BALWOIS 2008 – Ohrid, Republic of Macedonia – 27,31 May 2008 1/9 Hydrograph Analysis using Stable Isotopes in a Micro Scale Basin Inci Tekeli A.Unal Sorman Soil- Fertilizer and Water Resources Center Middle East Tech.Unv.Civil Eng.Dep. Research Institute, Ankara, Turkey Water Resources Lab. Ankara, Turkey Abstract Spatial and temporal distributions of precipitation show quite a variation in different regions of Turkey. In general, amount of precipitation is low in the central part of the country. It is necessary to store water to prevent drought as the precipitation is lower than expected. Should excess precipitation occur in the short term, basin management and flood control structures must be considered to prevent floods. In this situtation it is necessery to know the hydrologic properties (such as precipitation, runoff, evaporation) to obtain long term economic water storage structures. In this research, a stable environmental isotope study was carried out from analysis of water samples collected from rainfall, runoff (total discharge), springs (subsurface flows), and wells (groundwater). The research site was Güvenç basin located near Yenimahalle-Ankara having a drainage area of about 16.125 km 2 . The aim of the study was to investigate rainfall-runoff relationship for Güvenç basin using stable isotope method in the separation of hydrographs. Recorded total discharge hydrographs are separated to their components using isotopes (oxygen-18, deuterium) contents. It was found out that the contribution of subsurface flow originating from various sublayers of the basin are important in hydrograph separation using isotope method of approach. Keywords: Rainfall-runoff, hydrograph separations, stable isotopes, Guvenc INTRODUCTION Spatial and temporal distributions of precipitation show quite a variation in different regions of Turkey. In general, amount of precipitation is low in the central part of the country. It is necessary to store water to prevent drought as the precipitation gets lower than expected. Besides, in case of receiving excess precipitation more than the expected in a short term, catchment management and flood control structures must be considered to prevent the floods. In this case, it is necessery to know the hydrologic properties (such as precipitation, runoff, evaporation) to obtain long term economic water storage structures. For these reasons, there are many representative hydrologic basins established in Turkey by the Soil and Water Resources Research Institutes. In these basins, the precipitation and runoff values are recorded continously and relationships between these values is investigated for the last twenty five years. Güvenç basin, the study area, is one of these representative basins. On this basin, a streamgaging, station and five raingages have been installed to collect runoff and rainfall data. A small dam was constructed at the outlet of the basin to store and provide water for agricultural areas. The aim of this study is to investigate the proportion of storm water during distinct hydrological events and also to analyze the hydrograph separation. In addition, we sought to better understand the behaviour of individual precipitation events using isotope method and to compare the results with a semi-log graphical method. Water balance can be used by isotope composition of the hydrologic cycle to idendify the important processes of flow patterns that cause the generation of streamflow in a basin. BASIN DESCRIPTION The Güvenç Basin is located about 44 km north of Ankara. The 16.125 km 2 basin is situated between 1053 m at the river sampling site and 1458 m (Figure 1). A 90° V-notch weir (W-17), located at the outlet of the basin, continuously records stream discharge. The main channel (Kayaönü river) is a forth order, perennial stream at the outlet. Five standart raingages are located within the basin which are used to record continuously the amount of precipitation (Figure 1). The data is than transformed to the Research Institute. The mean annual precipitation depth of basin is 497.9 mm (1984-2001), 33,3 % of the precipitation falls during spring season. Spring runoff is the dominant hydrological event in this region. The mean annual runoff is 119,32 mm with 32, 63 mm surface runoff, 14, 68 mm subsurface and 72,01 mm groundwater (Tekeli and Babayigit, 2002). Topography is mature with the 21 %