تعیین پارامترهای اساسی در فرآیند ذوب برف در تخمین آب‌نگار سیل در حوضه آبریز کارون

نویسندگان

چکیده

هیدرولوژی برف پیچیده‌تر از باران بوده و معمولاً به صورت مستقیم در تولید رواناب اثرگذار نیست. در حوضه‌های کوهستانی ذوب برف دربرگیرنده بخش اصلی جریان کل در خروجی حوضه است. پس در این بررسی،‏ به واسنجی و صحت‌یابی دو پارامتر درجه حرارت بحرانی و ضریب درجه روز در حوضه آبریز کارون،‏ به عنوان یکی از مهم‌ترین حوضه‌های کوهستانی ایران پرداخته شد. شبیه‌سازی سیلاب با نرم‌افزار HEC-HMS انجام شد. بهترین مقدار ضریب درجه روز و درجه حرارت بحرانی برای منطقه حوضه آبریز رودخانه کارون تا خروجی پل شالو به ترتیب برابر 2‎/3 میلی‌متر بر درجه سانتی‌گراد روز و 5‎/2 درجه سانتی‌گراد به دست آمد. بررسی خصوصیات هیدروگراف سیل‌های منتخب در این بررسی و ارزیابی مقادیر به دست آمده بر اساس شاخص‌های آماری مجذور مربعات خطا و ضریب تبیین نشان داد که با واسنجی پارامتری درجه حرارت بحرانی و ضریب درجه روز به عنوان پارامتر‌های مؤثر بر رواناب حاصل از ذوب برف برازش خوبی بین هیدروگراف مشاهده‌ای و شبیه‌سازی شده حاصل شده است. همچنین با در دست داشتن مقادیر بهینه شماره منحنی،‏ زمان تأخیر و تلفات اولیه به عنوان پارامترهای مؤثر در پیدایش رواناب مستقیم در حوضه می‌توان برازش دقیقی بین هیدروگراف مشاهده‌ای و محاسباتی با مدل HEC-HMS در شبیه‌سازی رواناب حاصل از ذوب برف و بارش در منطقه ایجاد کرد.

کلیدواژه‌ها


عنوان مقاله [English]

Determining the basic parameters in snow melt process for estimating flood hydrograph in Karoun basin

نویسندگان [English]

  • Fatemeh Karandish
  • Jahangir Porhemat
  • Kioumars Ebrahimi
چکیده [English]

Mountainous area has a considerable share in supplying water demand in the arid regions, because the snow consists a large part of the rainfall in such regions which could be saved in the area located in the high elevations for a long time. Therefore, snowmelt runoff consists a large part of the total runoff in the mountain watersheds. The snow hydrology is much complicated than rainfall hydrology, since there is always a lag time in occurring snowmelt runoff in the basins' outlet. Thus, finding out simple methods for calculating such runoff is so important. In this research, after calibrating and validating the critical temperature and degree-days coefficient, the HEC-HMS model was applied to simulate the snowmelt runoff in Karoun basin, which is one of the most important mountainous watersheds in Iran.
In this research, a large part of Karoun watershed up to PolShaloo outlet was selected. This region is located in the south-west of Iran, which lies between 50o23'37" to 51o54'29" E longitude and between 30o16'50.4" to 31o32'18.3" N latitude. In this area, about 50 percentage of total precipitation falls as snow and usually occurs during November-May. The HEC-HMS model was applied for runoff simulation. In this regard, first the whole basin was divided into eight sub-basins entitled Marghak, Barez, Pataveh, Kata, Solegkan, Armand, DazakAbad and PolShalou. Then, based on the Digital Elevation Mode, the sub-basins' physiographic characteristics were extracted in Arc-View environment and using HEC-GeoHMS extension. The values of curve number, lag time and initial loss were calibrated and validated for each of the sub-basins using the observed snow-free runoff events. Thereafter, the hypsometric maps with 100-meter elevation intervals were prepared. The snowmelt pattern was determined for each of the elevation bands by using hypsometric maps and the required thermal data. Moreover, the snow water equivalent was calculated in the first day of the selected snowy events based on the daily calculations of snowmelt. After providing the required input data, the HEC-HMS mode was run for the snowy events and the critical temperature and degree-day coefficient were calibrated and validated for the study area.
The hypsometric maps with 100-meter elevation intervals revealed that 22, 70 and 8 percent of the study area are, located in the areas with the elevation ranges of 693-2000, 2000-3000 and >3000 meter, respectively. Based on the observed data in the snow survey stations in the study area, the correlation coefficient between snow water equivalent and elevations was higher than 90 percentage. Therefore, this equation was applied for determining the snow water equivalent in the beginning of the selected snowy events. The daily calculations of snowmelt based on this equation showed that in the Mach 17, 1998, which is the first day of the first selected snowy event, all the regions with elevations higher than 1943 meter is covered by snow and the non-snow regions covers less than 20 percentage of the study area. The results of the sensitivity analysis of the HEC-HMS model for calibrating the basins' physiographic parameters in the non-snow events revealed that the curve number, initial loss and lag time parameters had the most effect on the outlet discharge. The calibrated values for these parameters ranges among 76.6-91, 5.02-13.96 millimeter and 155-571.12 minutes, respectively. After calibrating and validating the basins' physiographic parameters, the snowmelt parameters were calibrated and validated based on the observed runoff during March 17-19, 1998, which is one of the largest snowy event in the study area. In this stage, the results of the sensitivity analysis demonstrated that critical temperature and degree-day coefficient had the most effect on three characteristics of flood hydrograph, including the peak discharge, the average flood depth and the time of peak discharge occurrence. The increase in the critical temperature usually led to the peak runoff delaying. The exception was for Armand sub-basin due to its high area in which, the increase in the critical temperature, the peak discharge of the flood hydrograph was increased mainly due to increasing the snow melt rate and subsequently the flow rate. The best values for critical temperature and degree-day coefficient, through the calibration process, were obtained 2.5 oC and 3.2 millimeter per oC-day, respectively. Based on the calculated criteria indices in the validation process, applying these values led to the acceptable errors in estimating snowmelt runoff for the flood event occurred through March 25, 1998 to April 8, 1998.
The results of this research showed that among different physiographic characteristics, the curve number, initial loss and lag time parameters had the most effect on the average flood depth, the peak discharge and the time to the peak discharge for the non-snow events. Moreover, the snowmelt runoff is highly influenced by critical temperature and degree-day coefficient and therefore, the HEC-HMs results of simulating runoff in the mountain watersheds will be reliable if the suitable values are selected for these parameters. Based on the results of this study, 2.5oC and 3.2 millimeter per degree-days will be the best values for critical temperature and degree-day coefficient in the study area, respectively.

کلیدواژه‌ها [English]

  • Flood hydrograph-Degree-Day Coefficient-Critical Temperature-HEC_HMS-