ESTIMATION THE VOLUME OF RUNOFF USING NATURAL RESOURCES CONSERVATION SERVICE METHOD AND GEOGRAPHIC INFORMATION SYSTEM IN KOYA BASIN, SULAIMANIYA, IRAQ
Measurements of rainfall excess or runoff has an essential role for Koya basin, Kurdistan, Iraq. The city has undergone rapid development and increase in
population at last few years, correspondingly the runoff in Koya basin should be evaluated, Technically GIS model was used for processing a satellite images to define the boundaries of Koya basin, the study has divided the basin into eight catchment areas of well defined boundaries and streams, the total area of the basin is about 58.658 Km2. The land hydrologic covers (described in Table 1) are defined from satellite images processed in GIS. A hydrologic soil groups are defined based on predetermined field measurements of infiltration throughout the basin. The NRCS Curve Number method is used to estimate excess rain from rainfall data records, during last 15 years ago, a maximum rain water depth recorded at 28 February 2006 is 103 mm as a total daily depth, based on this historic hydrologic event, excess rain water depth has calculated for each catchment area. A maximum excess rain depth of 36.9258 mm has found in the catchment number (3) because it has minimum storage capacity (abstraction), while the maximum runoff volume of 724855 m3 has found in catchment number (1) because it has a biggest watershed area. The results of this study will form a data base for the purposes of planning and design for many hydraulic and hydrologic projects in Koya city.