The effect of climate change and gabion check dams on the amount of sediment yield of basin, case study: Dehbar

Predicting changes due to climate change and its possible consequences on hydrological processes of the watershed helps to solve the challenges facing managers and water resources planners in the coming period. The effects of this phenomenon and gabion check dams simultaneously on the sedimentation of Dehbar Basin have not been studied so far, so the aim of this study is to investigate this with the perspective of the next 30 years, using SWAT and LARS-WG models and the ability of these two models to simulate climate change and remove gabion check dams.

In this research, the performance of gabion dams as a reservoir in the Dehbar Basin, 10 km west of Mashhad and south of the city of Torghabeh, which has a cold semi-arid climate, is compared to the amount of sediment output from the basin for the next 30 years with the LARS-WG statistical model and SWAT hydrological simulation model has been evaluated. There are five gabion dams in this area. To predict the meteorological variables of the upcoming period at the level of Dehbar Watershed, after recalibrating the LARS-WG model, the HadGEM2 model and three scenarios of RCP8.5, RCP4.5 and RCP2.6 were used for the microscaling of meteorological data in the period of 2050- 2020. Kolmogorov Smirnov (K-S), t and F statistical tests were used to check the performance of the simulation results in this model. To prepare and implement the SWAT model, the map and digital model information of elevation, soil, land use, hydrological and meteorological data were used. The tank was used to introduce gabion dams to the model. In order to analyze the sensitivity, calibration and validation of the SWAT model, SUFI-2 semi-automatic algorithm was used.

The average values of the simulated precipitation are in good conformity with the observed values and the biggest difference is related to the months of February and April. Regarding the standard deviation values of monthly rainfall, the biggest difference belongs to the months of February and March. The average simulated minimum temperature is also in very good agreement with the observed values. Also, for the maximum temperature, a trend similar to the minimum temperature is seen. Also, the p-value obtained from the t-test for the aforementioned variables, there is no significant difference between the average temperature and precipitation data produced and the observed data, and the LARS-WG model has been able to calculate the average temperature and precipitation very well. To simulate monthly precipitation in all scenarios (RCP8.5, RCP4.5 and RCP2.6) in the horizon of 2050 in the first seven months of the year is lower than the monthly precipitation in the base period, but an increase in precipitation is observed for the second five months of the year. Also, in the horizon of 2050, the temperature will increase on average compared to the base period. To draw the curve of the sediment gauge, the method of batch average with modified FAO coefficient was used. The amount of observed sediment was calculated as 2.14 tha-1yr-1. For the calibration and validation of the SWAT model, at first, using CUP_SWAT software, the parameters that had a greater effect on the discharge and sediment output from the basin were identified. In order to analyze the sensitivity, calibration and validation of SWAT model, SUFI-2 semi-automated algorithm was used. After that, the effects of climate change on the amount of runoff and sediment in the basin were investigated using the validated SWAT model. The results show a decrease in rainfall, an increase in temperature and a decrease in runoff in the horizon of 2050. The precipitation changes for RCP4.5 and RCP2.6 scenarios are +9.3% and +3.1%, respectively, and -4.6% for RCP8.5 scenario. On average, gabion dams reduce 57.09% of sediment exit from the basin.

In this research, the effects of climate change on the sediment output from Dehbar Basin in the period from 2020 to 2050 and the effect of gabion dams in the existing conditions (presence of dams) and in the conditions of removal of these dams were investigated. In all scenarios, the minimum and maximum temperature increased in the 2020-2050 period compared to the base period. One of the negative effects of temperature increase is change in the amount and time-spatial pattern of precipitation. The results indicate that the SWAT model is capable of simulating hydrological processes and sedimentation in relatively small to medium watersheds with complex conditions such as Dehbar Basin, even with limited observational data, with acceptable accuracy. The increase in the amount of sediment at the outlet point of the watershed, despite the decrease in rainfall and runoff, indicates short-term rainfall with high intensity, which increases the occurrence of flooding conditions. From the changes in the amount of erosion and sedimentation in the horizon of 2050 and with different scenarios, it can be concluded that the climate change will affect the erosion of the basin in the future and the simulation model can be effective in predicting the erodibility. Therefore, the results obtained from the SWAT model provide the possibility of recommending its use in the region.