مقایسه ی روش‌های هیدروگراف واحد Scs و Uniform در برآورد حداکثردبی سیلاب حوضه ی عموقین

سیلاب در مطالعات مربوط به بهره‌برداری از منابع آب، ساخت سدها، مدیریت حوضه و مطالعات هیدرولوژیکی اهمیت زیادی دارد. بنابراین، تا حد زیادی دقت این مطالعات و ایمنی تاسیسات آبی بستگی به روش‌های مطاله سیلاب دارد. این مطالعه با هدف مقایسه ی روش‌های  هیدروگراف واحد scs و uniform  در تعیین بیشینه ی دبی سیلاب با استفاده از مدل wms در حوضه ی عموقین واقع در استان اردبیل، انجام شده است. مساحت حوضه ی مذکور 78 کیلومتر مربع بوده و برای تعیین مقدار cn منطقه از تلفیق نقشه ی کاربری اراضی و گروه هیدرولوژیک خاک (b ,c ,d) به‌دست آمده از تحلیل تصاویر ماهواره لندست 8 در نرم افزار idrisi32 به‌دست آمد که برابر 78.7محاسبه گردید. نتایج واسنجی (re=7.17، rmse=0.44) و اعتبار‌سنجی مدل (re=2.51، rmse=0.0042 ) با وقایع بارندگی رواناب در منطقه نشان‌داد که میزان حداکثر دبی سیلاب و حجم سیلاب برآورد شده توسط روش scs به‌خوبی با مقادیر مشاهده شده مطابقت دارد. همچنین نتایج حداکثر دبی روش‌های scs وuniform  در دوره ی بازگشت‌های 25، 50 و 100 ساله مقایسه شد و نتایج نشان داد میانگین برآوردهای روشuniform  تقریباً 5 درصد بیشتر از روش scs می‌باشد و بر اساس آزمون t استیودنت، تفاوت بین مقادیرuniform  و scs در سطح اعتماد یک‌درصد معنی‌دار نگردید. نتایج نشان‌داد در صورتی که زمان تاخیرحوضه با درنظر گرفتن سیلاب واقعی حوضه، محاسبه گردد، روش scs دقیق‌تر از روشuniform  قادر به شبیه‌سازی سیلاب در منطقه است، با توجه به حساسیت بالای روش scs به توزیع بارندگی، تحلیل بارندگی در منطقه ی مورد مطالعه، برای دستیابی به نتایج مطلوب لازم بوده و توزیع باران و توزیع زمانی آن باید حتی‌الامکان نزدیک به مقادیر واقعی در منطقه باشد.

Comparison of Uniform and SCS Unit Hydrograph Methods to Estimate Maximum Flood Discharge of Amughin Basin

1IntroductionFlood discharge is of high importance in studies regarding water resource exploitation, flood control, construction of dams, basin management, and hydrologic studies (Alzahrani et al, 2017). Therefore, to a large extent the accuracy of these studies and the safety of water constructions depend on flood study methods. Flood is a natural phenomenon that threatens the life and properties of a large number of people all over the world, and it is impossible to manage water resources in basins without the accurate determination of the peak flood discharge (Badri et al, 2017). The advances in flood estimation techniques have made it possible to use rainfallrunoff models to assess the hydrographic properties of the flood in watersheds and decrease the risks of the flood. Therefore, this study was carried out to compare the SCS unit hydrograph and Uniform methods in determining the peak flood discharge with WMS model in Amughin basin of Ardabil province.2MethodologyAmughin basin with an area of approximately 78 km2 is located in the northwest of Iran. The physiographic features were extracted using the basin map (scale: 1:25000) and WMS model. This study applied Arc GIS 9.2 and Idrisi32 software to obtain the properties of the studied basin using DEM (Digital Elevation Map) of the National Cartographic Center, NCC. Remote sensing methodology was utilized to study the  geographical land use changes occurred during the study period. Landsat images of TM and ETM+ of Amughin basin area were collected from the USGS Earth Explorer web site. After image preprocessing, unsupervised and supervised image classification were performed to classify the images into different land use categories. In general, soil hydrologic groups were divided into three subgroups of B, C, and D and CN value of 78.7 was estimated for the Amughin basin based on the geological examination, permeability, vegetation, and hydrologic conditions of the basin soil.3ResultsThe model calibration results showed that the simulated peak discharge and flow volume were in good correspondence with the observed values (RE%= 7.17, RMSE= 0.44). Thus, the calibration results were used for optimum values of parameters. The model was validated using two rainfall events and the model performance indices were acceptable in both cases (RE%= 2.51, RMSE= 0.0042) in SCS method. To evaluate and test model validation, two rainfall events, were used. That the model performance indices were acceptable. Distribution of CN amount in the area showed that the upstream flow had higher CN values and consequently increased flood volume in these areas. Based on the values of obtained CN, the amount of peak flood discharge was calculated for return periods of 25, 50, and 100 years.4Discussion and conclusionAccording to the results, the SCS model has good agreement with experimental results among the different methods used for estimating flood discharge in the northwest of Iran. In fact, this model requires calibration in the study region. In small watersheds in the northwest of Iran, the SCS model yields better results than the Uniform method because the conditions required for using this model are satisfied in these basins. Moreover, the results obtained from this method can be closer to actual values provided that the watershed concentration time is calculated more accurately. Our results also showed that the SCS model has a high sensitivity to rainfall distribution across the region and that  the rainfall across the region needs to be analyzed to obtain desirable results. Besides, the rainfall distribution and its time distribution should be close to the corresponding values in the region. A comparison between the obtained results of peak discharge from the SCS and Uniform methods in return periods of 25, 50, and 100 years revealed that the average estimates of the Uniform were approximately 5% higher than the SCS method. According to paired Ttest, the difference between Uniform and SCS values were not significant at a confidence level of 0.01. Overall, the results obtained from this method can be closer to actual values if the watershed lag time is calculated more accurately using the floods occurred in the studied basin.