Storing and sharing water in sand Storing and sharing water in sand rivers: a water balance modellingrivers: a water balance modellingapproachapproachEGU General Assembly 2009EGU General Assembly 2009

David LoveDavid LovePieter van der Pieter van der ZaagZaagStefan UhlenbrookStefan UhlenbrookRichard OwenRichard Owen

OutlineOutline• Alluvial aquifersAlluvial aquifers• Lower Mzingwane Valley study areaLower Mzingwane Valley study area• Data collectionData collection• Modelling: approachModelling: approach• Modelling: resultsModelling: results• ConclusionsConclusions

Alluvial aquifersAlluvial aquifers• Water stored in sand bed of riverWater stored in sand bed of river• Evaporation only in top 0.90 mEvaporation only in top 0.90 m• Recharged annually / by dam Recharged annually / by dam

releasesreleases

Alluvial aquifersAlluvial aquifers3 ha aquifer3 ha aquiferBlue: unutilisedBlue: unutilisedRed: 4 ha Red: 4 ha

irrigationirrigationYellow: irrigation Yellow: irrigation

plus managed plus managed releasesreleases

Lower Mzingwane ValleyLower Mzingwane Valley

• High water stressHigh water stress– PET 1800 mm/a, P 360 mm/aPET 1800 mm/a, P 360 mm/a

• Infrastructure favours commercial Infrastructure favours commercial farmersfarmers

• Catchment 5,955 kmCatchment 5,955 km22

– Irrigation 2,597 haIrrigation 2,597 ha– Smallholder irrigation 854 haSmallholder irrigation 854 ha

• Extensive alluvial aquifersExtensive alluvial aquifers

Data collectionData collection• Water users identified, interviewedWater users identified, interviewed• Dams identified from national database Dams identified from national database

and satellite imageryand satellite imagery• Field investigations of alluvial aquifersField investigations of alluvial aquifers• Choice of assumptions and data sources Choice of assumptions and data sources

giving a more conservative estimate of giving a more conservative estimate of water resource availabilitywater resource availability

• Problems with discharge availabilityProblems with discharge availability

Modelling: approachModelling: approach• WAFLEXWAFLEX

– Water balance modelWater balance model– 10 days time step10 days time step– Spreadsheet modelSpreadsheet model

Supply Demand

Operation of the reservoirs

Modelling: approachModelling: approach• Challenges with WAFLEXChallenges with WAFLEX

– Does not consider “losses”Does not consider “losses”– Groundwater is not consideredGroundwater is not considered

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

Modelling: resultsModelling: results• S0: Current scenario: System is S0: Current scenario: System is

under-utilisedunder-utilised• S1: Planned 1,205 ha: demand metS1: Planned 1,205 ha: demand met• S2: Use alluvial aquifers, irrigate S2: Use alluvial aquifers, irrigate

4,207 ha extra along the river, 4,207 ha extra along the river, demand metdemand met

Modelling: resultsModelling: results• S3: Without Zhovhe Dam: demand S3: Without Zhovhe Dam: demand

met: planned 1,205 ha & 1,256 ha met: planned 1,205 ha & 1,256 ha extraextra

• S4: Oakley Block instead of Zhovhe S4: Oakley Block instead of Zhovhe Dam: demand met: planned 1,205 ha Dam: demand met: planned 1,205 ha 2,008 ha extra2,008 ha extra

• S5: Oakley Block and Zhovhe Dam: no S5: Oakley Block and Zhovhe Dam: no benefit over S2benefit over S2

ConclusionsConclusions• S2 allows maximum benefit-sharing from S2 allows maximum benefit-sharing from

current systemcurrent system– Irrigated area tripledIrrigated area tripled– Decentralised, family-run irrigation along Decentralised, family-run irrigation along

river banks brings benefits to more peopleriver banks brings benefits to more people• S3 shows that Zhovhe Dam was not S3 shows that Zhovhe Dam was not

needed for current demand, official plansneeded for current demand, official plans• WAFLEX: flexible, infomation for planning WAFLEX: flexible, infomation for planning

from limited datafrom limited data• Flood plain aquifers offer further scopeFlood plain aquifers offer further scope

AcknowledgementsAcknowledgementsFundedFunded through the CGIAR through the CGIAR Challenge Challenge Program on Water and FoodProgram on Water and Food

With With additional funding by the additional funding by the International Foundation for ScienceInternational Foundation for Science

Acknowledgements –Acknowledgements –cooperation & datacooperation & data•Zimbabwe National Water AuthorityZimbabwe National Water Authority

–Charles Sakahuni, Mzingwane hydro-technicianCharles Sakahuni, Mzingwane hydro-technician–Felix Whinya, Zhovhe Dam BailiffFelix Whinya, Zhovhe Dam Bailiff

•Field assistants: Daniel Mkwananzi, Field assistants: Daniel Mkwananzi, Saddam Mkwananzi, Brighton SibandaSaddam Mkwananzi, Brighton Sibanda•Dabane TrustDabane Trust•Mzingwane Catchment CouncilMzingwane Catchment Council•Meteorological Services DepartmentMeteorological Services Department

Taboka / Siyabonga / Thank-Taboka / Siyabonga / Thank-youyou