European Water 37: 27-32, 2012. © 2012 E.W. Publications Comparative assessment of water management indicators in the Gediz river basin with foreseen climate scenarios B. Yilmaz 1 and N.B. Harmancioglu 2 1 Golmarmara Vocational School, Celal Bayar University, Manisa, Turkey; baris.yilmaz@bayar.edu.tr 2 Dokuz Eylul University,Department of Civil Engineering, Izmir, Turkey; nilgun.harmancioglu@deu.edu.tr Abstract: Gediz Basin is an agriculture-dominant river basin in the western region of Turkey along the Aegean coast. The most significant feature of the basin is water scarcity, which is due basically to competition for water among various uses (water allocation problems), mainly irrigation with a total command area of 110,000 ha versus the domestic and fast growing industrial demand in the coastal zone, and environmental pollution although the basin experiences significant droughts from time to time. Since the water resources in the basin are almost fully allocated to various uses, it is expected that the negative impacts of climate change will exacerbate the water crisis. In this study, the results of the project “Climate Change Scenarios for Turkey”, funded by TUBITAK (The Scientific and Technological Research Council of Turkey) are used in Water Evaluation and Planning System (WEAP), in order to assess the climate change impacts on water resources and water demand in the basin. The WEAP is forced to simulate the water system between 2010 and 2100 with the time series of temperature, precipitation, evapotranspiration and surface runoff data projected for the Gediz Basin. The data used are the simulation results of ECHAM5 (European Centre Hamburg Model version 5) general circulation model and RegCM3 (Regional Climate Model version 3) regional climate model. The study is further based on IPCC (Intergovernmental Panel on Climate Change) A2 and B1 emission scenarios. The results obtained by WEAP model are evaluated for three (30-year-long) periods, and comparative assessments under foreseen climate scenarios are given with respect to water supply reliability, unmet water demand and crop yield. Keywords: Climate scenarios, water management indicators, WEAP, Gediz River Basin. 1. INTRODUCTION Due to the rapid increase in population, economic development, climate change, improved living standards, urbanization, and industrialization, water managers have been faced with more complex water management problems in the early 21st century, and it is expected that coping with water problems will be harder in future. Thereby, understanding the possible impacts of climate change is of great importance for water resources management. In the Gediz River Basin, the water resources are almost fully allocated, and it is expected that the climate change impacts will exacerbate the water crisis in this agriculture-dominant basin (Yilmaz and Harmancioglu, 2010a). Recently, a lot of efforts have been made to better characterize and model the possible impacts of climate change (e.g. Onol and Semazzi, 2006; Karaca et al., 2006; MoEF, 2007, Ozkul, 2009); one of which is the project “Climate Change Scenarios for Turkey”, funded by The Scientific and Technological Research Council of Turkey (TUBITAK, 2008). Within the project, the detailed regional projections which can constitute the main inputs of the studies related with climate change impact have been developed. In this study, the project results downscaled to the Gediz River Basin are used in the Water Evaluation and Planning System (WEAP), in order to assess the climate change impacts on surface water balance of the basin. The water supply and demand interrelations in agriculture, which is the largest water consumer, constitute the main focus of the study. The WEAP is forced to simulate the water system between 2010 and 2100 with the time series of temperature, precipitation, evapotranspiration and surface runoff data obtained by web-based data dissemination system of the relevant project. The basic aim of the study is to reach a comprehensive assessment with respect to variations in supply reliability, unmet demand and crop yield versus changes in precipitation and temperature.