In this exercise, a wetland with a contributing rural catchment was modelled to demonstrate the necessity and benefits of simulating groundwater (GW) for post-development conditions using the newly developed GW modelling tool in VO. Under existing conditions, the wetland relied on recorded GW data to simulate hydrologic responses. However, for post-development scenarios, changes in land use and inflow patterns significantly altered the groundwater contribution to the wetland, making the recorded data unsuitable for accurate modelling.
Two post-development scenarios were analyzed: one without control measures, where a portion of the catchment was developed and represented using a StandHyd command, and one with control measures, where additional infrastructure, including a roof area and a RouteReservoir, was integrated. Simulated GW data for these scenarios were compared against the conventional approach of using recorded GW data from existing conditions.
!Note: In previous modelling practice, groundwater levels under post-development conditions were often assumed to remain unchanged due to the absence of GW measurements for future scenarios. However, this assumption neglects the fact that land-use change alters surface inflow, infiltration, and recharge patterns, which directly influence groundwater dynamics and wetland–groundwater exchange. The new VO Groundwater (GW) Simulation tool addresses this limitation by allowing users to generate post-development groundwater levels based on the modified inflows resulting from development. By linking groundwater response to changes in inflow, the VO GW tool provides a more physically consistent and realistic representation of wetland hydrology compared to traditional approaches that rely on static, existing-condition groundwater data.
*Regulatory Context and the Need for Hydroperiod Comparison
The comparison between the pre-development target hydroperiod and the unmitigated post-development hydroperiod is not optional; it is a regulatory requirement under the Toronto and Region Conservation Authority (TRCA) Wetland Water Balance Modelling Guidance. The objective of this comparison is to determine whether proposed land-use changes result in significant alterations to wetland hydrology and to inform the need for mitigation measures. TRCA requires that post-development conditions demonstrate minimal deviation from pre-development water balance and hydroperiod characteristics to ensure protection of ecological function.
The hydroperiod, defined as the temporal pattern of water level fluctuation within the wetland, is the primary indicator used to assess hydrologic impact. It reflects the cumulative interaction of all water balance components, including precipitation, surface inflow and outflow, evapotranspiration, groundwater inflow and outflow, and storage dynamics. Even small shifts in hydroperiod timing, magnitude, or duration can influence vegetation communities, habitat conditions, and overall wetland function. Therefore, graphical comparison of hydroperiods provides a clear and defensible basis for evaluating development impacts.
VO provides a built-in capability to generate hydroperiod graphs directly from continuous simulation results. Traditionally, users could produce these hydroperiod comparisons using observed groundwater data carried forward into post-development scenarios. However, this conventional approach assumes that groundwater conditions remain unchanged despite alterations in runoff, infiltration, and recharge caused by development. The new VO Groundwater (GW) Simulation tool improves upon this methodology by recalculating groundwater levels under post-development conditions based on modified inflows. As a result, the hydroperiod generated using simulated GW data more accurately reflects the true hydrologic response of the wetland to development.
