Conjunctive surface and groundwater use

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Aerial landscape in Okavango delta, Botswana. Lakes and rivers, view from airplane. Green vegetation in South Africa. Wetlands and groundwater.

Conjunctive Water Management (CWM) is a form of water resources management in which surface water, groundwater and other components of the water cycle are managed as a hydraulically connected system in order to maximize potential benefits from water in the short and long term. It is an essential component of Integrated Water Resource Management (van der Gun, 2020).  

CWM is especially important in river (and lake) basins, where the surface and groundwater are two parts of one water system. Accordingly, a change in one part of the system will imminently lead to a change in the other part. That holds in particular for alluvial, shallow aquifers which are in direct contact with surface water and less for deep-sited aquifers of an aquifer system.    

Monitoring of both surface and groundwater is crucial because it provides information on (state of the resources but also on) interaction between surface and groundwater. This interaction can be quite complex, primarily because dynamics of groundwater is usually much slower than of surface water (with exception of karst aquifers).  

Surface-groundwater interaction determines the total effective water reserves in the basin and fair estimation of reserves is necessary prior any management intervention. A double counting of reserves is not unusual error, having multiple consequences. Available reserves are calculated using the balance equation; inputs (e.g. rainfall) and outputs (e.g. abstraction) in the equation are based on observations and (modelled) estimations. Numerical models are often utilised, especially when prediction and scenario analyses is required. Groundwater component and its dynamics is commonly the least known component in the models. Integrated  models are powerful tools for a conjunctive water management when based on sufficient info and calibrated properly.   

Reference

  • Gun van der Jac, Conjunctive Water Management, UNESCO, 2000;  
  • Conjunctive use and management of groundwater and surface water, Thematic Paper, GW Governance project, FAO; 
  • Jonathan Lautze Conjunctive management of surface and groundwater; and 
  • in transboundary watercourses 2018, IWA.