مقايسة اثر تغيير كاربري اراضي و اقليم بر رواناب يك حوضۀ آبخيز كوچك كوهستاني (مطالعۀ موردي :حوضۀ آبخيز گرين)

Comparison of Land Use and Climate Change Impacts on Runoff in a Small Mountainous Catchment (Case Study: Garin Dam Catchment)

در اين تحقيق به منظور بررسي اثر تغيير كاربري اراضي و تغيير اقليم بر رواناب يك حوضة كوهستاني برفگير در زاگرس مركزي از مدل هيدرولوژي SWAT استفاده شد. كاربري اراضي حوضة آبخيز گرين در سال 1986 از ادارة منابع طبيعي همدان استخراج و در سالهاي 2000 و 2014 از تصاوير ماهوارة لندست 8 تهيه شد. نقشة كاربري اراضي در سال 2042 با استفاده از مدل ماركوف و CA ماركوف پيشبيني شد. براي پيشبيني اقليم آينده از مدل HadCM3 استفاده شد و خروجيهاي آن با مدل LARS-WG ريزمقياسنمايي شد. با توجه به ضريب نش- ساتكليف، ضريب تبيين، P-factor، و R-factor بهدستآمده در مرحلة واسنجي (بهترتيب برابر با 59/0، 60/0، 47/0، و 09/0) و مرحلة اعتبارسنجي (بهترتيب برابر با 71/0، 72/0، 59/0، و 02/0)، اين مدل داراي كارايي قابل قبولي است. نتايج نشان ميدهد كه اين منطقه تا سال 2042 شاهد افزايش 28/2درصدي مساحت جنگل و كاهش 07/2درصدي مساحت مرتع، روند كاهشي ميانگين بارش ماهانه و روند افزايشي ميانگين دما خواهد بود. همچنين، كاهش ميزان رواناب ناشي از تغيير كاربري اراضي (5/6درصد) نسبت به اثر تغيير اقليم در اين حوضة كوهستاني (7/10درصد) كمتر است.

Introduction Land use and climate change and its impacts on water resources and hydrological regime have always been the most important problems in recent decades in Iran. These environmental risks will have direct and indirect impacts on health, economy and society by accelerating the hydrological cycle, drought and flood. Some researchers have examined the impacts of climate and land use change on extreme rainfall, runoff and flood events. Water resources have been investigated in different basins using hydrological models and GIS in arid and semi-arid regions via different scenarios and strategies. They have identified a serious increasing trend of extreme rainfall and drought intensity and duration due to land use change and climate change. Western Iran has experienced an agriculture growth and land use change that can alter evaporation patterns and affect the more frequent occurrence of drought and flood extremes largely due to climate change in this mountainous region. Definitely, there is an increasingly notable challenge in management of water resources, prediction of future changes in land use and climate variabilities, and human activities. In a watershed, climate change and human activities both contribute to the hydrological cycle, and this result has been supported by many researches. In this study, climate change scenarios and land use change models are coupled with a hydrological model to study impacts of these changes on runoff in a mountain catchment in western Iran. Materials and methods Under the assumption that runoff is affected only by land use and climate changes, the effects of climate changes on runoff were studied using SWAT model. This hydrological model calibrated for the period of 2002 to 2007 and was then validated for the period of 2008 to 2010, and after that it is operated in base (2014) and future (2042) period. The required data are including a Digital Elevation Model (DEM), soil properties, vegetation, land use, climate observations, and discharge observations in Garin dam gauging station. Land use in the Garin Basin was extracted from the Natural Resources Department of Hamadan in 1986. These maps have been produced from Landsat 8 images in 2000 and 2014. Additionally, Land use map has been predicted for 2042 using Markov and CA Markov models based on transition probabilities. Curve number can reflect the capacity of runoff yield for the land cover with a continuous spatial distribution. Based on land use maps of two periods and soil type data of Garin catchment, CN distribution maps in the same periods were obtained with spatial interpolation. To predict the future climate, the HADCM3 model was used and its outputs were scaled up with SDSM model. SDSM Model used for down scaling of rainfall and temperature data obtained in Hadcm3 output for prediction of Garin future climate. Results and discussion The SWAT model is performed well in both the calibration and validation periods, accurately simulating the outlet flows according to the model performance criteria after the sensitive parameters were optimized. The simulation coefficients for calibration and validation are presented in Table 1 and 2. The results show that the forest area will be increased and rangeland will be decreased until 2042 (table3). The Result of Markov chain and CA Markov Chain analysis indicate that land use change will make less the runoff rate under A2 and B2 scenarios in 2042. The results reveal that climate change impacts on reduction of runoff is more than land use change during 2042 to 2050 compared with 2000 to 2010. Table1. The model criteria in Calibration and validation for discharge simulation in Garin catchment