Seeing the Whole Watershed: Probabilistic vs Deterministic Hydrology

by Sara Zieleman, Executive Director and Drew Brayshaw, Senior Hydrologist and Geoscientist at Statlu Environmental Consulting

The topic of probabilistic watershed assessments has seen growing interest in our community following a number of events featuring Dr Younes Ailila.  Dr Alila is a UBC professor whose research has significantly advanced understanding of applying probabilistic frameworks to hydrologic processes.  This topic has been discussed amongst our Directors, Community Advisory Panel Members, staff and professionals for some time and we want to share our considerations and current direction on hydrologic assessments. 

We routinely obtain watershed assessments to inform forestry planning.  The SCCF is committed to using the best available science to inform decision making, and yet a wholesale shift away from current methods used in hydrologic practice cannot be undertaken, and perhaps shouldn’t be. These are some of the reasons why.

Policy and Regulatory Requirements

Current regulatory frameworks in BC, including the Forest and Range Practices Act (FRPA) and Water Sustainability Act (WSA), require operationally defensible assessments that can be clearly interpreted and enforced. Deterministic methods provide transparent cause-and-effect linkages that regulators, practitioners, and communities can follow. Probabilistic outputs, though valuable for academic and exploratory work, are often less intuitive for decision-makers and are insufficient for compliance-based systems. 

Operational Scale and Data Limitations

Probabilistic hydrology demands robust, long-term datasets over large areas. On the Sunshine Coast and within the Community Forest, the data are not consistently available to support probabilistic modelling. Additionally, the community forest has many small catchments which are poorly represented in probabilistic frameworks. Deterministic methods—calibrated with local monitoring, expert judgment, and field evidence—remain the most reliable approach at forestry planning scales.

Geomorphic Science and Ground-Truthing

Deterministic assessments are not limited to peak flow calculations. They also integrate geomorphic science by examining how watersheds respond physically through sediment supply and movement, riparian forest cover, in-channel wood, and channel pattern. Deterministic approaches also rely on ground-truthing: when models suggest that flood frequency or magnitude is changing, practitioners look for evidence in the stream itself. If channels, sediments, or riparian areas are not showing corresponding changes, it prompts further questions—whether risks are offset, conditions are more resilient than expected, or the model is not accurately representing local processes. This field-anchored perspective ensures that assessments reflect actual watershed behaviour, not just theoretical predictions.  Findings from field verification can be significant, such as learning that a stream was completely cleared of

 wood for a flume in the 19^th century, or that recent beaver activity diverted a stream and effectively quadrupled the area of a neighbouring watershed.

Climate Change

Climate change is altering hydrology in predictable but locally variable ways. Deterministic/geomorphic methods already incorporate these signals by tracking how watershed processes are shifting over time and by adapting management prescriptions where needed. The additional context provided by geomorphic science and ground truthing allows for interpretation of whether observed changes reflect natural variability, legacy effects, or the influence of climate change.

Risk Communication and Accountability

While probabilistic methods are powerful for expressing uncertainty, they can complicate communication and enforcement where clear commitments on thresholds are required (e.g., peak flow increase limits, water quality standards). Deterministic frameworks provide concrete benchmarks that facilitate accountability. A complete shift to probabilistic approaches risks undermining trust by introducing conditional results which cannot easily be verified or enforced.

The Sunshine Coast Community Forest’s approach is to advance integration, not replacement. Deterministic and geomorphic methods provide the operational baseline for regulatory compliance and on-the-ground decision-making, while probabilistic tools can augment understanding at regional or strategic planning levels where uncertainty analysis and risk trade-offs are most useful. This blended approach allows us to benefit from cutting-edge research, contribute to the evolving field of probabilistic hydrology, while maintaining the use of deterministic methods that align with existing regulation and compliance requirements. Dr. Alila’s work makes a valuable contribution by challenging the status quo and highlighting the role of uncertainty in hydrologic science, but it is not yet a substitute for the comprehensive, field-based deterministic approach currently required in practice and may never be.  It could be something completely different, and we look forward to contributing to this science reaching its full potential to benefit community and forest resilience here on the Sunshine Coast.

Photo Credit: Drew Brayshaw, taken during field work for an optional watershed assessment. See all our watershed research and reports at www.sccf.ca/hydrology