Effects of land cover and climate changes on runoff and sediment yield from a forested catchment in northern Iran
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There have been more than 3700 recorded floods in Iran during the 50 years to around 2005, of which slightly more than half occurred during the final decade of this period. Over a longer period (1909-2004), floods in Iran caused economic losses in excess of US$3.5 billion. These floods, and associated erosion problems, have previously been attributed to land cover change and climate change. This research aimed to investigate future runoff changes in a small forested catchment in northern Iran for which appropriate and sufficient data exist to support such analyses. Approaches for estimating erosion and sediment production from the same catchment in the absence of relevant detailed data were also examined. The selected study site was the Casilian Catchment, with the town of Sangdeh at its centre. This is an elongated catchment approximately 17.8 km long with a mean catchment slope of 34.3% or 18.8°. The upstream half of the catchment is forested and, as such, could theoretically be susceptible to significant deforestation in the future. As such, it provides a suitable basis for the examination of hydrological implications – particularly runoff change – resulting from significant land cover change. To investigate the land cover change impact on runoff (volume and peak discharge of the outflow from the catchment), the approach used in this study was to use the Soil Conservation Service (SCS) curve number method with spatial catchment data using a Geographical Information System (GIS) and the SCS dimensionless unit hydrograph. The effects of rainfall characteristics and antecedent soil water content on runoff were also investigated. Runoff changes between observed period (1980-1986) and three different future periods (2011-2017, 2046-2052 and 2080-2086), for the Casilian Catchment, were estimated using the Hydrologic Engineering Centre-Hydrologic Modelling Systems (HEC-HMS) rainfall-runoff model. Thirteen climate models provided the required input variables (temperature, rainfall) to the rainfall-runoff model, under three different scenarios (A2, B1, and A1B) using the the ‘Long Ashton Research Station’ weather generator known as LARS-WG. This was used to downscale the Global Climate Model (GCM) to the Sangdeh Station to overcome the limitations of the coarse scale GCM output. Temperature was used to calculate potential evapotranspiration required for HMS rainfall-runoff model. The two empirical erosion models, known as the Erosion Potential Method (EPM) and Pacific South-West Inter-Agency Committee (PSIAC) methods, were evaluated to estimate sediment yield for the Casilian Catchment. Further, areas with high erosion intensity were identified, and factors affecting erosion were determined for the Casilian Catchment. In the EPM method, only three processes are used to prepare an erosion intensity map, but nine processes are considered in the PSIAC method. Runoff volume and the peak flow are considered to estimate erosion intensity and sediment yield in the PSIAC method, but not in the EPM method. Soil characteristics are also considered in erosion intensity and sediment yield estimation in the PSIAC method but not in the EPM method. The calculated mean annual specific sediment yield of the Casilian Catchment using the PSIAC method (482 t km-2yr-1) was closer to a measured value for the Talar Catchment (532 t km-2 y-1), of which Casilian is a subcatchment. Thus, the PSIAC-based erosion intensity map seems to be more representative of the erosion condition of the the Casilian Catchment. Slope, magnitude and intensity of rainfall, soil and land cover can be important factors affecting soil erosion of the Casilian Catchment. From this research, it was found that antecedent soil water content, magnitude and intensity of rainfall and the area affected by land cover change relative to the catchment size are important factors affecting runoff characteristics (volume of runoff and peak discharge). In the future, the mean rainfall totals (especially mean annual rainfall) and frequency of extreme rainfall events and rainfall intensity may increase due to an increase in atmospheric water vapour for the Casilian Catchment and probably other small catchments in northern Iran. Therefore, these catchments may be subjected to more flooding and erosion in the future as a result of changes in the rainfall characteristics. The findings of this research show that land cover change (e.g. deforestation and agricultural activities) should be given more consideration in the management of small catchments in northern Iran in the future, due to the consequences of climate change.