Model determination of non-point source phosphorus transport pathways in a fertilized grassland catchment

Abstract

A modified version of TOPMODEL was developed and applied to a 14·26 ha grassland catchment in Ireland in order to infer the significant pathways of soil-to-stream phosphorus transport. The physically based hydrological model generated pathway-specific information for three components of discharge: overland flow, shallow subsurface flow, and groundwater discharge. Shallow subsurface flow in this model consisted of lateral, unsaturated discharge and relied upon the same hydraulic conductivity versus depth profile used by groundwater discharge, thereby limiting the number of additional parameters that were required. Model output compared favourably with both observed stream discharge and a soil moisture time series that was measured at a location within the catchment. By bringing together, on an independent basis, model results with in-stream measurements of total phosphorus (TP) concentration, we found that the fraction of modelled stream discharge deriving from overland flow and shallow subsurface flow was a reliable descriptor of the observed TP concentrations. The analysis revealed shallow subsurface flow to be the dominant P transport mechanism, primarily because volumetric contributions to stream discharge from this zone were much greater than those from overland flow. Since the source area associated with shallow subsurface flow covers the entirety of the catchment, in contrast to only the near-stream area for variable saturated area overland flow, a catchment-wide strategy for managing fertilizer and slurry inputs for this grassland setting may have a greater potential for effectively reducing P export than would the implementation of targeted strategies alone. Copyright © 2005 John Wiley & Sons, Ltd.