Document Type : Research Paper

• soil erosion and conservation

EXTENDED ABSTRACT

Introduction

Erodibility represents the inherent susceptibility of the soil to detachment and transportation by rainfall and surface runoff, which was introduced as K factor in the Universal Soil Loss Equation (USLE). Assuming that the methods of determining the other USLE factors (i.e. R, LS, C, and P) are appropriate, it was hypothesized that the USLE nomograph or its derived classical equation is the best available prediction tool for the K factor. This research aimed to determine the K factor by direct measurement of soil loss at natural runoff plots (K_obs), and to compare it with the estimated K by the USLE nomograph (K_est). The research was carried out at the Sanganeh Soil Conservation Research site with an area of 20 ha located in the northeast of Iran, in dry rangelands.

Method

To consider different conditions in terms of slope steepness, soil, vegetation cover, surface rock fragments, and soil depth, 19 plots were installed throughout the site. The plots had lengths of 20 and 25 m and a fixed width of 2 m. Unlike the USLE unit plot, the plots were not disturbed and were in natural conditions. After installing the plots, the most important soil attributes and surface characteristics were measured in each hillslope. They include vegetation cover, primary soil particle size distribution and surface rock fragments, structural stability index, electrical conductivity, pH, organic carbon, cation exchange capacity, and carbonate calcium equivalent.

A recording gauge meteorological station with a measurement precision of 0.2 mm has been established to collect the characteristics of each rainfall event in the vicinity of the site. To perform calculations related to the rainfall erosivity index, the data of each event was converted to a time step of 5 minutes and then the relevant calculations were performed. During the research (1997-2000 and 2006-2009), 20 erosive rainfall-runoff events were recorded. The erosive events were defined based on Renard et al. (1997)’s conditions. To determine the concentration of sediment resulting from each rainfall event, after thoroughly mixing the runoff and sediment, a two-liter sample was taken using the valve at the bottom of the tank and transferred to the laboratory. Sediment samples were transferred to the laboratory and placed in an oven at 105  for 24 hours. The concentration of each sample was determined in terms of and finally, it was converted into  using the volume of the tank and the dimensions of the plot. Then,  calculated by   for three cases: 1) the long-term average of soil loss during the research period. 2) the year with the highest annual soil loss, and 3) events with the highest contribution to annual soil loss (high-intensity events), Finally,  was estimated by applying silt + very fine sand correction () by the classic equation of the USLE nomograph. and comparing to .

Results

The average intensity () of the 20 measured rainfall events during the seven years was 11.6 . The average maximum 30-minute intensity () was 16.7 . The annual rainfall erosivity index () was 45.8 . Based on the erosive events (20 events) in the 7-year research period, the average annual soil loss was about  in the whole site. While the minimum average annual soil loss was  (Plot no. 4), its maximum for was about  (Plot no. 15).

In general, no relationship () was observed between  and . The values ​​of long-term  varied in the range of 0.00004 (plot no. 4) to 0.00293 (plot no. 1) , while the values ​​of  were in the range of 0.056 (plot no. 18 and 19) to 0.077 (plot no. 5 and 6) . Using the seven-year average soil loss, the year with the highest contribution to the total soil loss, and soil loss induced by the event with the highest intensity (),  was at least 24, 7, and 6 times of , respectively.

Conclusion

The results showed that the value of  obtained from the USLE nomograph significantly overestimated the  value, leading to the rejection of the research hypothesis. The main reasons for this overestimation would be the low intensity of erosive events and the resulting uncertainties in the measurement of soil loss, in on hand, and determination uncertainty of the other USLE model factors (R, LS, C and P) in dry rangelands, on the other hand. The intensity of the rains used to develop the USLE nomograph was 63 . Events of this intensity rarely occur in arid regions. On the other hand, due to the variable duration of the drought period and fear in arid regions, the periods of 1-3 years, and even 7 years of measuring the K factor are associated with a high uncertainty and errors. Therefore, based on the overestimation of the USLE nomograph, further research is needed, especially with a longer statistical period or using a rainfall simulator with an intensity of >63  in the field conditions, to develop appropriate relationships for estimating the K factor in rangelands in arid regions.

Author Contributions

Ayoub Avizhgan: Design, Analysis, and Interpretation of data Writing- Original draft preparation, Visualization. Hossein Asadi: Conceptualization, Methodology, Design, Revision of the manuscript and Editing. Mahmood Arabkhedri: Design, Revision of the manuscript and Editing. Hamzeh Noor: Revision of the manuscript and Editing. Aliakbar Nazari Samani: Revision of the manuscript and Editing.

Data Availability Statement

Data can be sent from the corresponding author by email upon request.

Acknowledgements

We are grateful to the Soil Conservation and Watershed Management Research Institute of Tehran for for measurement and data collection.

Ethical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

The author declares no conflict of interest.