Water Stewardship Information Sources

Citation Sutherland, C and Yao, W. 2015. St Mary Lake watershed water availability and demand - climate change assessment. Prepared for North Salt Spring Waterworks.
Organization North Salt Spring Waterworks
URL http://www.northsaltspringwaterworks.ca/wordpress_water/wp-content/uploads/2015/06/St.-Mary-Lake-Hydrology-Study-Final-2015.pdf
Abstract/Description or Keywords St. Mary Lake is one of the water sources that the North Salt Spring Waterworks District (NSSWD) relies on to
supply about 5,500 customers on Salt Spring Island: more than half the population of the year round residents
on the Island (based on 2013 values). Since 2006, a low weir at the outlet of St. Mary Lake, owned and
operated by NSSWD, has controlled storage at St. Mary Lake to support summer water use and conservation
flows downstream in Duck Creek.
At 1.823 km² in surface area, St. Mary Lake is the largest lake on Salt Spring Island. The watershed upstream
of the outlet of the lake has an area of 6.47 km² and is primarily covered in second growth forest, rural
residential development and agricultural fields. The watershed is underlain by complex geology typical of Salt
Spring Island, including sandstone, shale, siltstone and conglomerates as part of the sedimentary rocks of upper
Cretaceous age belonging to the Nanaimo Group. The surficial soils of the watershed consist primarily of thick,
unconsolidated marine or fluvial deposits over compact unweathered till from the last glaciation.
In addition to licenced water withdrawals by NSSWD for water supply purposes, the lake also supports two other
small waterworks districts managed by the Capital Regional District as well as several smaller private residential
and agricultural water licences. At 90%, the NSWWD and CRD waterworks withdrawals make up the largest
percentage of the total licenced withdrawal limit at St. Mary Lake with the remaining water licences issued for
agricultural, industrial and individual domestic water uses. NSSWD holds four water licences which permit up to
264,507,500 imperial gallons or 1,202,000 m³ of water to be withdrawn from St. Mary Lake on an annual basis.
The total licenced withdrawal limit for all water licence holders on the lake is 1,593,000 m³. No water demand
forecasting based on population or land use was completed as part of this study as the water licence dictates
the maximum amount of water that could be withdrawn in the future.
In addition to water licence withdrawals, a minimum downstream environmental flow of 9 L/s is required to be
released to Duck Creek for conservation purposes. This is equal to an annual volume of up to 285,000 m³.
Therefore, the maximum combined water withdrawal and conservation flow that St. Mary Lake and the
watershed would have to support is equal to 1,878,000 m³ per year.
WATER BALANCE MODEL APPLICATION
Water balance (estimation of available water supply to the lake) and water budget (comparison of available
water supply with the licenced water withdrawal limit and the required minimum downstream environmental flow)
analyses were carried out in order to assess the capacity of existing storage to support licenced water
withdrawals for NSWWD and other water licence holders around the lake.
Often water balance assessments rely on hydrometric records to provide an indication of water availability in a
watershed. For the St. Mary Lake watershed, lake levels have been recorded since 1981 while discharge data
was recorded in Duck Creek downstream of the lake from 1980 to 1998. Unfortunately, the historical discharge
in Duck Creek is of relatively limited value for the water balance/budget given that most of the years of record
only include discharges for the spring/summer period. The few years in which the station was operated year
round have significant gaps in the daily flow record such that there is not a single year of continuous daily
discharge data.
Previous hydrological assessments carried out for St. Mary Lake relied on a relationship developed between
discharges recorded in Duck Creek and lake levels to extend the discharge record sufficiently to provide a reasonable period of record for analysis. However, the relationship between lake level and discharge in Duck
Creek has been modified by construction of the weir at the outlet of the lake in 2006.
Given the limitations of the historical water data for assessing water availability in St. Mary Lake, the water
balance was carried out using a hydrological model developed based on the USGS Water Balance Model
protocol. This model accounts for movement of water in a watershed through a series of theoretical storage
volumes representing important physical hydrological processes such as evapotranspiration, lake evaporation,
infiltration, soil moisture storage, groundwater storage and deep aquifer loss. The model converts inputs of
precipitation and temperature to net inflow into St. Mary Lake (the model output). The modelled net inflows
were used to assess available water supply to the lake under current climate conditions (1981 to 2010 climate
normal period) and predicted future climate conditions (2050s).
Information Type report
Regional Watershed Vancouver Island South
Sub-watershed if known Salt Spring Island
Aquifer #
Comments
Project status complete
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