Water diversion license limits for St. Mary Lake
DONALD O. HODGINS, Ph.D., P.Eng.Executive Summary:
The Province issues licenses for surface water withdrawal for a range of purposes. Each license specifies the maximum amount that can be withdrawn over a defined period of time, usually one day or one year.
For St. Mary Lake, all licenses total 573 dam3 for the critical June-October summer season1. However, foran extreme drought the available safe yield during summer is about 350 to 380 dam3, substantially less than allowed by license.
In view of such a large difference, the purpose of this investigation was to uncover the guiding principle used by the Province in allocating a water resource, and to confirm the basis for the numbers applicable to St. Mary Lake as of 2015.
What I have concluded is this: the basic design principle is to allocate water licenses up to a limit that is available for climate average hydrological conditions, taking critical environmental flow thresholds into account. The Province recognizes that the full allocation will not be met during droughts, and has implemented a complex procedure for restricting or denying withdrawals2. These include rights by precedence based on when the license was issued.
The design principle is readily confirmed for Salt Spring Island, and for St. Mary Lake, as shown in this note. The weaknesses are obvious: there are dry winters when the runoff is inadequate to replenish the fully allocated storage, and there are summer droughts that result in significant shortages of water to meet the licensed withdrawal total. Periods when restrictions come into force are inevitable.
Current demand appears to be well below the licensed total for St. Mary Lake. NSSWD and CRD both withdraw less than their licenses (approximately 460 dam3 annually), which together account for 90% of the allocated total (1593 dam3). However, summer demand (June-October) is roughly 340 dam3 without water restrictions, leaving a reserve of about 12% of the safe yield3
