WATER SUPPLY UPDATE — SOUTH SASKATCHEWAN SUB-BASIN
Date: March 18, 2025
Prepared by: SEAWA – Watersheds & Knowledge
SUMMARY
Early-spring conditions remain drier than average despite two late-season snow events. River flows have stabilized week-over-week but remain below the 25th percentile for this date. Reservoir storage is adequate for municipal demand; agricultural allocations will depend on April precipitation and melt timing.
KEY POINTS (HIGHLIGHTS)
• Basin precipitation (Water Year to Date, Oct 1–Mar 17): ~78% of normal.
• Snow Water Equivalent (SWE): 86% of median across eastern foothills; 72% on prairie sites.
• Mainstem discharge (Bow–Oldman–South Sask at Medicine Hat): 95–110 m³/s (≈ 22% below median).
• Reservoirs (combined major storage): ~64% full vs 68% 10-yr average.
• Groundwater spot checks: stable to slightly declining (−0.05 to −0.12 m since February).
• Drought code (provincial index proxy): moving from “Moderate” toward “High” risk if April remains dry.
CONTEXT & INTERPRETATION
Two modest snowfalls (Mar 4–6 and Mar 12–13) boosted SWE at higher elevations, but warm, windy periods accelerated sublimation and crusting on prairie snowpack, limiting runoff potential. Without at least 20–30 mm of basin-wide precipitation before mid-April, streams will likely peak earlier and lower than normal, increasing reliance on carry-over storage. Municipal systems report no near-term constraints; irrigation districts are modeling tiered start-up plans contingent on April inflows.
WHAT THIS MEANS
• Municipal: No restrictions anticipated short term; continue conservation messaging as a precaution.
• Agriculture: Expect conservative early allocations; on-farm storage and scheduling will be important if April remains dry.
• Ecosystems: Lower spring baseflows may limit fish passage in small tributaries; consider targeted monitoring of temperature and dissolved oxygen during low-flow windows.
METHODS (ABRIDGED)
• Streamflow: Provincial hydrometric network daily means, 7-day smoothing; percentile vs 1981–2010 baseline.
• SWE: Median of selected foothills stations; prairie sites via snow course and modeled raster.
• Storage: Operator weekly bulletins; values normalized to full supply level.
• Groundwater: Observation wells (unconfined) weekly manual checks, corrected to local datum.
UNCERTAINTIES
• Melt rate and rain-on-snow events could change runoff efficiency rapidly.
• Model spread remains high for late-March precipitation tracks.
RECOMMENDED ACTIONS (NEXT 2–4 WEEKS)
Maintain weekly reservoir coordination calls; prepare public explainer on spring runoff vs summer demand.
Activate enhanced tributary monitoring at three low-flow pinch points (site IDs: T-07, T-12, T-19).
Draft irrigation start-up advisory decision tree for release by April 5, pending new forecasts.