Conjunctive use and conjunctive management.

Physical / Chemical Interaction – water balance / quality implications

System Dimensions: time / flow rate factors

River basin / aquifer boundaries Allocation issues Institutional interactions Legislation concordance Practical management / monitoring

issues

All abstraction will have an impact on the total water balance and water quality.

Groundwater dependant ecosystems as discharge zones are often the first to register the impact of abstraction.

These impacts may be immediate and relatively simple in small local systems such as dambos or removed, long term and complex as in large regional systems such as rift valleys.

The processes and their duration / time scale need to be understood for effective management.

W E

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Groundwater

Dependent

Ecosystems

Small local featureseg Dambos

Regional featureseg Rift Valley

Decant site – Witwatersrand Gold 20 – 120 ML / daypH 2.1Fe ~ 5% SO4 ~11%

• River basin and the aquifer dimensions may not coincide, especially in sedimentary environments. • Significant volumes of inter-basin water transfer may occur. • Potential for river basin and aquifer management authority conflicts • Complex interactions take place and conjunctive management is needed.

River systems harvestrainfall from the basin and channel this water down the river, where it is stored in alluvial aquifers as alluvial groundwater

Improved infiltration =>High recharge rates =>Perennial water supply => Greater prosperity

No streams on Kalahari SandDense drainage pattern on gneissSame rainfall – different runoff

Effective water allocation requires not only a good working knowledge of the basin / aquifer water balances, but also good communication between the different managers.

Institutional and regulatory issues often impede effective scientific management of the water resource.

Surface water allocations tend to take precedence especially if the groundwater condition is not well known or understood.

Adequate knowledge of surfaceand groundwater resources needed for allocation decisions.

Communication and shared knowledge are essentialfor effective water allocation

The impact of surface water abstractions on groundwater flows => reduced recharge => declining heads => reduced discharges to GDE’s; reduced aquifer yields; increased pumping heads; etc...

Impacts of groundwater abstractions on surface water flows => reduced baseflows and drying out of streams; delay in start of river flow during rainy season; amelioration of floods

Many countries manage their water by means of River Basin Organizations (RBO’s). The role of groundwater in such organizations is often minimal.

Aquifer Management Organizations often operate as independent organizations without reference to surface flows.

Groundwater and Surface water are often managed by different departments (eg Geological Survey vs Dept Water)

The actual interactions between surface and groundwater are complex, and adequate data and advanced modelling are needed to understand these interactions.

Groundwater may not be mentioned at all in the water act.

Often regulation of groundwater is implied in the legislation rather than specified.

Separate acts, such as pollution control acts, are often used to protect groundwater; protection is frequently not included in the water act.

Interpretation and application of the legislation may require significant human resources and technical capacity to achieve compliance.

Where possible, the entire water cycle should be managed in an integrated way, with all inflows and outflows to / from the water balance accounted for.

Conjunctive use of surface water and groundwater allows managers to make use of the large volumes of surface water flows during the rainy season and the large stored groundwater volumes in the dry season.