Water Balance Methodology: "Watershed objectives start with the stream and end with the stream," says Jim Dumont, the Partnership’s Engineering Applications Authority

Note to Reader:

The “Beyond the Guidebook Primer Series” supports implementation of targets and actions listed in Living Water Smart: British Columbia’s Water Plan. The targets and actions establish expectations as to how land will be (re)developed so that stream and watershed health are protected and/or restored.  

Primer-on-Water-Balance-Methodology_Feb-2014_cover_200pThe Primer on Water Balance Methodology for Protecting Watershed Health is the fifth in a series of guidance documents that form the basis for knowledge-transfer via the Georgia Basin Inter-Regional Education Initiative (IREI). The foundation document for the series is Stormwater Planning: A Guidebook for British Columbia, released in 2002.

In the article below, Jim Dumont provides context for the Water Balance Methodology to explain its relevance and value. He is the Engineering Applications Authority for the Partnership for Water Sustainability in BC, the agency of record for the Primer Series.

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Water Balance Methodology Explained

Jim-Dumont1_June2015DSC_05358_120p“Protection of streams and fish has become an important public expectation which created a driving force altering our perceptions, aspirations, and treatment of the urban landscape. Ultimately this translates into a number of governing watershed objectives that may initially seem to be in conflict but upon further reflection they will be found to be in harmony,” states Jim Dumont.

Watershed Objectives

“The objectives of the Water Balance Methodology starts with the stream and ends with the stream which provides a true measure of success for environmental protection. Water Balance Methodology objectives include: Mimic-Water-Balance_Feb-2014

  • Maintain stream flows,
  • Maintain stream flows,
  • Improve water quality,
  • Provide flood Protection,
  • Prevent unforeseen impacts,
  • Prevent stream erosion, and
  • Replace the lost interflow system.”

“The Water Balance Methodology is based upon watershed and stream function and operation. Understanding how precipitation makes its way to the stream allow us to assess how a watershed and stream operate while analytically demonstrating impacts of development and the effectiveness of any mitigation works. The watershed function and the flow pathways that water takes has been documented in numerous publications and the time scales associated with each is readily describable.”

On the west coast, explains Jim Dumont, the Water Balance of many watersheds can be described as summarized in the table below:

How & How Long

West Coast Water Balance

How Water Gets to a Stream, and How Long It Takes

“The flow path of surface runoff sees precipitation entering the stream within minutes or hours with the discharges from saturated aquifers occurring with a very steady rate with the time from surface to stream being a duration that can be spread over years or decades,” continues Jim Dumont. “The often ignored interflow system is the flow of water in a very shallow unsaturated surface soil that will occur over a season to a year.”

“The importance of interflow has been highlighted in the new regulations established by the California State Water Resources Board. Closer to home we can find evidence of interflow as shown in the photo of a gravel pathway (below). There is a surface slope from right to left and the water ponding on the gravel pathway is not the remnant of rainfall; rather, it is from interflow surfacing on the right, flowing across the pathway and then re-entering the interflow system on the left.”

“This is a graphic example of the shallow nature of the interflow system and how easily it can be disrupted. Now imagine the effect of urban development upon a watershed’s natural interflow system.”


Rainwater Management

“The objective of Rainwater Management is to mimic the natural Water Balance and flow pathways naturally occurring in a watershed,” emphasizes Jim Dumont. “Consideration is given to water that was flowing by surface runoff, through the interflow system during a season and that which entered the saturated aquifers to discharge at some future date. This approach provides a level of assurance that:

  • Excess water will not be directed to the ground and would avoid potentially adverse impacts of excessive groundwater levels and discharges in areas lower in the watershed.
  • Summer flows will be maintained with an operating interflow system.
  • Downstream properties will not suffer an increased risk of flooding or flood damages.”

“The Methodology established watershed targets with verifiable calculations and the mitigation systems are optimized for bothcost and function. The analysis begins with calibration of continuous simulation models using long term climate records, not just selected storms or typical years. The effects of urban development are then estimated and the required mitigation measures are sized. This can be seen in the chart (below) which shows the flood frequency for one watershed and stream.”

“The results also yield estimates of stream flow durations (as shown in the second chart below) where the system has been optimized.”

The rain garden detail (below) illustrates application of the three performance targets for design – the source of the illustrated detail is the Metro Vancouver Source Control Guidelines.

The chart shows flood frequency for one watershed and stream

The chart shows flood frequency for one watershed and stream

The chart shows stream flow durations

The chart shows stream flow durations

What this Means

“The analytical approach used in the Water Balance methodology is verifiable, and allow the mitigation works to be optimized for size and cost while achieving the watershed objectives. The Water Balance Methodology is superior to approaches that rely upon simplistic capture targets unsupported by actual calculations to demonstrate effectiveness,” concludes Jim Dumont.

To Learn More:

Download Primer on Water Balance Methodology for Protecting Watershed Health. The Primer storyline is structured in five parts:

  • Part A: Watershed-Based Approach to Rainwater Management
  • Part B: Water Balance Methodology Explained
  • Part C: Science Behind the Methodology
  • Part D: How to Establish Targets
  • Part E: References

For a synopsis of each part, click on the following link to Table 1. Information is presented in a layered fashion to accommodate the interests of a continuum of audiences.

How Performance Targets for Storage, Infiltration and Flow Release are incorporated in a Rain Garden Design (image source: “Stormwater Source Control Design Guidelines 2012 , Metro Vancouver, British Columbia)

How Performance Targets for Storage, Infiltration and Flow Release are incorporated in a Rain Garden Design (image source: “Stormwater Source Control Design Guidelines 2012 , Metro Vancouver, British Columbia)