam: no benefit over S2

Conclusions S2 allows maximum benefit-sharing from current system Irrigated area tripled Decentralised, family-run irrigation along river banks brings benefits to more people S3 shows that Zhovhe Dam was not needed for current demand, official plans WAFLEX: flexible, infomation for planning from limited data Flood plain aquifers offer further scope

S2 allows maximum benefit-sharing from current system

Irrigated area tripled

Decentralised, family-run irrigation along river banks brings benefits to more people

S3 shows that Zhovhe Dam was not needed for current demand, official plans

WAFLEX: flexible, infomation for planning from limited data

Flood plain aquifers offer further scope

Acknowledgements Funded through the CGIAR Challenge Program on Water and Food With additional funding by the International Foundation for Science

Funded through the CGIAR Challenge Program on Water and Food

With additional funding by the International Foundation for Science

Acknowledgements – cooperation & data Zimbabwe National Water Authority Charles Sakahuni, Mzingwane hydro-technician Felix Whinya, Zhovhe Dam Bailiff Field assistants: Daniel Mkwananzi, Saddam Mkwananzi, Brighton Sibanda Dabane Trust Mzingwane Catchment Council Meteorological Services Department

Zimbabwe National Water Authority

Charles Sakahuni, Mzingwane hydro-technician

Felix Whinya, Zhovhe Dam Bailiff

Field assistants: Daniel Mkwananzi, Saddam Mkwananzi, Brighton Sibanda

Dabane Trust

Mzingwane Catchment Council

Meteorological Services Department

Taboka / Siyabonga / Thank-you