TECHNICAL REPORTS 548 A modified type of grassed waterway (GWW) with large hydrodynamic roughness has proven ability to reduce sediment load and surface runoff under conditions where best management practices on the delivering fields reduce sediment inputs that could otherwise damage the grass cover. It is unknown how such a GWW affects the loading of surface runoff with dissolved reactive phosphorus (DRP). he effect on DRP was tested in a landscape-scale study where DRP concentrations and loads in surface runoff were measured in two watersheds in which GWWs were newly installed and increased in effectiveness over time. Both watersheds were compared with paired watersheds without GWW installation; all watersheds were continuously monitored over 5 yr (1993–1997). Additionally, DRP concentrations were measured in open field and throughfall precipitation under growing grass and crops in field experiments, and DRP concentrations in surface runoff from straw covered surfaces were determined with laboratory rainfall simulation experiments. Dissolved reactive P in throughfall for the different cover types was highly variable, and the highest concentrations (up to 2.8 mg L −1 ) occurred especially during flowering of the respective crop and after frost events. Dissolved reactive P concentrations in runoff from straw-covered surfaces were slightly higher compared with those from bare soil. On average, there was a small difference in DRP concentrations between throughfall under growing crops and grass and in runoff from bare or straw covered soil surfaces. Hence, the introduction of a relatively small grassed area has little effect on the DRP concentration in surface runoff from the total watershed. his finding was supported by the watershed data, where watersheds with and without GWW showed similar DRP concentrations. No change in DRP concentrations occurred over the 5-yr period. Such GWWs will thus reduce the DRP load analogously to the reduction in total surface runoff. Efects of Hydrodynamically Rough Grassed Waterways on Dissolved Reactive Phosphorus Loads Coming from Agricultural Watersheds P. Fiener* Universität zu Köln K. Auerswald Technische Universität München E nrichment of surface water bodies with nutrients coming from diffuse sources, especially from agricultural land, has become a major environmental issue in many countries globally because the resulting eutrophication can cause serious ecological and economic damage. As a consequence, substantial effort has been made to reduce nutrient and sediment loads by the implementation of mitigation measures. Reduction of sediment and phosphorus losses are often considered together because a large proportion of phosphorus is bound to fine-grained sediment (Bechmann et al., 2005; Owens et al., 2007). Mitigation options include improvements in agricultural practices (e.g., no- till, contouring, adapted crop rotations, timely application of fertilizers [Abu-Zreig et al., 2003], and establishment of vegetated filter strips [Dorioz et al., 2006]). Studies to investigate the effects of vegetated filter strips (VFSs) on reducing surface runoff trapping of sediment and nutrients have mainly focused on VFSs located at the downslope end of fields. Ex- periments have been performed predominantly on field plots sub- jected to natural or simulated rainfall, as well as various inflow rates and sediment and nutrient inputs (e.g., Uusi-Kämppä et al., 2000; Borin et al., 2005; Gharabaghi et al., 2006; Deletic and Fletcher, 2006). Published studies have tested VFSs of various sizes, different slopes, and different soils and vegetation characteristics. Extremes with very wide filters and very steep slopes have not been widely tested (Dorioz et al., 2006). Moreover, except for a few studies (e.g., Blanco-Canqui et al., 2006; Verstraeten et al., 2006), shallow (not concentrated) inflow has been assumed; hence, in this respect, the optimum performance of the VFSs was often tested. Compared with VFSs, studies focusing on grassed waterways (GWWs), where concentrated flow passes through a long filter located along a thal- weg (the deepest continuous line along a valley or watercourse), are relatively rare (e.g., Hjelmfelt and Wang, 1997; Briggs et al., 1999; Fiener and Auerswald, 2003a, 2005). Studies of phosphorus trapping in VFSs report a wide range of effectiveness. Reduction of total phosphorus load (P tot ) after passing through the VFSs, which is commonly dominated by trapping of particulate-bound phosphorus (P part ), ranged between about 90% (e.g., Dillaha et al., 1989; Abu-Zreig et al., 2003; Abbreviations: AGNPS, Agricultural Non-Point Source Pollution Model; CNS, Cornell Nutrient Simulation; DRP, dissolved reactive phosphorus; GWW, grassed waterway; VFS, vegetated ilter strip. P. Fiener, Geographisches Institut, Universität zu Köln, Albertus Magnus Platz, D-50923 Cologne, Germany; K. Auerswald, Lehrstuhl für Grünlandlehre, Technische Universität München, Am Hochanger 1, D-85350 Freising-Weihenstephan, Germany. Copyright © 2009 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Published in J. Environ. Qual. 38:548–559 (2009). doi:10.2134/jeq2007.0525 Received 5 Oct. 2007. *Corresponding author (peter.iener@uni-koeln.de). © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA TECHNICAL REPORTS: SURFACE WATER QUALITY