- Delivering advice to farmers improves water quality in meso-scale river catchments.
- Detectable benefits for fine sediment and dissolved phosphorus take three years.
- Water quality responses to reduced pollutant loading are site-specific.
The object of the study was to evaluate the effectiveness of the Catchment Sensitive Farming (CSF) partnership, a voluntary pollution reduction scheme in England that between 2006 and 2018 provided advice to 19,776 farm holdings covering 34 % of the total farmed area in England.
Water quality data from 110 river monitoring sites in catchments ranging from 11 km2 to 2276 km2 was analysed using statistical regression models to compare pollutant concentration trends among sites that varied in the amount and timing of upstream pollution reduction activity.
Additional covariates were included to control for potentially confounding changes in pollution pressure caused by:
(i) background changes in cropping patterns and livestock numbers,
(ii) the uptake of land management measures delivered via other policy mechanisms, and
(iii) seasonal and weather-driven variation in water quality.
After delivering advice to farmers, there was a delay of at least two years before mitigation measures elicited a detectable improvement in suspended solids (SS) and orthophosphate (OP) at downstream monitoring sites.
Across all sites, the percentage reduction in SS concentration was 28 % of the modelled percentage reduction in fine sediment load. The strength of response was site-specific and changes in SS concentration varied from +12 % to -33 %.
In absolute terms, CSF was estimated to have reduced the mean SS concentration at sites in target catchments by an average of 4.4 %. Mean OP concentrations in target catchments decreased by an average of 13.0 % (95 % confidence interval 3.7–22.4%) but the overall strength of the water quality response to unit reductions in phosphorus loading was uncertain.
CSF measures reduced nitrogen loading by 5 % in target catchments, but there was no evidence that mean concentrations of total oxidised nitrogen (TON) decreased, probably due to the complex and site-specific time lags involved.
The combination of pollution load modelling and statistical water quality modelling used in the study provides a framework for testing and quantifying the effectiveness of land management interventions, individually and in combination, but also highlights the importance of establishing a clear counterfactual and accounting for spatially-variable, lagged responses.
Source – Science Direct