The dynamics of hydrological connectivity, phosphorus and sediment delivery - a field and modelling approach

• Main Author: Thompson, J.
• Published: Queen's University Belfast 2013
• Subjects: 363.7363
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.602940

This thesis tests the critical source area (CSA) hypothesis that heterogeneities in hydrological connectivity and pollutant mobilisation result in limited areas of a catchment which export disproportionate pollutant loads. This hypothesis was tested at the field and catchment scale over a 3 year period at a field site in Hillsborough, Co. Down, and 3 catchments in Co. Down and Co. Louth. At the field scale, this thesis demonstrated that the CSA hypothesis may be suitable to predict loads for certain fractions of P, yet may not be suitable to predict the loads of nitrogen and particulate P. Research at the catchment scale used sediment tracing of sediment sources to validate CSA modelling. Results indicated that no variation in median land use weightings was needed to predict CSAs at the larger catchment scale, however, for the two smaller study catchments, variation in median risk weightings was considerable which may indicate the importance of non-linear connectivity processes at this scale. In all instances, arable land consistently generated the highest risk of sediment loss across all catchments and sampling times. The application of continuous monitoring using turbidity sensors to estimate suspended sediment fluxes was also used to understand the ecological significance of suspended sediment concentrations (SSC). Results indicated that frequency of exceedance above European guidelines were equivalent to 4.1 % and 17.8% of the monitoring period in the Down Catchment and Louth Lower Catchment, respectively. Analysis of the frequency and duration of each event exceeding the FFD guideline value revealed that in both catchments periods of exceedance were skewed towards events of short duration, typically below 5 hours. Reviews of the literature documenting the response of aquatic biota to SSC revealed that the majority of these studies had dosage values far exceeding those reported within the two catchments reported here. Data presented in this thesis furthers understanding of CSAs in agricultural catchments and highlights existing research that is needed. This understanding is essential, particularly in light of current environmental legislation from Europe that necessitates improvements in water and biological quality