Document Type : Research Paper
Authors
1 Soil Science Dep., Faculty of Agriculture, University of Tehran, Karaj, Iran
2 Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran.
3 Department of Soil Science, Faculty of Agriculture, University of Tehran, Karaj, Iran
Abstract
Keywords
Main Subjects
Introduction
Soil erosion, as a major type of land degradation, poses a growing threat to both the environment and human societies. Due to the various types and complex dynamics of erosion processes, the use of empirical models such as the Revised Universal Soil Loss Equation (RUSLE) has become essential. Which estimates soil erosion based on key variables: (R, K, LS, C and P). The WaTEM/SEDEM model is a spatially distributed soil erosion and sedimentation model derived from RUSLE that also incorporates a sediment transport capacity equation to predict sediment delivery within drainage networks. In this study, the risk of soil erosion was mapped and sediment yield was estimated using both the RUSLE and WaTEM/SEDEM models in Hajighoshan watershed, located in Golestan Province.
Method
This study covers a 15-year period (2002–2017) in the Hajighoshan watershed of Golestan Province. Soil loss was estimated using RUSLE. The R factor was determined using the Modified Fournier Index, while the K factor was calculated using the Romkens et al. (1987) equation. The LS factor was derived using the Moore and Wilson (1992) equation, and the C factor was calculated using the Normalized Difference Vegetation Index (NDVI) through the Google Earth Engine platform. The P factor was set to 1 for the entire region, due to the absence of implemented soil conservation measures. To estimate the suspended sediment load at the basin outlet, hydrological and sedimentological data from the Temer station were analyzed over the study period.
The 2004 version of the WaTEM/SEDEM model was used to estimate soil erosion and sediment yield. Model inputs included a digital elevation model (DEM), a land use/land cover map, and RUSLE parameters. All input data were converted to the IDRISI software format (.rst) and standardized to a uniform pixel size of 20 × 20 meters to ensure cell alignment.
Results
The Modified Fournier Index ranged from 20 to 91, the K factor varied from 0.0371 to 0.0437 ton h MJ⁻¹ mm⁻¹, the average LS factor was 3.98, and the C factor ranged from 0.003 to 0.98. The soil erosion risk estimated by the RUSLE model ranged from 0 to 2000 tons per hectare per year across the Hajighoshan watershed, with an average of 61 tons per hectare per year. The annual sediment yields of the watershed showed significant variation (0.270 to 16 million ton per year), mainly due to changes in rainfall patterns and erosivity across different years. These variations were influenced by the intensity of rainfall events, which in turn affected the occurrence of floods in different years. The gross soil erosion estimated by WaTEM/SEDEM showed a high correlation with RUSLE at the watershed level, but it had a systematic overestimation. While the mean values were closed, there was no a good agreement between the annual specific sediment yield estimated by WaTEM/SEDEM and those calculated based on sediment rating curve and flow discharges. Comparing the gross soil erosion and sediment yield estimated by the WaTEM/SEDEM model, suggested that a large portion of the eroded soil was redeposited within the watershed and did not reach the watershed outlet.
Conclusion
The modelling of soil erosion and sediment yield using the WaTEM/SEDEM model is based on average annual rainfall erosivity, which does not fluctuate significantly between years. However, sediment yield is highly influenced by extreme rainfall events and flooding. Therefore, the sediment output estimated by the WaTEM/SEDEM model does not correspond well with the observed annual sediment yield, indicating a weak correlation between model predictions and actual sediment delivery from the watershed. Nevertheless, soil loss estimates from the WaTEM/SEDEM model showed a strong correlation with RUSLE model results across the watershed, though the WaTEM/SEDEM model tended to systematically overestimate erosion. While the spatial pattern across the watershed of soil erosion risk estimated by RUSLE is very useful in prioritization and planning of soil conservation practices, it is necessary to measure and monitor soil loss in the field/hillslope scale for calibration of the models. Soil conservation in mountainous watersheds is considered one of the key priorities in natural resource management. Thus, scientific analysis and the adoption of sustainable methods can contribute significantly to the preservation and improvement of soil conditions in these sensitive ecosystems.
Conceptualization, H.A. and M.A.; methodology, H.A., H.Z. and M.A.; software, H.Z.; validation, H.A., H.Z. and M.A.; formal analysis, H.Z.; investigation, H.Z.; resources, H.A.; data curation, H.Z.; writing—original draft preparation, Z.A.C.; writing—review and editing, Z.A.C., H.A. and M.A.; visualization, H.Z. and Z.A.C.; supervision, H.A. and M.A.; project administration, H.A.; funding acquisition, H.A. All authors have read and agreed to the published version of the manuscript. All authors contributed equally to the conceptualization of the article and writing of the original and subsequent drafts.
Data available on request from the authors.
The authors would like to thank the University of Tehran for supporting the research.
The authors avoided data fabrication, falsification, plagiarism, and misconduct.
The author declares no conflict of interest.
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