SIMULATION OF RUNOFF AND PHOSPHORUS TRANSPORT IN A CARPATHIAN CATCHMENT, SLOVAKIA Y. B. LIU a , J. CORLUY a , A. BAHREMAND a , F. DE SMEDT a , J. POOROVA b and L. VELCICKA ´ b a Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium b Slovak Hydrometeorological Institute, Jese ´niova 17833, 15 Bratislava, Slovakia Republic ABSTRACT A comprehensive, GIS-based modelling approach is developed to estimate runoff and phosphorus transport within a watershed at a daily time step. The approach relies on the use of GIS data for deriving major critical model parameters that exhibit distinct spatial variability over the catchment. Surface runoff is calculated by a modified rational method, which depends upon rainfall intensity, soil moisture status, slope, land-use and soil characteristics. Phosphorus loading is estimated as a function of the runoff volume and the event mean concentration for different land use categories. A diffusive approximation method is used to trace runoff and phosphorus transport to the basin outlet. The modelling approach is tested in the Margecany catchment, Hornad River basin, Slovakia, to simulate runoff, phosphorus loading, and its transport on a daily time scale using data observed between 1995 and 2000. Satisfactory results of the hydrographs and phosphorus concentration at the basin outlet are obtained, though more efforts regarding the phosphorus cycling and its biochemical reactions need to be clarified by further research. Copyright # 2006 John Wiley & Sons, Ltd. key words: runoff prediction; phosphorus loading and transport; WetSpa; GIS; Margecany Received 20 April 2005; Revised 15 October 2005; Accepted 3 February 2006 INTRODUCTION The water quality of the Ruzin reservoir, situated in the middle reach of the Hornad River valley, Slovakia, is affected by the loading and transport of phosphorus, both spatial and temporal, resulting from a wide range of point and diffuse sources upstream of the reservoir. Land use, particularly agriculture, is considered to be a significant diffuse source of phosphorus export during high flow conditions. Other inputs are point sources from sewage and industrial discharges that deliver high concentrations of nutrients directly into the waterbody. Phosphorus has been shown to be the primary nutrient affecting the eutrophication of freshwater bodies (Lemunyon and Gilbert, 1993). The loading of excessive nutrients causes accelerated eutrophication, the growth of excessive nuisance algal and aquatic weed, etc., which are harmful to the local ecosystem and are difficult to reverse (Dorioz et al., 1998). Monitoring and modelling of phosphorus transport are therefore critical for the protection of Ruzin Reservoir, which is important for water supply, tourism and recreation in eastern Slovakia. Particulate phosphorus (PP) constitutes the major proportion of phosphorus transported from cultivated land, which is about 75–90% of the total phosphorus (TP) (Sharpley et al., 1995). However, runoff from grass or forestland carries little sediment, and is therefore generally dominated by dissolved phosphorus. Watersheds with mixed land use including significant forested areas often have similar amounts of dissolved and sediment-bound phosphorus losses (Miller, 1979). Losses of sediment-bound phosphorus from watersheds are variable from site to site as well as over time at a particular site depending on climate, soil and management factors. As rainwater falls on the soil surface, some particles are detached from the soil mass and are transported by water running off the ground surface. The large particles settle at varying locations in the field, whereas the small particles may be discharged into small streams and are subsequently transported into larger streams and lakes. A high proportion of the annual phosphorus transport can take place during a very limited time period, usually in connection to the high water flow. RIVER RESEARCH AND APPLICATIONS River Res. Applic. 22: 1009–1022 (2006) Published online 29 September 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rra.953 *Correspondence to: Y. B. Liu, Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium. E-mail: lyongbo@uoguelph.ca Copyright # 2006 John Wiley & Sons, Ltd.