Critical source area controls on nutrient and sediment transfers in agricultural catchments

Abstract

Sediment and nutrient transfers from agricultural catchments have been linked to water quality pressures globally. Particularly in Northern Ireland (NI), where in 2023, the Lough Neagh algal bloom highlighted the impacts of agricultural phosphorus (P) and sediment pollution. This thesis aimed to develop a scientific evidence base for including critical source area (CSA)science in future agri-environment schemes (AES), to mitigate P and sediment transfers to surface waters. To achieve this aim, fine-scale, geo-spatial methods were used across four catchments in NI to quantify the impact of CSAs and spatially targeted mitigation measures. Terrestrial laser scanning surveys determined annual streambank export coefficients of 0.19 – 0.21 t m-1 sediment and 0.065 – 0.087 kgm-1total phosphorus from unfenced cattle access points to watercourses. These coefficients were scaled to quantify potential benefits of fencing installed through a national, NI AES and were estimated at up to 10 kt yr-1 and 4 t yr-1, respectively. Catchment carrying capacities for above agronomic optimum soil test P (STP) and hydrologically sensitive areas (HSAs) were investigated for instream soluble reactive P and a range of 15 – 44% (proportion catchment area above optimum STP) was determined. When coincident with above optimum STP, all HSAs posed a water quality risk (carrying capacity < 1%). Measured soil moisture data were compared to modelled soil topographic index values using interpolation to validate the delineation of HSAs (at2 m resolution). The sample size of 100 points (34 points ha-1) was insufficient in areas dominated by microtopography. Resampling determined that at least 1,832 points (626points ha-1) would be required at this scale. The findings of this research enforce the need for spatially targeted mitigation measures to improve the efficacy of AES. This work is of importance to all stakeholders from policy makers and catchment managers to farm advisors and landowners.

Thesis embargoed until 31st October 2026

Date of Award	Oct 2024
Original language	English
Supervisor	Paul McKenzie (Supervisor), Phil Jordan (Supervisor) & Joerg Arnscheidt (Supervisor)