WATER BALANCE PERFORMANCE TARGETS: “The flow-duration relationship is the cornerstone of British Columbia’s Water Balance Methodology. As understanding has grown, the methodology has evolved.” – from Water Balance Approach on Vancouver Island (released by the Partnership for Water Sustainability in January 2018)
Note to Reader:
In January 2018, the Partnership for Water Sustainability in British Columbia released the 7th in the Watershed Case Profile Series. The storyline for a Water Balance Approach on Vancouver Island is built around three regional Water Balance demonstration applications.
Storylines touch lightly on technical matters, yet are grounded in a technical foundation. The objective in ‘telling a story’ is to engage, inform and educate multiple audiences – whether elected, administrative, technical or stewardship. Stories in the series are presented in a magazine style to make it easier to read, comprehend and absorb technical information. Stories are designed to connect dots.
Water Balance Performance Targets
The centrepiece of Stormwater Planning: A Guidebook for British Columbia is the Water Balance Methodology (WBM). It is applied to set performance targets that would prevent flooding and protect stream health. The flow-duration relationship is the cornerstone of the WBM. As understanding has grown, the methodology has evolved (see chronology below).
Managing urban watersheds as an integrated Water Balance system – three flows, each with a different time scale – requires that performance targets be determined by means of verifiable calculations. Only then can mitigation measures be analyzed and optimized for cost and effectiveness.
Case Study Experience
Vancouver Island demonstration applications have shown how to downscale performance targets that, over time, would restore the natural flow-duration pattern in a stream situated within an urbanized or urbanizing area. Real-world success would be defined as reduced stream erosion during wet weather, and sustained ‘environmental flows’ during dry weather.
It has been a building blocks process to link rainfall, the landscape, streamflow, groundwater and sustainable service delivery. The timeline for this process is illustrated and synthesized below.
Verifiable Calculations
Quote extracted from Representative & Experimental Basins – What Next?. a paper published in the Hydrological Sciences Bulletin in 1976.
Flow-Duration Relationship
A flow-duration image is included below for illustrative purposes. It presents the total picture for Shelly Creek, which is the demonstration application for the Nanaimo region.
The results of flow-duration analyses for a range of four scenarios tell us that:
- The mean discharge for the natural hydrology condition of the Shelly Creek watershed is approximately 1 litre per second per hectare (Lps/ha); and this rate was exceeded for 6,900 hours during the 21 year period of analysis.
- The 2-year return period natural flood discharge of 7.9 Lps/ha was exceeded for just 10 hours during the 21 year period. Thus, this cannot be the rate of discharge causing stream erosion.
- Using 5.2 Lps/ha as an illustrative example, the graph shows that continued alteration of the watershed landscape due to land development would ultimately result in a possible 14-fold increase in the duration of erosion-causing discharge rates, that is: from 58 hours under natural conditions, to 312 hours for existing conditions, to 782 hours in future.
Land development changes the water balance proportions. Simply put, interflow is transformed into surface runoff. This has consequences for stream health.
The Water Balance Methodology addresses the alterations to the land surface and its land use while providing solutions that would maintain stream health within a developed watershed.
The Water Balance Methodology also recognizes the potential change in the paths followed by rainwater in the hydrologic cycle and establishes the methodologies required to protect stream and watershed.
To Learn More:
Download a copy of the Primer: https://waterbucket.ca/wp-content/uploads/2012/05/Primer-on-Water-Balance-Methodology-for-Protecting-Watershed-Health_February-2014.pdf
