بررسی اثر تغییر کاربری اراضی بر هیدروگراف سیل رودخانه کشف‌رود با تحلیل نتایج روش Scs-Cn

تغییر کاربری اراضی و به طور کلی پوشش اراضی تاثیر مستقیمی بر تغییر رژیم هیدرولوژیکی دارد و ویژگی‌های سیل در هر منطقه می‌تواند تابع کاربری اراضی آن حوضه باشد. این تحقیق با هدف تعیین میزان تغییر کاربری اراضی طی 28 سال (1366 تا 1394) و تاثیر آن بر هیدروگراف سیل رودخانه کشف‌رود انجام شد. برای این منظور، نقشه کاربری اراضی با استفاده از نرم‌افزار erdas imaging 2014 از تصاویر لندست tm سال 1366 و oli سال 1394 با دقت 87.3 و در 10 کلاس کاربری استخراج شد. سپس با تلفیق نقشه‌های کاربری اراضی و گروه‌های هیدرولوژیک خاک در محیط gis نقشه شماره منحنی رواناب، حاصل شد. در مرحله بعد وقایع بارشرواناب در مدل hec-hms با استفاده از روش scs-cn شبیه‌سازی شد و پارامترهای هیدرولوژیکی مربوط به سال 1366 و 1394 واسنجی و اعتبارسنجی شد. نتایج نشان داد به علت تغییرات کاربری اراضی که در جهت کاهش مساحت مراتع متوسط و افزایش مساحت مراتع فقیر اتفاق افتاده است، متوسط شماره منحنی حوضه در این مدت از 77.5 به 78.4 افزایش یافته است. نتایج شبیه‌سازی نشان داد که میزان دبی اوج و حجم سیلاب حوضه کشف‌رود طی دوره مورد مطالعه به طور متوسط به‌ترتیب 15.2 و 13.7 درصد افزایش یافته ولی زمان رسیدن به دبی اوج هیدروگراف سیل تغییری نداشته است.

The Effect of Land Use Changes on the Flood Hydrograph in the Kashaf-Rood River by Analyzing of SCS-CN Results

Introduction: Runoff estimation is one of the main concerns of hydrologists and plays a key role in various engineering calculations and designs. Many factors such as climate, topography, soil properties, land cover, etc, are involved in producing surface runoff. Land use and land cover changes have a direct impact on the hydrological cycle in the ecosystem. The most common model of surface runoff estimation is the curve number model developed by the US Soil Conservation Service (SCSCN). Accurate estimation of its important parameters increases its precision and performance. Land use is one of the most important parameters of this model.Remote sensing (RS) and geographic information system (GIS) technologies are used in order to increase its speed and accuracy of estimation. One of the problems that have occurred in the KashafRood Basin is the extensive land use changes that may cause changes in peak discharge and surface runoff volume. In this study, due to the great importance and impact of land cover change on increasing flood risk, the effects of land use change over 28 years (from 1987 to 2015) on flood hydrograph characteristics were investigated.،Materials and Methods: The KashafRood basin is a part of the GharaGhum basin. The total area of the basin is 16779 square kilometers with the highest and lowest elevation of 3235 and 378 meters above sea level, respectively . The length of the KashafRood River from the highest point to the outlet of the basin is about 374 km and its average and gross river slope are 0.0028 and 0.0043 m/m, respectively. The digital elevation model was used to calculate the topographical properties, hydrological properties and geometrical corrections required on satellite images. In this research, the data of the Global Digital Elevation Model (ASTER) with a spatial accuracy of 30 m was used. Also, the soil hydrologic group map prepared in GharaGhum water resources balance studies was used. Since no land use change occurs in the short term and can be detected at long intervals, a 28year interval was chosen for satellite imagery. In general, five images of Landsat satellite are needed for full coverage of the KashafRood Basin. For the oldest data, Landsat 5 images and for the latest data, Landsat 8 images were used. ERDAS IMAGINE 2014 software was used to digitally process satellite images. The images were classified in three methods: The Minimum distance, Mahalanobis distance and the Maximum Likelihood. In order to select the appropriate method, after applying different classification algorithms for the image of 2015, the accuracy of their classification was evaluated and, the image of 1987 was also classified based on the selected method. By combining soil hydrological group and land use map derived from Landsat satellite imagery using ArcGIS 10.3 software, the curve number maps for 1987 and 2015 were prepared. In the present study, the US soil conservation service standard curve number method (SCSCN) was used to calculate the amount of rainfall and losses in the HECHMS model. For the calibration of the HECHMS model, four flood events at the bridge of Khatun KashafRood hydrometric station with relatively concomitant precipitation were selected. Three flood events were used for calibration and one flood event for validation.،Results and Discussion: The images were classified into three methods: The Minimum distance, Mahalanobis distance, and the Maximum Likelihood. Comparing the results of these three methods showed that their overall accuracy in evaluating and identifying land use was 78.5, 83.7 and 87.3, respectively. Thus, the maximum likelihood algorithm was used to classify the images and the image of the year 1987 was classified with this method. Ten land use classes were identified in the study area. The results showed that during the 28 years of study, the area of rocky lands and rangelands did not change. The highest percentage of change was due to water zones, poor rangelands and residential lands, which increased by 189, 143 and 50 percent, respectively. The highest amount of increase in the area occurred in the poor rangelands, which 1514 km2, and the highest decrease occurring in moderate rangelands which is 1278 km2. By combining soil hydrological group maps and land use maps in ArcGIS software and using standard tables, the curve number maps for 1987 and 2015 were prepared. The weighted average of the curve number in the mean moisture conditions for 1987 and 2015 was 77.5 and 78.4 units, respectively. After performing the calibration and validation steps, the HECHMS hydrological model was used to investigate the impact of landuse change on the flood hydrograph of the KashafRood River between 1987 and 2015. According to the results, in all four events which were studied, landuse changes have increased the peak of discharge and the flood volume over the 28 years of study. On average, the peak flood discharge in 2015 was 15.2% higher than the peak flood discharge in 1987, and similarly, the flood volume increased by 13.7% during the study period.،Conclusion: In conclusion, it can be derived that in recent decades, landuse changes which were caused by human interference, affected the flood characteristics and increased the risk of flooding in the KashafRood river. Therefore, land use must be managed and prevented further destruction of natural resources to prevent flooding in the area.