Historical failure to recognise the connection between surface water and groundwater and to manage river and groundwater systems conjunctively has led to over allocation of water resources. Examples of river systems that have been depleted by groundwater pumping, for example, are well documented.

Although conjunctive management of surface water and groundwater is preferred, it requires information on the magnitude of the interaction between the two reservoirs. While numerous different methods have been used to measure the exchange flux, the usefulness of many of these methods is limited by the very high spatial and temporal variability of the fluxes involved.

Methods that have been used successfully to estimate spatial variations in groundwater discharge to streams, over scales of interest to water managers, include water and solute mass balances. Groundwater discharge to streams can be determined from differences between river flow gaugings made at different points along the river, and from measurements of conservative ion concentrations. Sensitivity is greatest, however, when tracers are used whose concentration in the groundwater greatly exceeds that in surface water. This condition is often best met for the naturally occurring radioactive element radon (222Rn). Since radon in surface water is lost by exchange with the atmosphere (which is low in radon), a large contrast in concentration with the groundwater is maintained.

Measuring changes in groundwater discharge with time is more difficult, because the required frequency of measurements means that only parameters that can be measured remotely are suitable.

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