مقایسه‌ی کارایی مدل واعظی و نموگراف ویشمایر برای برآورد فرسایش‌پذیری خاک در اراضی لسی (مطالعه موردی حوزه آبخیز عرب‌قره‌حاجی استان گلستان)

این پژوهش، به‌ منظور ارزیابی عامل فرسایش‌ پذیری خاک در اراضی لسی شرق استان گلستان و در حوزه آبخیز عرب‌قره‌حاجی به مساحت 2695 هکتار انجام شد؛ بدین منظور، از هر جزء واحد اراضی شامل تراس رودخانه‌ای، فلات لسی و تپه، در مجموع 42 نمونه ی سطحی (100) سانتی متر برداشت شد. سپس خصوصیات فیزیکوشیمیایی خاک شامل بافت خاک، آهک خنثی، ماده آلی، درصد ذرات شن درشت و ﻣﺠﻤﻮع درﺻﺪ ﺳﻴﻠﺖ و شن ﺧﻴﻠﻲ رﻳﺰ در آزمایشگاه اندازه ‌گیری شد. هدایت هیدرولیکی اشباع خاک نیز در آزمایشگاه توسط دستگاه اتوماسیون هدایت هیدرولیکی اشباع خاک با روش بارافتان اندازه‌ گیری شد. در این پژوهش فرسایش پذیری واقعی خاک، به وسیله ی شبیه ‌ساز باران در صحرا اندازه ‌گیری و تمامی نقشه‌ های موردنیاز در نرم افزار gis و با روش idw ترسیم شد. در بررسی بافت خاک مشخص شد که 90 درصد نمونه ها، دارای بافت سیلتی لوم و 10 درصد، دارای بافت لومی است. همچنین میانگین آهک در حوزه ‌ی مورد مطالعه، در حدود 29 درصد اندازه‌ گیری شد که به ‌نسبت زیاد است. نتایج نشان داد که روش واعظی، میزان فرسایش ‌پذیری خاک را در حدود 30 برابر کمتر از روش نموگراف ویشمایر برآورد می‌کند. نتایج آزمون f و گیمز هاول نیز نشان داد که با احتمال 95 درصد، بین مقادیر فرسایش ‌پذیری از روش واعظی با داده ‌های واقعی اختلاف آماری وجود ندارد؛ در حالی که بین روش ویشمایر با داده‌ های واقعی اختلاف آماری مشاهده می شود. نتایج روش رگرسیون چندگانه‌ ی خطی و روش گام ‌به‌ گام نیز نشان داد که به ترتیب شاخص های درصد سیلت، موادآلی و آهک خنثی، مهم ترین پارمترهای تاثیرگذار در تعیین فرسایش ‌پذیری خاک است (r2=0.9648). در این پژوهش به‌دلیل بالا بودن درصد آهک خنثی در تعدادی از نمونه ‌ها (بیش از 30 درصد)، محاسبه‌ ی عامل فرسایش پذیری خاک با استفاده از روش واعظی ممکن نبود. درنتیجه، استفاده از این روش برای مناطق مشابه نیازمند واسنجی و بروزرسانی است.

Efficiency comparison of the Vaezi model and Wischmeier nomograph for estimating soil erodibility in loess soils (Case study: Arab-Qareh-Haji watershed in Golestan province)

Extended abstract1 IntroductionOne of the important parameters for identifying the erosionsensitive points is the use of soil erodibility factor, which is determined by two methods; direct and indirect methods. The direct method for calculating soil erodibility has good accuracy. However, it is economically expensive. Therefore, this method is not considered by experts. The indirect and conventional method for calculating the soil erodibility is the Wischmeier nomograph method, which was presented for noncalcareous and semiwet soils of the United States. Since the majority of the soils in Iran are calcareous, hence in this research, the Vaezi method, which is designed for this type of soil, was used. Loess is one of the most important Quaternary sedimentary formations in northeastern Iran, which has variable sedimentation and erosion potential due to its physicochemical properties. This study was conducted to evaluate and compare the estimated values of soil erodibility (K) from two methods of Wischmeier nomograph and Vaezi method in calcareousloess soils of ArabQarehHaji watershed in the east of Golestan province.2 MethodologyThe study watershed with an area of 2595 hectares is located in the northeast of Golestan province. To perform this research, after the initial studies, the components of the land unit in the study area (including river terrace, loess plateau, and hill) were selected as the working unit. For this purpose, 42 surface samples (010 cm depth) were collected from each part of the land unit. In this study, the standard laboratory methods were used to determine the soil physicochemical parameters. Soil physicochemical properties, including soil texture, neutral lime, organic matter, coarse sand, very fine sand+silt, and soil structure, were measured in the laboratory. The saturation hydraulic conductivity of soil in the laboratory was estimated using the falling head method. In this research, the actual soil erodibility was measured using a rain simulator in the field (in plots of 1 m2). Then, spatial changes of soil erodibility in the study area were plotted using GIS software and inverse distance weighting (IDW) method. Ftest and GamesHowell at the probability level of 0.05 were used to compare and determine the statistical difference between the estimated erodibility values (the Wischmeier nomograph and Vaezi methods) with actual data. To estimate the most effective parameter in the soil erodibility using the Vaezi method, multiple linear regression and stepwise methods were used. Statistical tests were performed using Minitab software.3 Results The average silt particles in the soil were measured about 60% in the ArabQarehHaji watershed, which plays an important role in the soil erodibility. In this watershed, the ratio of fine sand to coarse sand was calculated about 18, and this can be one of the main causes of runoff in this area. Soil texture assessment in this watershed revealed that around 90 percent of soil samples have silt loam texture, and the rest of the samples has loam one. Also, the average lime in study area was estimated at about 29%. This amount is higher than the standard lime (15%) in the soil to create stable aggregates in the wet state. Then, soil erodibility values were determined by three methods: Wischmeier nomograph, Vaezi method, and rain simulator (actual data). The results showed that the value of soil erodibility using Wischmeier nomograph was estimated at about 30.4 and 18.8 times higher than for the Vaezi method and actual data, respectively. The results of Ftest and GamesHowell test with 95% probability showed that there was no significant difference between the soil erodibility from the Vaezi method and the actual data, while there was a significant difference between the Wischmeier method and the actual data. Results of multiple linear regression and stepwise methods showed that the most important variables in estimating the soil erodibility are silt, organic matter, neutral lime factors, and coarse sand, respectively (R2 = 0.9648, R2adj = 0.9539). In this study, it was not possible to estimate the soil erodibility factor by Vaezi method due to the high percentage of neutral lime for a number of samples.4 Discussion ConclusionsIn this study, the average lime was estimated at about 29%. This amount is higher than the standard lime (15%) in the soil to create stable aggregates in the wet state. Therefore, it seems that excess lime in the wet soil can reduce the stability of aggregates. In this watershed, a large number of tunnels and gully were observed, which indicates that this soil is erodible. The statistical analysis results showed that the soil erodibility values from the Vaezi method in this watershed were relatively close to the actual data but were less estimates, while the Wischmeier nomograph method was significantly different from the actual data and was overestimated. However, for areas where the amount of neutral lime was over 30%, the Vaezi method was not able to calculate the soil erodibility factor. Hence, for the areas where the percentage of neutral lime is lower than 30%, the use of the Vaezi method is more suitable than the Wischmeier nomograph method, and for areas where the percentage of neutral lime is higher than 30%, the model needs to be calibrated and updated.