Investigating the impacts of rapid urbanization on basin hydrology and sediment loads with land change model and climate change scenarios

Climate change and land use/land cover (LULC) change are among the most critical environmental factors affecting the water cycle and sediment transport at the watershed scale. In this study, the hydrological impacts and sediment transport outcomes of rapid urbanization, particularly due to the construction of an airport in the watershed, were investigated. These effects on the Alibeyköy Watershed, which is a drinking water basin, were evaluated based on LULC change modeling and climate change scenarios. Within the scope of the study, future LULC changes were projected using the Multilayer Perceptron-Markov Chain (MLP-MC) method. Climate projections were generated by considering five different Global Climate Models (GFDL-ESM2M, HadGEM2-ES, IPSL-CM5A-LR, MIROC, NorESM1-M) and two different climate scenarios (RCP 4.5 and RCP 8.5). The SWAT (Soil and Water Assessment Tool) watershed model was calibrated for the simulation of streamflow and sediment loads and was validated for different periods. In addition, recent sediment load data were obtained through field sampling and laboratory analyses. These data were used in an additional validation process to verify the model's performance. High accuracy was achieved in estimating the LULC change and this was supported by the Kappa value being determined as 0.95. In the SWAT watershed model streamflow simulations, the NSE value was found to be above 0.5 for both calibration and validation periods, indicating satisfactory results. Although the NSE value in the additional validation using the sediment dataset created through field sampling and laboratory analyses was below 0.5, the trend agreement between observation and simulation was interpreted as acceptable. The findings were addressed from three perspectives: First, by comparing the LULC conditions before and after the airport construction, the hydrological and sediment load-related outputs of this impact were evaluated using the watershed model. The findings showed that the airport construction increased surface runoff and accelerated erosion and sediment transport on a regional scale. Then, future LULC projection scenarios were integrated into the watershed model to predict sediment accumulation in the Alibey Dam reservoir. It was concluded that the observed sediment accumulation in the Alibey Dam may lead to an increase in dead storage in the long term, but does not pose a critical threat to the total reservoir volume in the short and medium term. Finally, as a result of the combined scenario analyses of future LULC and climate change, a decreasing trend in average streamflow values was observed. This decrease was mainly attributed to climate change, while the increase in sediment loads was largely found to result from LULC change.