Stationarity Is Dead: Why PNW Stormwater Standards Are Changing

For decades, stormwater design in the Pacific Northwest has relied on a fundamental assumption: that historical precipitation patterns are sufficient predictors of future conditions. This assumption—known as stationarity—is now dead, and regulatory frameworks are shifting accordingly.

The End of Stationarity

The concept of stationarity in hydrology assumes that natural systems fluctuate within an unchanging envelope of variability. When it comes to stormwater design, this meant using historical rainfall records to size infrastructure. But climate science has made it clear: the envelope is changing.

In 2008, researchers published a landmark paper declaring "Stationarity is Dead" in the context of water management. The Pacific Northwest, with its reliance on both summer drought patterns and intense winter precipitation, is experiencing these shifts in real-time.

What's Changing in PNW Regulations

Washington and Oregon municipalities are increasingly adopting climate-adjusted design standards:

Intensity-Duration-Frequency (IDF) Updates

Traditional IDF curves based on 50-year historical records are being replaced with climate-adjusted projections. This means:

• Increased design storm volumes for 24-hour events
• Modified recurrence intervals accounting for projected increases in extreme precipitation
• Combined sewer overflow (CSO) system pressure from more frequent high-intensity events

Low Impact Development (LID) Emphasis

Regulations are pushing harder on LID requirements as a resilience strategy. Permeable surfaces, bioretention, and distributed infiltration are becoming mandatory, not optional, because they provide buffer capacity against uncertain future precipitation patterns.

"The assumption of stationarity is no longer viable for long-term infrastructure planning. We must design for a range of possible futures, not a single historical past."
— PNW Municipal Stormwater Manager

Practical Implications for Developers

If you're in the design phase of a project, consider:

1. Check your jurisdiction's latest stormwater manual: Many are in the process of updating IDF curves. Using outdated standards risks non-compliance if your permit review extends over manual revision dates.

2. Factor in longer review timelines: Municipalities are struggling with how to apply new standards to projects already in the pipeline. Expect questions about climate-adjusted design even if your original submittal used historical standards.

3. Budget for infrastructure upsizing: Climate-adjusted standards typically require larger detention facilities, increased infiltration capacity, or both.

4. Document design decisions: If you're using historical standards based on submittal timing, clearly document this. Administrative records that don't explain design basis may create vulnerability during appeal or re-review.

The Regulatory Gap

One of the biggest risks right now is the gap between what some municipalities are requiring and what others haven't yet adopted. A project in jurisdiction A may face stricter requirements than jurisdiction B, even for similar site conditions. This creates:

• Uncertainty in early project scoping
• Potential for appeal on disparate treatment grounds
• Need for early engagement with regulatory staff

Looking Ahead

The shift away from stationarity isn't a one-time update. It's an ongoing process of recalibration as climate projections refine and observational data accumulates. Projects designed today may face re-evaluation of their stormwater systems as standards continue to evolve.

The key is building administrative records that demonstrate thoughtful consideration of climate-adjusted design, even if you're using current (not yet updated) standards. This protects your project timeline if regulations change during permit review.

This is part of our ongoing series on regulatory risk for PNW development. For project-specific analysis, contact us through our project intake form.