Why do we study island “water budgets”?

Freshwater availability on an island is finite (and it is not from Mount Baker...). The big controls are island area (to capture rainfall; relief to trigger orographic rain; vegative cover to help capture and hold precipitation; and evaporation-transpiration) and “the subsurface” (rocks where freshwater storage takes place - aquifers, and where “groundwater” recharges). A water budget is basically a consideration of freshwater in (rainfall) vs freshwater out (via multiple natural and anthropogenic mechanisms), measuring/monitoring what is an appropriate "in" - "out" balance over time (think deposits cash, moving freely; bonds longer term; and withdrawals, but hopefully staying in credit!).

If the island’s freshwater catchment system was always “instantaneous” (it rains, it flows to lakes, the sea or sinks into the ground) and there were no subsurface “leaks” the natural components of a water budget for the island would be a simple “in” (rainfall) and “out” (flow to sea) calculation. Unfortunately, not all raindrops land and become instantly available (to a lake, to a creek at sea outfall, to a groundwater well, etc). The availability of that drop of water can be minutes to years. Therefore, flow to sea needs to be measured and groundwater systems do leak (as “baseflow”), so are partly “lost".

Determining what freshwater is available (the “IN”), and is not available (the “OUT”), is important for both for “areal water budgets” and for “groundwater recharge potential” modelling, and supports island groundwater /freshwater resource management for planning purposes (and protection and stewardship).

The FWC flow and chemistry information being gathered provides information on how much of the rainwater that the island receives is:

- "surface water flow” from simple stream run-off - leaving the island “directly" by surface water creek outflows to the sea

- "groundwater creek (in and out) flow" from shallow and deep aquifers, leaving the island “indirectly" via groundwater outflows into creeks and this water outflowing to the sea - a groundwater baseflow.

Where we have enough year-round data, our measured FWC water chemistry helps with determining the various flow component(s), and we are able to pin down what is “groundwater". For more on the "FWC Oakton chemistry reveals groundwater contributions” story-line have a look at the recent 2020 FWC summer project study by Simon Fraser University. Understanding groundwater components of our surface water systems is valuable information, for informing areal water budget/balance models, and for our understanding of groundwater recharge potential. For more reading on this surface-groundwater topic, please refer to the following reports:

• Salt Spring Island Groundwater Recharge Potential Report, 2019 GW Consultants, sponsored by the IT

• Salt Spring Aquifer Mapping and Monthly Groundwater Budget Analysis for Aquifers on Salt Spring Island, sponsored by FLNRO and the MoE

Our FWC data also gives insights into anthropogenic influences in our creeks/watersheds, and can be used in baselines (water quality and quantity) for watershed restoration projects (eg at Xwaaqw’um), and in climate change monitoring. More on these in a future background reading article.