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TECHNICAL REPORT
Landscape and Watershed Processes

Modeling Phosphorus Concentrations in Irish Rivers Using Land Use, Soil Type, and Soil Phosphorus Data

Landscape and Watershed Processes

^a Teagasc, Johnstown Castle Research Centre, Wexford, Ireland
^b Environmental Protection Agency Regional Inspectorate, The Mall, Castlebar, Mayo, Ireland

* Corresponding author (kdaly{at}johnstown.teagasc.ie)

Received for publication December 5, 2000. Modeling diffuse phosphorus (P) loss may indicate management strategies to minimize P loss from agricultural sources. An empirical model predicting flow-weighted phosphorus concentrations (MRP) was derived using data collected from 35 Irish river catchments. Monitoring records of riverine P and stream flow data were used to calculate MRP values averaged for the years 1991–1994. These data were modeled using land use, soil type, and soil P data. Soil type in catchments was described using soil survey classifications weighted according to their P desorption properties from laboratory results. Soil test P concentrations for the studied watersheds were obtained from a national database. Soil P levels were weighted based on the results of field experiments measuring P losses in overland flow from fields at different soil test P levels. The 35 catchments were statistically clustered into two populations (A and B) based on differences in soil type, specifically, soil hydrology. Catchments in Cluster A had predominantly poorly drained soils and comparatively higher MRP concentrations (0.03–0.17 mgL^-1) than Cluster B areas (0.01–0.7 mgL^-1) with mostly well-drained soils. Regression equations derived for A and B type catchments predicted MRP values with 68 and 62% of the variation explained in the models, respectively. Data extracted for the rest of the country were applied to the models to delineate areas at risk on a national scale. While the models were only moderately accurate they highlighted the influence of land management, specifically, high production grassland receiving high P inputs, in conjunction with the effect of soil type and soil hydrology on the transport of P to surface waters.

Abbreviations: dmf, daily mean flow • MRP, flow-weighted phosphorus concentrations • ortho-P, molybdate reactive orthophosphate