Source to Stream : technical summary

The Source to Stream tool is a high‑resolution, map‑based application developed to support understanding of how pollutants originate within catchments, move across the landscape and potentially impact river-water quality. It integrates national datasets on terrain, land cover, soils, climate, agriculture and infrastructure to visualise the spatial distribution of key pollutants, sediment, sediment‑bound phosphorus, nitrogen and road runoff, alongside the pathways by which they may be transported to rivers.
The tool applies established, empirical modelling approaches to estimate long‑term average pollutant losses:
• sediment loss is calculated using the widely adopted ‘Revised Universal Soil Loss Equation’ (RUSLE)
• phosphorus loss is derived from sediment estimates combined with soil geochemistry
• nitrogen loss is estimated from atmospheric deposition and inorganic fertiliser inputs, with a representative proportion assumed to be lost via surface runoff
Road runoff pollution is characterised using an integrated pollution classification, which combines multiple pollutants into a single ranking to identify road segments with relatively higher pollution potential.
Sediment, sediment‑bound phosphorus and nitrogen pollutant sources are routed across the landscape using a digital terrain model to represent surface flow pathways, enabling the tool to highlight both where pollution is generated and how it may reach rivers. Indicative pollutant concentrations in rivers are also visualised by combining modelled pollutant loads with estimates of surface runoff, providing a proxy representation of potential in‑stream impacts.
The Source to Stream tool is designed as a screening and exploratory platform. It supports interpretation of spatial patterns, identification of pollution hotspots and evidence‑based discussion of catchment pressures and potential interventions. However, it relies on simplified assumptions, including:
• the use of long‑term average conditions
• representation of only surface transport pathways
• the assumption that all generated pollutants are delivered to rivers
As a result, outputs represent relative patterns and potential risks rather than absolute or observed values. They are not intended for regulatory, compliance or site‑specific decision making.
Overall, the tool provides a coherent and accessible framework for integrating multiple sources of diffuse pollution into catchment‑scale analysis, supporting improved understanding of environmental processes and helping to inform prioritisation of further investigation and management actions.