422 Hydrology of die Mediterranean and Semiarid Regions (Proceedings of an international symposium held at Montpellier. April 2003). IAHS Publ. no. 278, 2003. Processes of pesticide dissipation and water transport in a Mediterranean farmed catchment MARC VOLTZ, XAVIER LOUCHART, PATRICK ANDRIEUX Laboratory on the Interactions between Soils, Agrosystems and Hydrosysterns (LISAH), National Institute for Agricultural Research (INRA), place Viala, F-34060 Montpellier Cedex I, France voltz@ensam.inra.fr BERND LENNARTZ Institute of Soil Science, University of Rostock, Justus-von-Liebig-Weg 6, D-18059 Rostock, Germany Abstract The contamination of soil and runoff water by diuron was monitored over several years at the field and the catchment level in a Mediterranean vineyard zone. Several aspects of pesticide behaviour in semiarid environments were observed. Pesticide dissipation in the soil was largely influenced by periods of drought, and large temporal changes in pesticide desorption properties occurred. The main pathway of pesticide losses at the field scale was the intense overland flow generated by heavy Mediterranean rainfalls. Soil treatment had a major influence on herbicide losses. Conventional tillage limited the contamination of surface waters in comparison to no-tillage, since it increased infiltration and restricted the use of herbicides. At the catchment scale the concentrations in pesticides of surface waters were similar to those at the field scale, due to the dominant Hortonian behaviour of the catchment. However, over a season the pesticide losses by runoff water were much smaller at the catchment scale because of important channel losses. Key words catchment scale; field scale; herbicides; overland flow; semiarid climate; tillage; vineyard INTRODUCTION Pesticides are widely used in modern agriculture in most countries throughout the world, and in a large range of environments. However, environmental monitoring increasingly indicates that trace amounts of pesticides are present in surface and underground water bodies, far from the sites of pesticide application. To find strategies for limiting the contamination of the surrounding environment, the processes affecting the persistence and transport of pesticides have been studied widely for many years. However, so far, the majority of studies has been performed in temperate or humid conditions, and consequently there is a need for studies about pesticide behaviour in other environments. This is especially so in semiarid or Mediterranean areas for two main reasons. Firstly, the risk of contaminating waterbodies by pesticides is not less severe in semiarid climatic conditions (e.g. Albanis et al, 1992; Lennartz et al, 1997) than in others (e.g. Franck & Sirons, 1979; Ng et ah, 1995), and it has potentially large social consequences given the limited water resources. In effect, the intense rainfall events of semiarid climates, combined with often discontinuous soil cover by crops,