Conjunctive Use

Even though there is no consistent definition of conjunctive use of water, it is most commonly referred to as the combined use of ground and surface water (FOSTER et al. 2010). The natural hydrologic connection between groundwater and surface water is used in order to improve water reliability (see also the water cycle). This practice can become particularly important for mitigating problems related to climate change, such as increased drought or overexploitation and decreases in groundwater availability (see also water resources assessment and water pollution) (DUDLEY & FULTON 2005).

Two main usage phases of conjunctive use are differentiated:

• Recharge: when the water table is high, the use of surface water is to be maximised. The recharge of groundwater can be enhanced artificially by surface water recharge and subsurface water recharge.
• Recovery: During dry season, water is drawn from groundwater resources (for possible measures to draw groundwater, see groundwater sources).

It is important to keep recharge and recovery in balance (see also water balance estimation). However, as there are usually several users (e.g. farmers) relying on the same water source, individual use may lead to overexploitation of groundwater. This is a widespread problem, because conjunctive use often occurs unplanned and by default. To enhance the benefits of this practice, better coordination between the users is needed, which is part of conjunctive water management discussed below (WORLD BANK 2006).

Conjunctive water management

Adapted from DUDLEY & FULTON (2005)

Conjunctive water management is an improved form of conjunctive use. Additional to the combined use of surface water and groundwater, components of groundwater management tools, such as water balance estimation and water resources assessment, are added. In comparison to conjunctive use, conjunctive water management helps to better manage water use between different parties, leading to well-prepared recharge and recovery phases and therefore to a lower probability of harming groundwater sources.

Today, conjunctive water management is less frequently implemented than conjunctive use, because of lacking institutions or coordination mechanisms (see also building an institutional framework) (WORLD BANK 2006).

If usage is done unplanned, groundwater can be exploited or salinity of groundwater may increase, leading to diminished agricultural productivity. Increased salinity may also be a threat to the security of drinking water supplies (FOSTER et al. 2010).

Conjunctive use of water is often done by default and therefore without special knowledge. To optimise the effectiveness of conjunctive use, system managers should be defined and well trained (see also developing human resources). Management of conjunctive use is best done at the local level where the special needs and conditions are known and the interested actors can be involved. Also, the potential for conjunctive use varies with differing hydrogeological settings, including such factors as average rainfall and geomorphological position. For further information on variations of the dynamics and constraints of conjunctive use (see FOSTER et al. 2010).

Achieving well-managed conjunctive use can be impeded by institutional constraints, economic considerations or the political climate (see also creating policies and a legal framework and building an institutional framework). It is therefore necessary to consider these factors during the implementation process. Participatory monitoring and evaluation can help to enhance the needed learning process and coordination within the community. On-going monitoring is recommended in order to constantly improve conjunctive use.