Surface water and groundwater: understanding the importance of their connections

Introduction

A basic understanding of the various components of the hydrological cycle has been known for the last 2000 years. Marcus Vitruvius Pollio, writing in the first century BC in Book VIII (water supply) of de Architectura, clearly understood concepts such as evaporation and condensation, the infiltration of rainfall into the ground and the source of springs. The interested reader can find an excellent web page on the work of Vitruvius prepared by Bill Thayer (<http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/home.html>). High school texts on geography and science will have a conceptual diagram of the hydrological cycle similar to that shown in Figure 1. With such a well-known concept, it is surprising that we should be emphasising again the interconnected nature of surface water and groundwater in 2009. Yet it is clear from policy laid down in the last 20 years that the degree to which surface water and groundwater are interconnected has somehow slipped from general awareness. A symptom of the divergence that has occurred is even the spelling now accepted in common parlance in Australia. Groundwater is one word, while surface water is two words!

Figure 1 Hydrological cycle.

There are many possible explanations for this collective lapse. It is frequently argued from a surface water perspective that it is hard enough to quantify the major surface fluxes (rainfall, runoff and evapotranspiration) and that small errors in those are of the same magnitude as that of surface water exchange with groundwater so that the latter can be ignored. From a groundwater perspective, modelling studies frequently concentrate on efforts to match hydraulic heads by varying the hydraulic properties within the aquifer (transmissivity and storage) with only passing interest in recharge or discharge, which are considered to be of second-order importance and handled as a fixed boundary condition. Educators must also take some responsibility for the artificial separation of teaching the various components of the hydrological cycle. It is common practice for surface water hydrology to be taught in engineering programs where the emphasis has been on estimates of runoff quantity and flood height for civil engineering application, whereas groundwater resource analysis has fallen more into science programs.

The problem is a worldwide phenomenon that grew particularly in the second part of the last century, particularly, perhaps, as a result of above average rainfall in several parts of the world during this time. Inaccuracy in hydrological cycle representation was obscured by an excess of water. With lower rainfall and further extensive development of water resources, combined with a growing appreciation of the importance of groundwater-dependent ecosystems such as springs, base flow to rivers and wetlands, the significance of surface water connectivity to groundwater has been rediscovered.

The lack of general awareness concerning interconnectivity prompted Winter et al. (1998) to publish a seminal work Ground Water and Surface Water—A Single Resource that has been extensively used by the groundwater community to better understand the problem. The International Association of Hydrogeologists (IAH) has been at the forefront of attempts to better represent the integrated nature of surface water and groundwater resources.

The papers in this thematic issue were first presented at a joint New Zealand Hydrological Society (NZHS)–IAH Conference in Auckland during November 2005 where the conference topic was ‘Where Waters Meet.’ The nine papers selected present various aspects of the new techniques required to investigate interchange in detail and specific studies undertaken to better characterise different aspects of surface water and groundwater connectivity.

Papers from Brodie, Baskaran, Ransley & Spring (Seepage meter: progressing a simple method of directly measuring water flow between surface water and groundwater systems), Baskaran, Ransley, Brodie & Baker (Investigating groundwater–river interactions using environmental tracers) and Baskaran, Brodie, Ransley & Baker (Time-series measurements of stream and sediment temperature for understanding river–groundwater interactions: Border Rivers and Lower Richmond catchments, Australia) describe available techniques for the investigation of aspects of connected waters, particularly the movement of water beneath a stream bed into the aquifer and vice versa.

The important interface between groundwater and the ocean is described in papers by Glamore & Indraratna (Tidal forcing groundwater dynamics in a restored coastal wetland: implications of saline intrusion) who consider the problem of acid-sulfate soils, and Vogwill, Williamson & Kote (Hydrogeology of the coastal mining area, Lihir goldmine, Papua New Guinea) who look at the fascinating problem of abstracting water from an oceanic island where relict volcanic activity produces very high thermal gradients. Anderson, Timms & Glamore (Optimising subsurface well design for coastal desalinisation water harvesting) show how seawater can be abstracted from an aquifer for use as a feed source for desalinisation.

White (Avon River Springs catchment, Christchurch City, New Zealand) gives a detailed consideration of the interconnection of stream flow and aquifer water in a description of the Avon Springs Catchment in New Zealand. The use of 3D geological modelling techniques demonstrates the detail of geological data required to successfully characterise the processes. Ivkovic, Letcher & Croke (Use of a simple surface–groundwater interaction model to inform water management) consider the same problem but from a modelling approach in their study of Coxs Creek in northern New South Wales. Finally, Acworth & Timms (Evidence for connected water processes through smectite dominated clays at Breeza, New South Wales) consider the problem of deep drainage beneath irrigation fields and an underlying aquifer, and show that even though there may be a considerable thickness of clay present, a significant connection still exists between the surface applied irrigation water and water in an aquifer at 16 m depth.

Abstracts to the Auckland Conference were originally invited in February 2005 and were considered by a Publications Committee established by the New South Wales Branch of the IAH. Committee members comprised Ian Acworth, Noel Merrick, Wendy Timms, Michael Williams, Warwick Wood and Don Wooley. The abstracts were reviewed by this committee and successful authors asked to prepare a full paper for the conference. These papers were then peer reviewed in full by two members of the committee. Of the 89 full papers presented at the conference, 25 were concerned with the theme of surface water and groundwater connectivity. As the theme generated considerable interest, it was decided to publish selected papers in a special proceedings volume, and a thematic issue of the Australian Journal of Earth Sciences seemed an appropriate vehicle for this. Authors were again asked to extend their papers where appropriate, and the final papers were reviewed by the Guest Editor.