Introduction

Surface water and groundwater are often treated as separate entities. However, almost all surface water is in continuous interaction with groundwater. In a few isolated cases there is virtually no interaction between the two, but in the majority of cases there is substantial interaction, albeit highly variable, temporally and spatially. Often surface water streams gain water from groundwater systems, and as a result extractions from groundwater will reduce streamflows. Sometimes the reverse is true, and groundwater is replenished by leakage from the streamflow channels (and/or from inundated floodplains); in these cases it is the withdrawal of water from streams reduces the recharge to groundwater.

The interaction between surface water and groundwater is hidden from view, and historically we have tended to manage the two resources separately. As a result we have often double-accounted and even double-allocated the same resource – once as surface water and a second time as groundwater – even though physically we are dealing with the same parcel of water.

We have often not recognised this interaction because groundwater moves very slowly beneath the surface. The time taken for groundwater extractions to influence streamflows may range from days to many decades. Thus, the interaction we become aware of today might be the legacy of actions taken many years earlier.