Scale matters: what Ousewem's whole-system modelling tells us

Ousewem’s whole-system modelling now allows us to explore a critical question:

What scale of action is needed to influence downstream flood risk and water quality across an entire catchment?

The answer is clear. To influence flood risk and water quality at York, nature-based solutions need to be delivered at significant scale and in the right locations across the upstream catchment.

A catchment-scale challenge

The SUNO catchment is vast, hydrologically and geologically complex. Water reaching York has flowed through multiple rivers, tributaries and landscapes meaning that the downstream outcomes depend on the cumulative effect of interventions across the catchment.

Click to see an enlarged bar chart showing average peak flow reduction at the Skelton flow gauge (opens in new window), upstream of York, for 8 different types of nature-based solutions.

What does 'enough' look like?

The modelling shows a clear relationship between scale and impact.

Measures including:

• woodland creation
• soil and land management
• floodplain reconnection
• storage features
• other nature-based approaches

These measures can reduce flood risk and improve water quality, but the modelling indicates that downstream change requires them to be implemented extensively and strategically.

The modelling suggests that increasing woodland cover by more than 20% beyond current levels could be required to achieve approximately a 10% reduction in peak flow at York, while also contributing to reductions in diffuse agricultural pollutants.

Click to view an enlarged map showing the extent of woodland coverage needed in the SUNO catchment (opens in new window) that might achieve a 10% reduction in peak flow at Skelton, upstream of York (opens in new window).

That is an ambitious target.

More importantly, it illustrates the scale of coordination, investment and long-term partnership needed to move from individual projects towards catchment-wide outcomes.

Why this matters

Local modelling helps us understand how interventions perform in specific places, as our work at the Hauxwell Estate has shown.

Whole-system modelling asks a different question: "What happens when interventions work together across an entire catchment?"

More than flood resilience

Flood resilience is a primary driver for Ousewem, but the modelling also highlights the wider benefits of delivering nature-based solutions at scale.

The same interventions that help slow and store water can also contribute towards:

• improved water quality
• reduced sediment and nutrient loading
• biodiversity recovery
• soil quality improvements
• habitat connectivity
• carbon storage
• more resilient land management

This matters because future investment and policy decisions are unlikely to be driven by a single outcome.

Catchment-scale resilience depends on understanding how multiple benefits can be delivered through the same interventions.

Click to view an enlarged bar charts showing percentage reduction to concentrations at the model outlet in York (opens in new window) for 8 different types of nature-based solutions.

Modelling is not the whole story

Like any model, a whole-system model simplifies reality. It helps us understand scale, relationships and opportunities across a catchment. It does not prescribe what should happen in every location.

Ousewem therefore, combines strategic modelling with local evidence, delivery and monitoring. Its value is not simply in estimating reductions in peak flow or pollutant concentrations. It helps decision makers understand the scale of action, coordination and investment required across the catchment.

What this means for future decisions

The value of the modelling is the evidence it provides for future prioritisation, planning, funding and delivery.

As Ousewem approaches the final year of its current funding, this evidence will help inform longer-term approaches to prioritisation, funding and delivery.

The challenge is no longer simply demonstrating that nature-based solutions can contribute to flood resilience and environmental improvement. It is understanding how evidence, partnerships and investment can work together to deliver those benefits at the scale required.

Local evidence helps us understand what works. Whole-system modelling helps us understand what it takes to make a difference across an entire catchment.

Scale matters.