Water Stewardship Information Sources

Citation Sutherland, C and Yao, W. 2015. Maxwell Lake, Rippon Creek, and Larmour Creek watersheds 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 Maxwell 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). The Maxwell Lake watershed is located on the North and East faces of
Mount Maxwell. Water can be diverted from neighbouring Rippon and Larmour Creek watersheds to Maxwell
Lake by means of a weir structure and pipeline. The water licence allows diversion only during the winter
months (November 1st to March 31st).
The lake has been licenced for use as a water supply since 1914, first by the Ganges Power and Water Utility
and then after 1948 by NSSWD. A dam and spillway were constructed at the outlet of the lake in 1995 to raise
full supply level by approximately 1.0 m. In 2003, the spillway was raised by an additional 0.6 m. The total
licenced withdrawal limit for Maxwell Lake to the NSSWD water system is 663,729 m3
per year. NSWWD hold
water licences to store up to 1,077,642 m3
at Maxwell Lake. However, currently the total physical storage at the
lake is 819,000 m3
. The total volume of water that can be diverted from Rippon Creek and Larmour Creek from
November 1st to March 31st is 448,812 m
3
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 current water withdrawals and licenced water withdrawal limit) analyses were carried out in
order to assess the capacity of existing storage to support both current water withdrawals and to support
potential future water withdrawals from NSWWD.
The water balance was carried out using a hydrologic model based on the USGS Water Balance Model. 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 Maxwell Lake, Rippon Creek and Larmour Creek (the model output). The
model was used to assess available water supply to the lake under current climate conditions (1981 to 2010
climate normal period) and projected future climate conditions (2050s).
To confirm if the model accurately represents hydrological processes in the watershed, the model was run and
compared to an independent estimate of lake inflow known as back-calculated net inflow. Unlike watershed
runoff which is the total inflow to the lake, back-calculated net inflow is the estimated difference between
watershed runoff (including direct precipitation on the lake) minus evaporation from the lake surface and
seepage losses. Net inflow is the water that is available to either be withdrawn from the lake or to replenish lake
storage. In some summer months, net inflow is negative when evaporation from the lake surface is greater than
watershed runoff and direct rainfall on the lake.
The back-calculated net inflow is based on lake level records and estimated outflow from Maxwell Lake for the
period from 2010 to 2013. Outflow from the lake was estimated based on lake-level-versus-discharge
relationship developed for the weir and discharge measurements collected in 2014 and 2015. Net inflow was
adjusted to account for estimated diversion volume from Rippon Creek and Larmour Creek during the diversion
period.
The comparison between the water balance model results and the back-calculated net inflow indicate that the
model simulates the back-calculated net inflow, and thus the hydrological processes, at Maxwell Lake with a reasonable level of confidence. The percentage difference between the average total annual back-calculated
net inflow and the average total annual modelled net inflow is 8% and -5.1% for 2013 and 2014, respectively.
Information Type report
Regional Watershed Vancouver Island South
Sub-watershed if known Salt Spring Island
Aquifer #
Comments
Project status complete
Contact Name
Contact Email