FLASHBACK TO 2007: At Water Balance Model Partners Forum, Ted van der Gulik explained Performance Targets for rainfall capture in the context of paradigm-shifts
Note to Reader:
The desire to mitigate environmental impacts has provided a driver for the ‘green infrastructure’ movement in British Columbia. This movement is water-centric, is founded on a natural systems approach, and is influencing infrastructure policies, practices and standards.
The Inter-Governmental Partnership held a Water Balance Model Partners Forum in March 2007 so that Partners could share success stories and lessons learned in implementing green infrastructure.
Development of the Water Balance Methodology for establishing Performance Targets has facilitated implementation of an Integrated Strategy for managing the complete rainfall spectrum.
Runoff-Based Approach to Drainage Modelling Connects the Dots Between Source Controls and Stream Health
Stormwater Planning: A Guidebook for British Columbia pioneered the application of performance targets to rainwater management, and the Water Balance Model enables users to test their achievability. For a performance target to be implemented and effective, it must be quantifiable. It must also have a feedback loop so that adjustments and course corrections can be made over time. To be understood and effective, a performance target needs to synthesize complexity into a single number that is simple to understand and achieve, yet is comprehensive in scope. A runoff volume-based performance target fulfills these criteria.
As explained by Peter Law of the Ministry of Environment, “When we wrote the Guidebook, we recognized there is a material difference in the characterization of surface runoff that originates from an individual development site versus flow that 
you see at a catchment or watershed scale. What you see in a watercourse is the total flow – that is, water that flows overland plus water that moves through soil until it daylights. This is one reason why Chapter 6 in the Guidebook emphasizes the need for flexibility in setting performance targets, and states that performance targets should be customized for individual watersheds and catchments, based on what is effective and affordable in the context of watershed-specific conditions.”
The output from the current version of the Water Balance Model is ‘surface runoff’, whereas the output from the Water Balance Model powered by QUALHYMO will be ‘total flow’. This expanded capability is a key to determining what level of rainfall capture may be effective and affordable. “It is important for everyone to keep in mind that the application of a performance target approach is essentially about due diligence and doing the right things to capture rain where it falls. The performance target approach is intended to be the exact opposite of a prescriptive approach. The latter is counter-productive when it leads to rigid standards that have the effect of inhibiting creativity and innovation”, added Peter Law.
The First Paradigm-Shift
In 2002, the purpose in introducing performance targets for rainfall capture was to facilitate implementation of the Integrated Strategy for managing the complete rainfall spectrum. To create a mind-map for practitioners, the rainfall spectrum was defined in terms of three tiers, with each tier corresponding to a component of the integrated strategy, namely: light showers, heavy rain, and extreme storms.
Defining rainfall tiers enabled a systematic approach to data processing and identification of rainfall patterns, distributions and frequencies. Establishing the Mean Annual Rainfall (MAR) as a reference point provided a convenient way to divide the total number of rainfall-days per year into three groupings. The relevance of the MAR is simply that it established a starting point for assessing the achievability of rainfall capture. In 2002, focussing on the MAR facilitated a paradigm-shift in the state-of-the-practice because it demonstrated that rainfall capturewas achievable: the number of rainfall-days up to and including the MAR equated to 90% of the total annual rainfall volume.
The Next Paradigm-Shift
According to Ted van der Gulik, IGP Chair, “The Inter-Governmental Partnership envisions the next paradigm-shift in the state-of-the-practice will revolve around the way we relate runoff volume management to stream erosion and water quality. 
Volume is something that local government has control over, and it is measurable. We have observed that the power of the Water Balance Model process lies in the conversations that result from users generating a single number – the percentage of rainfall that becomes runoff – that represents the synthesis of any particular scenario. Comparison of scenarios creates understanding, especially when the focus is on the hydrologic implications of the assumptions that underpin those percentages.”
“Several qualitative indicators can be utilized in assessing the potential for erosion or sediment accumulation within a watershed”, elaborated Jim Dumont, the Partnership’s Engineering Applications Authority, “The methodology selected for the Water Balance Model powered by QUALHYMO is based upon shear stress as applied to the stream banks over time. This is a measure of the energy available to cause erosion in a stream. Continuous simulation is the key to generating scenario comparisons. ”
“Normal sediment loading from a stable urban watershed is in the range of 0.10 to 0.61 tonnes per year per hectare of watershed. It is therefore normal and expected that a stream will carry some sediment on a regular basis. Because sediment transport is a natural process, it should not be disrupted without anticipating some consequences. So, when simulating the build-up and wash-off of sediment and first-order decay contaminants from a watershed, the general objective is to identify what combination of rainfall capture measures will maintain a natural level of annual suspended sediment loading”, explained Jim Dumont.
Implementation
In his closing remarks at the Partners Forum, Ted van der Gulik provided this perspective: “Drainage planning has been evolving over the decades. We used to refer to stormwater management because the objective was to manage a handful of extreme rainstorms. Now we use the term rainwater management because it is all-encompassing. Looking back only a few years, it is amazing how little we really understood about the rainfall-runoff process.”
“At this Partners Forum, you learned about the fundamental differences between rainfall-based thinking and runoff-based thinking, and what the implications mean for an environmental protection strategy. Because the runoff-based approach holds the key to assessing environmental impacts in watercourses and the effectiveness of mitigation techniques, integration of the Water Balance Model and QUALHYMO means local governments will now have a runoff-based tool for source control evaluation and stream health assessment.”
“While the beta-testing is underway over the next six months, the leadership of the Inter-Governmental Partnership urges the Water Balance Model Partners to identify exactly how they will implement Beyond the Guidebook in their municipalities.”
To Learn More:
To read the complete and comprehensive story about the 2007 Forum, click on Inter-Governmental Partnership Rolls Out ‘Beyond the Guidebook’ at Water Balance Model Forum
