Integrated modelling is emerging as a powerful tool to assess and understand the interactions between surface water and groundwater processes. Traditional un-coupled, steady state modelling approaches often do not provide the necessary temporal scale needed to offer insight into many surface water/groundwater interactions.  For example, vernal pools that fluctuate seasonally both in volume and extent depending on the depth to water table can significantly affect runoff generation. Transient, high resolution modelling has required the development of new software tools to process, visualize, and communicate model results. Together, these successful projects illustrate how an integrated GSFLOW model can provide the insight and understanding needed to assess today’s complex water management challenges.

The identification of significant groundwater recharge areas (SGRAs) has been an important element of recent Source Water Protection initiatives in Ontario. Several techniques for identifying ESGRAs, and establishing the linkage between the recharge zones and ecologically significant discharge areas were investigated using watershed scale groundwater models. Particle tracking is an accepted methodology for visualizing and understanding groundwater flow paths in areas with complex, three-dimensional flow fields. In summary, the reverse particle tracking combined with the cluster analysis technique provides an approach that can be used to identify recharge areas of true ecological significance.  This map can be used to quantitatively contour, visualize and compare these areas on a watershed by watershed basis, both under existing or future development or climatic conditions.

The modelling team applied an integrated groundwater and surface water modelling strategy distributed at a high-resolution to assess groundwater supply risks in southern Ontario, Canada.  In this paper, we first demonstrate an approach to routing surficial flow based on a cascading flow approach. It is further possible to demonstrate how minor changes in groundwater table elevation can have a large influence on surface water processes.  By integrating groundwater and surface water processes, groundwater feedback mechanisms become implicit, resulting in drastic impacts to surface water runoff production. We conclude with a discussion of the limitations of such an approach, but will hopefully convince the audience that the benefits outweigh the limitations of alternative approaches.  These efforts were accomplished to assess municipal water supplies and are rooted in the robust regulatory system for clean water protection in Ontario.

Integrated groundwater/surface water modelling offers numerous benefits, but not all models have the same ability to represent the complex and detailed interaction between the groundwater system and surface water bodies such as wetlands, stream channels and lakes. This paper presents the issues, and select examples, showing how the new USGS GSFLOW model handles this interaction.  GSFLOW represents lakes and water bodies as part of the subsurface model domain, thereby allowing them to interact with multiple aquifer layers, each with distinct lateral and vertical conductance terms. In summary, these examples show that considering “surface water bodies” as an integrated part of the (subsurface) model domain is necessary to effectively simulate and understand groundwater/surface water interaction.

Hydromodification due to urbanization usually results in a larger peak event stream discharge, a change in typical event volume, a reduced lag time between rainfall and stream discharge events, and a more complex falling hydrograph. These hydrologic changes preferentially affect smaller magnitude, higher frequency events and can have a negative impact on stream habitat quality and biotic diversity. Recently, Environment Canada data have been extracted to create a high-resolution (15-minute interval) instantaneous stream flow dataset dating to the late 1960s for many Ontario gauge stations. This suggests that the increased detention introduced by with stormwater management systems (designed to mitigate the effects of urbanization) can substantially decrease the peak magnitude of storm events below that of pre-urban conditions.

The Lake Simcoe Protection Plan contains policies requiring that each subwatershed in the Lake Simcoe Watershed undergo a Tier 2 Water Budget and Stress Assessment to understand the current supply, demand, and how the watershed will likely respond to future conditions such as increases in water demand or drought. The Tier 2 assessment showed that study subwatersheds have a low potential for stress under existing and future water use conditions. The recommended policies will require the pre-development water balance to be maintained, the pre-treatment of contaminated run-off prior to infiltration, and environmental compliance approvals guidance document be amended to reflect the importance of protection ESGRAs as a priority consideration prior to approving stormwater works.

Integrated surface water/groundwater modelling is a preferred approach for impact assessment, land development and watershed management studies. One central question is whether it is appropriate to independently calibrate the surface water and groundwater sub-models, or is it better to proceed to integrated calibration early in model development process. Integrated calibration has some additional benefits. Parameter values are generally more tightly constrained when forced to match both the surface water and groundwater response. Better data management strategies are also critical when individual model runs can produce over a terabyte of output. Finally, some of the greatest management and calibration challenges involve integrating the various project team members, process conceptualizations, and even the separate terminologies that have developed in each discipline.