Flood Intensification due to Changes in Land Use

The non-stationarity in runoff regime may be attributed to various causes such as climate change, land use change, and man-made runoff control structures. Degradation of land use can induce significant impact on infiltration and surface roughness leading to higher flood discharges. This study aims at quantifying possible effects of land use changes and identifying flood source areas for future flood control planning in the Golestan watershed located northeast of Iran. A preliminary trend analysis on the annual maximum flood record of three stations inside the watershed showed that two stations were subject to anthropogenic change. This is while no trend could be detected in the annual maximum rainfall records in the region. Using a calibrated event-based rainfall-runoff model, flood hydrographs corresponding to land use conditions in 1967 and 1996 were simulated and relative changes in the peak flow of the two subsequent conditions were determined for different return periods. The results showed that the impact of land use changes on the flood peak discharge is considerably greater in some subwatersheds. Two limiting land use scenarios were also considered to investigate the envelope of future flood peaks in the watershed. By successively eliminating subwatersheds from the simulation process in a method titled "unit flood response”, the contribution of each subwatershed to the outlet flood peak was quantified. Contribution, per unit area, to the outlet flood peak was the basis to rank the subwatersheds in terms of their flood potential.