Evaluation of Effect of Land Use Change on Flood Hydrograph and Peak of Flow in Atashgah Basin of Ardabil

Watershed management undoubtedly influences all water balance equation parameters. One of the most important parameters that influence runoff is land use change. Atashgah Basin, one of the most important sub-basins of Yadyboolikkchay, has an area of 40.50 km2 and is located to the east of Sabalan Mountain. In this study the effect of land use change on flood hydrographs was evaluated for the time period between 1968 and 2011. Land use map from 1968 was prepared using aerial photographs and land use map for 2011 was prepared using land use map of Ardabil province and the Google Earth package. To estimate maximum daily precipitation, data from 9 rain stations were used. Maximum daily precipitation data were prepared, completed and normalized. 8 probability distribution functions were evaluated using 3 statistical criteria. Maximum daily precipitation was calculated for 2, 5, 10, 25, 50 and100 years return periods using a log-normal probability distribution function. For simulation, the WMO rainfall pattern and the SCS rainfall-runoff model were selected. Results showed that dense rangeland areas had decreased and area of residential and agricultural land had increased, which have cause an increase in curve number during the 43-year period. Hydrograph simulation results for the two ends of the interval showed that maximum flood flow increased greatly with increase in return period, at low return periods (2 to 10 years). There was a small difference between 1968 and 2011 in terms of flood flow at mentioned return periods. Difference between flood flow of 1968 and 2011 increased with increas in return period (decreased permeability). Results, also showed that, land use change of Atashgah basin within 43 years decreased soil permeability and increase runoff.