Analysis of the relationship between sediment budget components and topographic metrics at the hillslope scale in control sub-watershed of the Khamsan representative-paired watershed

Introduction and Objective: Given the critical importance of soil and humanity's reliance on it for food production worldwide, especially in developing countries, soil erosion and degradation have become significant challenges. The average soil erosion in the country is estimated to be around 16.5 tons per hectare. In Iran, soil erosion is approximately 2 to 2.5 times higher than the average in Asia and 5 to 6 times higher than the global average, predominantly occurring in slopes and hilly areas due to poor vegetation cover. Considering the progress made in measuring the amount of erosion and deposition, as well as the cost and inaccuracy of the old methods, including the use of erosion pins and plots, today the use of radionuclides, especially 137Cs, to measure the amount of erosion and deposition and Sediment budget components is of great concern. One of the important advantages of these methods, which has attracted the attention of researchers especially in recent decades, is the possibility of measuring soil redistribution due to water erosion in slopes and watersheds and even on the scale of laboratory plots. Accurately estimation of sediment budget components is essential for estimating erosion rates and planning effective soil conservation measures in any region. Therefore, this research aims to estimate the sediment budget components at the hillslope scale using topographic metrics.
Materials and Methods: Due to the possibility of using the results of previous researches and the availability of a digital elevation model with a spatial accuracy of one meter obtained from a drone photogrammetry, the present study was conducted in the control sub-watershed of the Khamsan representative paired watershed in Kurdistan province. To investigate the effect of the slope angle and curvature on erosion and sedimentation, soil sampling was done at 31 points in a 200-m grid with a systematic-random method and also at 48 points on six transects. After preparation of the soil samples including air and freeze drying and passing through a 63 micron sieve, 293 grams of soil were sent to the laboratory in special containers for the measurement of 137Cs. Also, distribution maps of topographic metrics were extracted using the digital elevation model in ArcGIS 10.8 software. Then, the soil erosion/redistribution map was prepared using the 137Cs inventory points and work unit approach and then, the sediment budget components including total erosion, total sedimentation, net erosion and sediment delivery ratio were calculated for 14 hillslopes in the studied sub-watershed. Then, in order to check the normality of the data in the 14 studied hillslopes, the Kolmogorov Smirnov test was used, and according to the normality of the data, the correlation between the topographic metrics and the sediment budget components at the hillslope scale was checked using the Pearson correlation test. Finally, regression equations were presented to estimate the sediment budget components using topographic metrics.
Results and Discussion: According to the results obtained in the 14 studied hillslopes, specific total erosion ranged from 0.93 to 7.52 and specific total deposition ranged from 0.03 to 0.51 t ha-1 yr-1, which shows that a significant percentage of erosion has been removed from the hillslopes. Specific net erosion was also calculated between 0.79 and 7.45 t ha-1 yr-1, which indicates the sediment delivery ratio between 0.84 and 1.00 for the investigated hillslopes. Results indicate a significant negative correlation between erosion (total and net) and sediment delivery ratio with slope angle, topographic factor, and stream power index. This highlights the dominant effect of dryland farming on increased erosion in gentler slopes and downstream areas. In contrast, erosion is minimal in the steeper, upstream regions due to less soil disturbance and rangeland usage. Consequently, the significance of land use and its changes along the slope profile were considered in interpreting the regression relationships and equations.
Conclusion and Suggestions: The inner area of the Khamsan watershed includes a low-slope plain with the predominant land use of rainfed agriculture, which continues to parts of the surrounding steep slopes, and similar conditions can be seen in the sub-watersheds, including the control sub-watershed. Land use changes from the upstream to the downstream of the hillslopes around the rainfed lands have relatively similar conditions, and therefore the regression equations presented in this study can be used in the hillslopes of the Khamsan representative paired watershed and other similar watersheds Finally, it is suggested to extract topographical metrics using digital height models with different spatial accuracies and use them in estimations. It is also recommended that a land management plan be developed for the studied area, taking into account the significant role of land use in erosion and sediment production, with adjustments and land use changes implemented based on the potential of each section.