Analysis the impacts of extreme precipitation events on Severe erosion of rainfed lands in Asdli defile, North Khorasan

Document Type : Complete scientific research article

Authors

1 Corresponding Author, Assistant Prof., Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran

2 Researcher, Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization, Tehran, Iran

3 Professor, Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran

4 Associate Prof., Khorasan Razavi Agricultural and Natural Resources Research and Education Center, Mashhad, Iran.

5 Assistant Prof., Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran.

Abstract

Background and objectives: Soil is a valuable and irreplaceable natural resource that provides essential ecosystem services and performs various environmental functions to support life on Earth. Nevertheless, one-third of the world's soils have been degraded, and annually, between 25 to 40 billion tons of soil are lost due to erosion. This situation significantly impacts the productivity, resilience, and sustainability of agricultural and environmental systems. Many studies focus on erosion in sloped agricultural lands due to its significant impacts on soil fertility and potential negative effects on downstream water quality. Soil erosion on sloping lands is considered one of the major challenges in natural resource management, particularly in agricultural and mountainous regions. In these areas, intense and continuous rainfall leads to surface runoff and an increased rate of soil erosion. Plowing on sloping lands, aimed at improving agricultural conditions, can weaken soil structure and increase the risk of erosion, particularly during heavy rainfall events. Considering climate change and the increase in the frequency and intensity of extreme rainfall events, it is essential to study and analyze the impact of these rainfall patterns on soil erosion in such areas. Therefore, considering the importance of soil in sustainable security and food health, as well as the changes occurring in global and, consequently, Iran's climatic conditions, the present study focuses on the impact of extreme rainfall on soil erosion in the Asadli Pass region of North Khorasan Province, particularly the erosion caused by the rainfall event on June 5, 2007.

Materials and Methods: The study area is located in the Asadli Pass, 25 kilometers south of Bojnord in North Khorasan Province, at a geographical longitude of 57°35′39″E and a latitude of 37°29′47″N, with an elevation ranging from 1700 to 1900 meters above sea level. The region experiences an average annual rainfall of 363.5 millimeters and has a cold semi-arid climate in the Aladagh Heights along the Bojnord-Esfarayen route. The predominant land use in this area is rangeland and dry farming. The geological formations of the region are sedimentary and belong to the Paleozoic through Cenozoic eras, with Jurassic and Neogene formations constituting the majority of the geological structure in the area. To document the effects of the extreme rainfall event, field visits and surveys were conducted to identify the topography and record deep linear erosion features. Additionally, historical imagery from Google Earth was used to examine the evolution of these erosion features. Due to the absence of a local rain gauge station, daily rainfall data from the nearest synoptic station in Bojnord and remote sensing data from GPM satellite images with an 11 km resolution were utilized to investigate extreme precipitation events. After selecting the appropriate station and obtaining the data, the quantitative and qualitative characteristics of the collected data were initially examined in the R environment using the Climpact2 package to calculate extreme indices and conduct climate analyses. Furthermore, extreme precipitation indices (10 indices based on the CCL/CLIVAR Expert Team's recommendations) were calculated and analyzed using R software and the ClimPACT2 package. The trend analysis of the extreme indices used was conducted based on the Mann-Kendall test. In this test, positive and negative Z-statistics indicate increasing and decreasing trends in the examined data series, respectively.

Results: Field investigations and Google Earth imagery revealed that deep linear erosions outside agricultural lands in Asadli Pass have existed for a long time. However, human activities, such as tilling along the slope, destruction of natural grass cover, and extreme precipitation events, have exacerbated the erosion. Field studies showed that after the intense rainfall in June 2007, deep erosive rills and temporary gullies with an average width of 40 cm, a depth of 30 cm, and an average spacing of 150 cm were formed, resulting in erosion exceeding 1000 tons per hectare on a tilled slope in Asadli Pass, North Khorasan. Daily rainfall data from the Bojnord station indicated that intense showers occur somewhat frequently during the warm season in this region. The maximum 24-hour rainfall recorded at this station during the 1978-2023 period was 54 mm in September 2000. Notably, there is a relatively high frequency of extreme precipitation events in this region, especially during the warm season, which, combined with weak vegetation cover, sets the stage for severe soil erosion. Records show that more than 5 mm of rainfall occurred on June 5, 2007, at the Bojnord synoptic station. GPM satellite images for this date revealed that the rainfall in Asadli Pass, amounting to 8-10 mm, was higher than that recorded at Bojnord station.

Conclusion: Based on field observations, historical data analysis from the Bojnord station, interpretation of Google Earth and GPM images, and the unique climatic and environmental conditions of Asadli Pass, it can be concluded that the study area is highly susceptible to extreme precipitation events and soil erosion. Therefore, it is recommended that converting rangeland to rainfed agriculture be avoided, annual tilling be minimized, and perennial or cover crops be prioritized for soil protection on such slopes. Despite the low probability of such extreme events, their impacts are significant and far-reaching. Documenting these occurrences is crucial for better understanding the role of intense precipitation in soil erosion and geomorphological changes in arid and semi-arid regions. The results of this study and similar research emphasize the importance of proper land management and the use of sustainable agricultural practices to prevent land degradation and preserve the quality of natural resources. Furthermore, considering the issues caused by water erosion, adequate measures must be taken to reduce the rate of this phenomenon and ensure the sustainable management of water and soil resources.

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References

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Journal of Water and Soil Conservation
Volume 32, Issue 1
April 2025
Pages 81-104

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