MODELLING THE EFFECTS OF ANTHROPOGENIC ACTIVITIES AND CLIMATE CHANGE ON A SMALL CATCHMENT IN AN AEGEAN ISLAND Alexandra Spyropoulou, George Tsirtsis University of the Aegean, Department of Marine Sciences Abstract A watershed model, including a runoff and an erosion component, was applied to a small watershed in the Eastern Mediterranean. The catchment is drained by ephemeral rivers flowing into the gulf of Kalloni, Island of Lesvos in the Aegean sea. The model estimates the daily nutrient loading of the receiving water body, taking into account both the environmental (geomorphology, land uses) and the socioeconomic (population, tourist activities, animal farming, industry) characteristics of the watershed. The model, which is forced by the daily rainfall height, was run for one typical year and calibrated using field data, collected along the route of the main rivers draining the catchment. The contribution of the various loading sources was estimated by the model, emphasizing on the importance of the anthropogenic activities for nutrient loading of coastal ecosystems and therefore, for the development of eutrophication episodes. An analysis of available meteorological data was also performed in order to assess the effects of global change on the frequency and intensity of rainfall events and possible scenarios were formulated. These scenarios were simulated and their effects on runoff and erosion were estimated. Key words: Watershed modelling; Anthropogenic activities; Climate change; Eastern Mediterranean 1. Introduction Human activities often induce unpredictable and not easily controllable changes in the hydrological regime and the nutrient budget of river basins (Kuusemets and Mander 2001). These activities include agriculture and fertilization, dispersion of untreated sewerage, animal farming and industrial by-products. According to the source and the transport route that the resulting pollutants follow to reach the receiving water bodies, two kinds of sources can be distinguished, the point and non-point. Point sources can be monitored at their point of origin and therefore their management is relatively easy, whereas non-point sources are rather diffused and uncontrollable (Arhonditsis et al. 2002a). The nutrients resulting from both types of sources enrich the receiving water bodies and eutrophication phenomena are often observed, especially in enclosed bays, gulfs or lagoons. Therefore, the monitoring and control of the various sources of nutrients is of primary importance for managers and decision makers in the framework of Integrated Coastal Zone Management (Kontogianni et al. 2006). Small Eastern Mediterranean watersheds undergo rapid hydrological cycles characterized by a rather short rainfall period during winter and a dry summer. The rivers draining these basins are ephemeral flowing mainly from November to mid-April (Arhonditsis et al. 2002b). The foreseen climate change is expected to strongly affect the water budget of these small watersheds influencing runoff, erosion, nutrient transport, as well as the groundwater reservoirs. In order to assess these changes in the framework of policy making, possible scenarios are formulated and watershed models are used to test the scenarios in a quantitative manner (Komatsu et al. 2007, Lin et al. 2007, Wu et al. 2007). In the present work, a time series of rainfall data for a small Mediterranean island is analyzed. Possible trends due to the climate change are assessed and two scenarios are formulated representing the recently observed trends and the reference condition. The two scenarios are simulated using a runoff and a soil erosion model for a small catchment in the study area and a comparative analysis is performed for the amounts of runoff, nutrient leaching and soil eroded, between the two scenarios. 2. Methodology 2.1 The study area The gulf of Kalloni is a shallow semi-enclosed water body (mean depth of about 11 m), located in the island of Lesvos, Greece in Eastern Mediterranean (Figure 1). It is connected to the open sea through a narrow, shallow and long channel (Spatharis et al. 2007a). The climate of the island is typical Mediterranean characterized by a hot-dry summer from May to September and mild-rainy winter for the rest of the year. The surrounding watershed of 413 Km 2 is covered, according to the CORINE database, mainly by grasslands