HOW WATER REACHES A STREAM: “The ‘Water Balance’ – what do urban drainage practitioners mean, really, when they use that phrase,” asks Jim Dumont rhetorically

Note to Reader:

Jim Dumont is the Engineering Applications Authority, Partnership for Water Sustainability in British Columbia. He is co-author of the Urban Hydrology chapter in the 50th anniversary update of Dr. Ven Te Chow’s Handbook of Applied Hydrology, Second Edition. This covers scientific and engineering fundamentals and presents all-new methods, processes, and technologies. 

He is on the Advisory Committee for the US-based Center for Infrastructure Modelling and Management, Under an agreement with the US Environmental Protection Agency, has been created to provide sustainable research, development and outreach for water infrastructure modeling, initially focusing on two foremost modelling tools – known around the world by the acronyms EPA SWMM and EPANET.

Over the past four decades Jim Dumont has undertaken watershed planning and stream assessment which has led to the design and implementation of terrestrial mitigation and aquatic enhancement systems. This knowledge complements the extensive urban infrastructure planning and design required to provide for the population growth of communities.

Jim Dumont promotes transforming the State of the Art into the Standard of Practice to provide added value and sustainability in our everyday lives. For more than a decade, he has taught courses on rainfall-runoff modelling and drainage facility design for professional development programs in British Columbia and across Canada. Jim Dumont is also on the expert faculty assembled by Forester University to provide online professional development.

In the article below, Jim Dumont provides a perspective on what he commonly sees in practice when drainage practitioners use the term ‘water balance’.

One Term, Four Different Meanings

The ‘Water Balance is a term that has been widely adopted by many; however, there are also many different meanings and methods for its application. In this article, I describe four different approaches to a so-called ‘water balance approach’, and these are:

  • The Silver Bullet Approach
  • A Little Bit of Science
  • British Columbia Water Balance Methodology
  • Paralysis By Analysis

For each approach, I provide a very simple introduction so that the reader will have a sense of what each approach involves.

The Silver Bullet Approach

The simplest approach, and the one that has widespread adoption, is to compare the surface runoff from natural conditions with runoff urban conditions. This is overly simplistic as it ignores many of the hydrologic processes within a watershed. This approach is used at a site level and does not include evaluation of impacts to the streams in the natural environment.

I refer to this as the ‘Silver Bullet’ approach because so many people want to believe that the watershed processes are this simple even though we know they are not.

A Little Bit of Science

Another approach that one sees is based on applying just a little bit of science. This one is truly dangerous because of the over simplification of the approach. This approach has been widely adopted, by the United States Environmental Protection Agency (EPA), the Province of Ontario, and several other jurisdictions. It is used to compare natural and urban watershed using the equation P = R + I + ET + S, where

  • R is surface runoff
  • P is precipitation,
  • I is infiltration into the ground,
  • ET is evapotranspiration, and
  • S is the change in groundwater volume

This approach is overly simplistic as most agencies assume that changes in the S value will be zero. This results in a reversion to the very simplistic comparison of surface runoff under different watershed conditions. While this approach has been used for watersheds, it is typically applied at a site level. Due to the limited information that is utilized in the analysis, it does not include evaluation of impacts to the streams in the natural environment.

British Columbia Water Balance Methodology

In 2002, the government of British Columbia released Stormwater Planning: A Guidebook for British Columbia. This established a new direction for urban hydrology and drainage engineering. Introduction of the Water Balance Methodology has enabled the setting of performance targets for rainfall capture, runoff control and groundwater recharge.

The approach we have been adopting here in British Columbia adds additional detail to describe the subsurface flows within a watershed as they are important to the health of a stream. We find that this is the simplest view of the processes that can be used to describe how the streams are impacted. We include the two components of soil water; the first is the groundwater in the aquifers, and the second is the shallow surface soil reservoir (interflow system) that is easily damaged and seldom considered in mitigation strategies.

Protection of watershed and stream health in the urban environment ultimately depends on maintaining the natural proportion of rainwater entering streams via three pathways: overland runoff, shallow interflow and deep groundwater flow. Due to the nature of the analyses the duration of flow and magnitude of floods are basic to the results and recommendations for mitigation of development impacts. The innovation of the Water Balance Methodology is in the way it integrates and applies standard scientific and engineering principles which are not typically used in typical engineering design of municipal infrastructure.

We make the evaluation of flow duration and flood discharges in the watershed and stream an integral part of the analysis in establishing the mitigation strategies that will be recommended. This approach has been validated as it has been mandated for urban stormwater plans in both California and Washington.

To Learn More:

 Click on Water Balance Methodology – Using Continuous Simulation to Protect Urban Watersheds and Stream Health to watch the Forester University on-demand webcast of a lecture by Jim Dumont.

Download the Primer on Water Balance Methodology for Protecting Watershed Health, 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.

Paralysis by Analysis

There is a fourth and very complex approach that would involve a detailed analysis of the flow patterns beneath the ground. There are some in regulatory positions who will always seek the complete answer to a problem before moving to a solution and will not proceed with any action to mitigate urban impacts until all of the studies and designs are complete. The cost of obtaining that type of information, and then carrying out an assessment, is often used as an excuse to do nothing.

Given what we do know, we would caution against use of this approach. The reason? The return on investment would not justify the high cost of investigations necessary to obtain the very detailed information needed to accurately describe the sub-surface conditions for very complex computer models.

At some point in the future, and for some watersheds, there may be a need to go to this level of detail. For the foreseeable future, however, we believe urban hydrologists are able to obtain a reasonable understanding of watershed operation by using a slightly simpler set of information and analysis techniques – such as the pragmatic approach that is demonstrated by the British Columbia Water Balance Methodology.