تغييرات مكاني رواناب، رسوب و آستانه شروع رواناب در حوزه آبخيز قره‌شيران اردبيل

Spatial variations of runoff, sediment and runoff threshold of Gharehshiran watershed in Ardabil Province

تعدد عوامل مؤثر در خصوصيات حوزه آبخيز منجر به تغييرات مكاني و زماني در فرآيندهاي توليد رواناب و رسوب مي‌شود. پژوهش حاضر با هدف ارزيابي تغييرات مكاني مقادير رواناب، رسوب و آستانه شروع رواناب با استفاده از داده‌هاي شبيه‌سازي بارش در حوزه آبخيز قره‌شيران اردبيل انجام گرفت. شبيه‌سازي بارش با استفاده از دستگاه باران‌ساز در 45 نقطه در سازندهاي مختلف زمين‌شناسي حوزه آبخيز انجام و پس از اندازه‌گيري مقدار رواناب و رسوب در هر نمونه متغيرهاي مورد مطالعه با روش كريجينگ درون‌يابي و تغييرات مكاني ارزيابي و ارتباط مكاني مقادير اين متغيرها مورد بررسي قرار گرفت. نتايج نقشه‌هاي درون‌يابي نشان داد كه سازندهاي بخش‌هاي بالادست حوزه آبخيز (گدازه‌هاي داسيتي و تراكيتي) در آستانه‌هاي پايين (17/3-99/1 دقيقه) رواناب توليد مي‌شود و به سمت سازندهاي بخش‌هاي پايين‌دست (مانند پادگانه‌هاي آبرفتي قديمي) آستانه توليد رواناب (25/7-13/6 دقيقه) افزايش مي‌يابد. مقدار رواناب در بالادست حوزه (گدازه‌هاي داسيتي و تراكيتي)، زياد (25/7-07/6 ليتر بر مترمربع) ولي مقدار رسوب كم (66/1-25/1 گرم بر ليتر) و در پايين‌دست حوزه (مانند پادگانه‌هاي آبرفتي قديمي) مقدار رواناب توليدي كم (50/3-20/2 ليتر بر مترمربع) و مقدار رسوب توليد شده بيشتر (5/3-25/2 گرم بر ليتر) مي‌باشد. نتايج ارتباط بين مقادير تغييرات رسوب، رواناب و آستانه شروع رواناب در نرم‌افزار Surfer نشان داد كه در آستانه‌هاي بالاي 4 دقيقه و رواناب‌هاي بين 5/5-5/2 ليتر، مقدار رسوب بيشتر از حدود 2 گرم بر ليتر مي‌باشد و به‌طور كلي مي‌توان گفت كه در آستانه‌هاي پايين اثر توليد رواناب بر توليد رسوب در حوزه بيشتر است و در آستانه‌هاي بالا اثر توليد رواناب كاهش مي‌يابد.

Diverse factors affect the characteristics of the watershed that lead to spatial and temporal variations in the runoff and sediment production processes. Runoff and sediment are the main important elements in the hydrological cycle, and their changes directly affect river systems and sedimentary environments; and their spatial and temporal variations change the morphology of the rivers. Due to differences in soil characteristics, source materials and geological formations, vegetation and slope in different parts of a region, the amount of runoff and sediment produced in these areas can vary with spatial variations. The purpose of this study is to evaluate the spatial variations of runoff and sediment and runoff threshold using rainfall simulation data in the Gharehshiran watershed in Ardebil Province. Considering the importance of spatial distribution of sampling points across the catchment area, the locations of the samples were determined, taking into account the access path to the points, as well as sampling in different formations through determining the boundaries of the study area. The field experiments and simulation of precipitation were carried out using a 1×1m rainfall simulator in 45 points in different geologic formations of the watershed area. The amount of runoff and sediment were measured in each experiment along with recording the threshold time of runoff generation. The measured variables were mapped and interpolated by using Kriging method over the study area. To assess the accuracy of the interpolation results, 7 samples were selected randomly and the Root Mean Square Error (RMSE), Mean Absolute Error (MAE) and Mean Bias Error (MBE) statistical measures were calculated by comparing observational and estimated values. Then, the correlation between the studied variables in various geological formations was evaluated using Pearson correlation analysis. The relationship between sediment and runoff amount, and runoff threshold time were also evaluated using a triple diagram model. The results of the interpolated maps showed that the lowest values of runoff time threshold (1.99-3.17 min) were observed in the geological formations of upper part of the watershed having dacite and tracite igneous, volcanic rocks. While the runoff time thresholds were increased (6.13-7.25 min) in the low land areas with the old alluvial terraces. The amount of generated runoff in the upper hillslopes of the watershed with dacite and tracite rocks was estimated as (6.07-7.25 lit/m2), and the amount of sediment was low (1.25-1.66 g/l). Meanwhile, in the lower parts of old alluvial terraces, the amount of runoff production was low (2.20-3.50 lit/m2) and the amount of produced sediment was higher with values of (2.25-3.5 g/l). The results of correlation analysis showed that the correlation coefficients between runoff threshold and runoff volume were significant at 0.01 significant level (r = -0.802). Also, a significant negative correlation (r = -0.672), were observed between runoff and sediment values. The relationship between the runoff time threshold and the sediment content was positive at significant level of 0.01 (r = 0.900). The results of interdependency between the sediment, runoff and runoff time threshold values using triple diagram models showed that the sediment amount was about 2g/l at high runoff time thresholds of 4 minutes with 2.5-5.5 lit/m2 runoff amounts. In general, it can be said that the sediment production in the study area is strongly under the effects of runoff amounts in lower time thresholds of runoff. As a remark, the results pointed out that the internal relationship of runoff and sediment production are affected by a variety of effective factors which requires comprehensive studies to reach a final conclusion.