Evaluating of Land Use Changes on Hydrological Characteristics of Basins and River Flood Plains (Case Study: Kashkan Basin in Lorestan Province)

Land use change is one of the most important factors affecting runoff from the basin, which changes the factors affecting water infiltration. According to the statistics collected by the Forests Organization, from the beginning of 2011 to the first six months of 2015, 552 floods occurred in Iran. Land use change is one of the important factors in changing hydrological flow, basin erosion and biodiversity destruction. Lack of attention to land use change and exploitation of natural resources due to disturbing the biological balance has been one of the effective factors in increasing flood zones. Therefore, it is possible to take a step towards equilibrium by knowing the trend of land use changes in order to lead ecosystems and simulate the hydrological reaction of the watershed with less error. Mathematical models have a great ability to estimate runoff and its flood plain and provide users with effective information by recognizing the factors and parameters affecting the flood. Such models provide the relevant managers and experts with considerable ability to manage the basin before floods, and even crisis management and rescue during floods. In this study, in order to investigate the effect of land use changes on hydrological characteristics, for an approximate distances of 40 km from the Kashkan River in Lorestan Province, first land use maps for 1989 and 2014 were extracted using satellite images of 5 and 8 June of the years 1989 (TMC1) and 2014 (Landsat8) with a spatial resolution of 30 m. Then, the number of the basin curve and lag time were measured using the help maps in order to calculate the runoff height in the hydrological model. On the other hand, the runoff precipitation simulation was performed through considerin other required parameters. In order to correspond to the land use extraction periods, two normal periods related to 1989 and 2014 were selected for simulation. To evaluate the efficiency of the model in simulating daily flood values, the coefficient of determination error criteria (R2), dispersion index (SI), volume error percentage (V%), peak error value (  %), and error peak time ( ) were considered. In the next step, in order to locate and adapt different land uses to flood plains in different return periods; the flood maps of these zones were extracted in different return periods and placed on land use change maps. And the area of land uses in different return periods was calculated for the studied periods. In order to achieve this goal, the first step involves preparing input information for HEC-RAS software. To prepare the required geometric layers of RAS and calculate the flood plains, the TIN of the area from a 1:150 digital map (mapped by Lorestan Regional Water Organization) was prepared and in order to achieve the peak discharge with different return periods, two 18-year statistical periods (from 1972 to 1990 and from 1999 to 2017), and maximum instantaneous discharge of Doab Veysian hydrometric station were used. The Manning coefficient was determined by using Cowan's (1956) method and geometric data. This data includes obstacles and structures in flow path that obtains and simulated in software. To analyze the flow in a steady way, the HEC-RAS software and HEC-geo-RAS extension in Arc-GIS environment have been used. The EasyFit software has been used to prepare peak discharges for different return periods. The results of land use changes during the period of 1989-2014 have shown that the area of land uses such as rangelands and forests has decreased and the area of land uses such as wastelands and human structures has increased. As a result, the value of the calculated curve number has increased from 45 to 68.4, and the lag time has increased from 41 to 24.3 mm, which indicates the flooding of the basin. The area of different uses on the flood maps in the period of different returns showed the residential areas in the new period compared to the old period, which were flooded in the 50-year return period. They are now flooded due to increase in the flood discharge and the development of rural areas along the river during a short 25-year return period; Even the new farms in the river side is danger of flood in shorter period such as 10-year.