year 10, Issue 4 (Winter 2021)                   E.E.R. 2021, 10(4): 39-57 | Back to browse issues page

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Eidi R M, Jandaghi N, Ghareh Mahmoodlu M, Ghorbani Vagheie H, Mohammadi A. 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). E.E.R.. 2021; 10 (4) :39-57
Assistant Professor in Engineering Hydrology, Rangeland and Watershed Management Department, Faculty of Agriculture & Natural Resources, University of Gondad Kavous, Gondad Kavous ,
Abstract:   (501 Views)
Extended abstract
1- Introduction
One of the important parameters for identifying the erosion-sensitive 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 non-calcareous and semi-wet 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 calcareous-loess soils of Arab-Qareh-Haji watershed in the east of Golestan province.
2- Methodology
The 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 (0-10 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. F-test and Games-Howell 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 Arab-Qareh-Haji 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 F-test and Games-Howell 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, R2-adj = 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 & Conclusions
In 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.
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Received: 2020/10/30 | Published: 2021/01/29

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