تغييرات ماهانه شدت و تمركز رواناب سطحي در رودخانه‌هاي حوضه ي گرگانرود استان گلستان

The Monthly Changes of Surface Runoff Intensity and Concentration in the Gorgan-Roud River, Golestan Province

چكيده تحليل تغييرات رژيم جريان و تداوم آن و تعيين عوامل موثر در بي‌نظمي‌ها از پيش‌نيازهاي اصلي مديريت و بهره‌برداري بهينه از رودخانه‌ها به‌ عنوان يكي از منابع اصلي آب مصرفي مي‌باشند. در اين پژوهش ميزان تغييرات ماهانه حجم رواناب در 20 ايستگاه‌ هيدرومتري واقع در استان گلستان در يك دوره ي 38 ساله (????-????) با استفاده از شاخص تغييرات سالانه (ضريب تعديل توزيع سالانه و شدت تمركز) مورد ارزيابي قرار گرفت. براساس داده‌هاي ماهانه دبي در سال‌هاي مختلف، حجم رواناب ماهانه و سالانه ي ايستگاه‌هاي مورد مطالعه در طول دوره ي آماري محاسبه گرديد. هم‌چنين روند شاخص تغييرات سالانه با استفاده از آزمون من-كندال مورد تجزيه و تحليل قرار گرفت. نتايج پژوهش نشان داد كه از نظر توزيع ماهانه، حداكثر مقادير حجم رواناب ايستگاه‌ها در فصل بهار و خصوصاً در ماه فروردين اتفاق افتاده است كه با مقادير بالاي بارش بهاره و اثر آن در ميزان آبدهي بالاي رودخانه‌ها در اين فصل در ارتباط بوده است. هم‌چنين، بيش‌ترين مقادير حجم رواناب در ايستگاه‌هاي آق‌قلا، قزاقلي و بصيرآباد به‌ترتيب داراي متوسط حجم رواناب سالانه (9/33، 5/33 و 6/32 ميليون مترمكعب) مي‌باشند. در حالي‌كه بيش‌ترين يكنواختي در وقوع رواناب در ماه‌هاي سال مربوط به ايستگاه‌هاي نوده‌خاندوز، تمر، گاليكش و قلي‌تپه به‌ترتيب با مقادير ضريب تعديل توزيع سالانه برابر 19/0، 21/0، 23/0 و 24/0 درصد بوده است. كم‌ترين شدت تمركز رواناب نيز مربوط به ايستگاه‌هاي نوده‌خاندوز و تمر به‌ترتيب برابر با مقادير 26/0 و 25/0 درصد مي‌باشند، در صورتي‌كه ايستگاه‌ راميان (با مقدار شاخص 62/0 درصد) داراي بيش‌ترين شدت تمركز بوده است. نتايج حاكي از رابطه ي مستقيم و معني‌دار ميان ضريب تعديل توزيع سالانه ي دبي و شاخص شدت تمركز با ضريب همبستگي 60/0 مي‌باشد. بنابراين بيش‌ترين روند كاهشي و افزايشي در طول دوره ي آماري با استفاده از آزمون من-كندال به‌ترتيب در ايستگاه‌هاي شيرآباد و نوده‌خاندوز در سطح معني‌دار 05/0 مشاهده شده‌اند.

Introduction Considering the relative stability of the physical characteristics of a watershed, the variability of the precipitation over space and time, and the direct relationship between rainfall and runoff, the variations of runoff can be expected and analyzed to understand the nature of variability. Determining changes in the amount of runoff caused by rainfall and detecting the time of riversʹ floods can provide a prediction of floodsʹ occurrence and, consequently, reduce their damages. The increasing importance of water resources management in recent years, erosion, and sediment highlights the need for understanding the riversʹ behavior and regimes. Regarding the changes in the river flow rate, estimating temporal and spatial variations of runoff changes can be effective in determining and controlling the dependent processes of soil erosion in a watershed and river bank, droughts, floods, and water quality and utilization. The analysis of the river flow variability, its duration and influencing factors, is necessary for an optimal river management/operation as the main sources of water uses. Methodology The monthly and annual runoff volumes of different stations were calculated based on the monthly discharge data in different years during the study period. Then, the variability indices were used to study the seasonal variations in the runoff volume at each hydrometric station. Next, using Annual Distribution of Regulating Coefficient and Concentration Rate indices, the seasonal variation in runoff volume of twenty river gauge stations located in Golestan Province were evaluated in 38 years. The values of Annual Distribution of Regulating Coefficient indicated the uniformity/non-uniformity of changes in runoff volume at the studied river gauge stations. In addition, the annual variation of runoff volume was plotted in triple diagram models based on average runoff volume and time variables. The Kriging method was also used to draw the triple diagram models using two independent variables in a surfer environment. The Annual Distribution of Regulating Coefficient and Concentration Rate indices were considered as dependent variables. The variability of the implemented indices were analyzed over a time period of 38 years. Results and Discussion According to discharge data in different years, the monthly and annual runoff volumes of the stations were calculated during the study period. Based on the monthly spatial distribution, the results showed that the maximum amount of runoff volume of the stations were observed in March. The highest amount of surface runoff amounts occurred in Aghghala, Ghazagli, and Basirabad which respectively had an average annual runoff of 33.9, 33.5, and 32.6 million cubic meters. The highest uniformity in runoff occurrence was related to Nodehkhandoz, Tamar, Galikesh, and Gholitappeh stations, respectively with an annual Distribution of Regulating Coefficient of 0.19, 0.21, 0.23, and 0.24. The lowest Rate of runoff concentration was at Nodehkhandoz and Tamar stations respectively with 0.26% and 0.25%. The results also indicated a direct and significant relationship (R2 = 0.60) between Annual Distribution of Regulating Coefficient and Concentration Rate (p < .05). Ramian station had the highest Concentration Rate with a value of 0.62%. The highest significant decreasing and increasing trends, in Mann-Kendall test, were observed at Shirabad and Nodehkhandoz stations Conclusions According to the findings, there was a correlation between the annual distribution of regulating coefficient and the concentration rate. The higher values of the Annual Distribution of Regulating Coefficient and the Concentration Rate of runoff volume can be attributed to physiographic properties of watershed such as its slope, vegetation, and soil permeability. In other words, the process of changes in the runoff volume at these stations can indicate the temporal and spatial variations of precipitation, human protection measures such as dam construction in the basin, or the amount of permeability during the statistical period. In conclusion, with the non-uniform distribution of runoff volume in different months of the year, it can be expected that variations between the minimum and maximum values of runoff volume will also be high. Indeed, the higher the uniformity of the monthly distribution of runoff volume, the lower the variations between the minimum and maximum changes in the runoff volume. Variations in the amount of monthly runoff in the studied area can be related to the characteristics of the area, the hydrological response, and land use (agricultural land plowing season), as one of the main factors controlling runoff.