The Impact of Land Use and Climate Change on Watershed Runoff Model SWAT (Case Study: Watershed Garin)

Introduction
In a natural ecosystem, changing the environmental conditions of that ecosystem influences hydrological responses such as flooding and the extent of erosion and sedimentation of the area. One of the models used to investigate the effect of land use change and climate change on SWAT runoff, the SWAT model is a hydrological simulator and a continuous and semi-distributive time-space model with a physical base. Understanding the relationship between land use change and its causative factors and its secondary effects on hydrologic regimes provides essential information for land use planning and sustainable management of natural resources. Investigating the amount and trend of the changes and its effect on hydrological processes in the basin is a way to predict the state of future changes and provide more effective plans for sustainable development of water resources in the basin. The construction of the Garin Dam in the Garin Basin and the risk of filling the sediment reservoir with sediment and reducing its useful life due to seasonal floods and the effect of basin land use and climate change on the reason for choosing this area for this research. The purpose of this study was to study the land use and climate change in the studied watershed and determine the effect of these changes on the runoff rate of this watershed in order to better manage it.
Study  of Area
Garin dam dam is located in the province of Hamedan and is located in the mountain range of Zagros mountains. This area includes the catchment area of Sarab Gamasiab River to the Garin Reservoir Dam and its area is up to the 22,000-square-meter Garin Garin Dam, the Garinland basin is mainly mountainous and its range of elevation ranges from 1833.9 to 3429.2229 meters above sea level.
Materials and Methods
SWAT model input data include climatic and hydrological data (daily precipitation, maximum and minimum temperature, relative humidity, wind speed, dew point and solar radiation), which is ten years in the study of statistics related to the synoptic stations Skinheads Became Topographic maps, digital elevation model (DEM), soil and land use are also needed as model inputs. A digital elevation model (DEM) was extracted using a topography map of 1: 250,000 Garin River basin. Calibration and validation of the SWAT model in SWAT CUP software. The study used calibration data from 2002 to 2007 and 2008 to 2010 for validating the model. In order to determine the degree of sensitivity of flow parameters in the model SWAT using SUFI2 software SWAT CUP sensitivity analysis for 24 parameters election, the results of the sensitivity analysis on the Elimination of parameters that has the less sensitive they are, the calibration process decision It is accepted. According to the P-value and T-Stat criteria, the sensitivity of the parameters is determined. Land use maps of 1986, 2000, and 2014 were prepared in the previous stages, and the Markov chain and the CA Markov filter were used to map the land use in 2042. In this research, the outputs of the Hadcm3 model were used to predict Garin's future climate. In this research, the SDSM statistical method was used to fine-scale the output of the general atmospheric circulation models. The SWAT model was used in the range of calibrated parameters to simulate runoff from climate change in Garin basin under two scenarios A2 and B2. After micro-sampling, the SWAT model was converted and the model was analyzed for the scenarios. Then, the results of model implementation with different scenarios and the results of model implementation with the current climate conditions were compared
Discussion and results
Regarding the results of statistical indices, NS index is equal to 0.95, P factor and R factor were respectively 0.47 and 0.03 respectively, and the coefficient of determination (R2) for simulated and simulated floodguns was 60 / 0. Accordingly, the results were confirmed in the calibration phase. The validation phase was conducted to verify the correctness of the selection of parameters during the calibration period for the period 2008-2010. Given that the Nashatcliff coefficient for Garin's catchment area at calibration and validation stage was equal to 0.95 and 0.66, respectively, the results were satisfactory and the SWAT model was able to simulate surface runoff in Garin River Basin. In general, due to increased forest use due to increased permeability and water drainage to the surface and deep water aquifers and increased evapotranspiration, the amount of runoff has decreased. Regarding the results of temperature, rainfall and runoff of the next period, it can be seen that in the months when rainfall is reduced and the temperature increased, the amount of runoff in the coming period also decreases. The main reasons for this discrepancy can be attributed to the difference in the intensity of land use change as well as the extent of the altered land area, which, given the mountainous nature of the area in the Garinland basin, can be compared to other areas with flat lands with agricultural uses. It is concluded that the effect of climate change in the Garin dam basin is greater than the change in land use due to its mountainous nature.
Conclusion
The results of the study of the effect of land use change on runoff in the Garin basin indicate that the amount of runoff is decreasing daily and monthly in this catchment area. Also, the results of the study on the effect of climate change on runoff in the Garinwestern basin indicate that the amount of runoff is daily and monthly in this catchment area. Considering that in both scenarios A2 and B2 the monthly average temperature, especially in the first and last months of the year, has an increasing trend and rainfall has decreased in the spring and winter, this decrease can be attributed to the increase in temperature which Following this, evaporation also increases and decreases in rainfall in this catchment area. Regarding the results, it can be seen that the average monthly runoff in months when rainfall decreased in January, February, February, April, May and December, and in the months when rainfall increased As of June, July, August and September, the amount of runoff will increase compared to the current period. It is also observed that the effect of land use change on the reduction of runoff in the upcoming period is lower compared to the change effect under A2 and B2 scenarios and will affect the climate change of the runoff more flatly and the reduction of runoff is more affected by climate change. According to the information obtained from these predictions, it is possible to properly manage the watershed and adopt appropriate management measures in accordance with the conditions of this watershed and to prevent unauthorized land use changes and reduce the damage caused by The phenomenon of climate change.