Determination of Sub-Basins of Turkey Using GIS Techniques

Turkey has been divided into 26 basins for studies related to water resources development. Surface areas of the basins range between 6,300 – 127,500 km², the average being approximately 30,000 km². Compared with nested watershed classification of U.S., the average size of Turkish national basins corresponds to the third-level (basins). Currently, a forth-level (sub-basins) with an average size of 1,820 km² reside below the basins in U.S. classification. Even these sub-basins are found to be too large to adequately serve many water resource management needs. Two additional levels (watersheds and sub-watersheds) are currently under development in order to solve this problem. Taking this fact into consideration, currently existing basins of Turkey are supposed to be too large for effective water resources management and there is a need for delineation of sub-basins in a scientific manner. In this study, the boundaries of proto-type sub-basins are determined by using DEM-based automated delineation techniques. For this purpose, 30 arc-seconds SRTM30 DEM is re-projected into an equal area projection, hydrologic sinks are filled and inaccurate elevation values are corrected. In order to prevent invalid delineation of closed basins that cover approximately 10% of the country, water bodies located at the center of these basins are extracted from Global Land Cover Characteristics dataset of USGS and excluded from the process. Among the alternatives, Deterministic-8 (D-8) algorithm with modifications by Garbrecht and Martz is used to calculate flow direction and flow accumulation grids. Using these grids, a set of candidate sub-basins are generated by applying different area thresholds. A threshold of 1,000 km² resulted in 359 sub-basins with an average size of 1,858 km², both of which are found to be appropriate and manageable for nationwide usage. Similarity of the average size of the delineated sub-basins to that of U.S. sub-basins also supports this result.