JWSS - Journal of Water and Soil Science

The annual soil erosion in different regions of the world has been estimated using various empirical and numerical methods whose accuracy is very dependent on their utilized parameters. One of the most common methods in the evaluation of the mean annual soil erosion especially in sheet and furrow regions is the USLE method. In this relationship, almost all factors that normally affect the soil loss process such as land cover, slope, precipitation, soil type, and support practice parameter of soil have been employed but, in this research, it was shown that the accuracy of this method in mountainous areas covered by rock and snow is somewhat low. To do this, a part of the Tibet plateau in China, where observation soil loss data were available, was selected for investigation. To implement the numerical and analytical analysis, many maps including DEM, NDVI, orientation, soil type, mean monthly and annual precipitation for 30 years were collected. For increasing the accuracy of the model, the cover management parameter was extracted from high accuracy NDVI maps and all USLE parameters were calculated in ArcGIS. The final results were shown that the amount of annual soil loss which was estimated by the USLE method is more than the observed data which were collected by Chinese researchers. This is because the large areas of the study area are covered by lichen and snow where soil loss due to the erosion process is very low but these regions cannot be recognized from NDVI maps. Also, the analysis of the NDVI maps was shown that the relationships of Fu, Patil, and Sharma were not suitable for soil loss estimation in elevated mountainous areas. If the other relationships such as Lin, Zhu, and Durigon are used for the regions with a height of more than 5500 m, a new correction coefficient needs to be used for the C factor which was calculated as 0.2 for the study area.

Rainfall erosivity is a key factor in soil erosion models. Although Iran’s climate is predominantly hot and dry, the occurrence of intense and heavy rainfall events, although infrequent, can cause significant damage to soil resources. In the Universal Soil Loss Equation, rainfall erosivity is calculated based on rainfall intensity. In this study, 45 years of hourly rainfall data from the ERA5 reanalysis dataset were used to estimate rainfall erosivity across Iran. The results revealed that the spatial distribution of rainfall events in Iran is highly heterogeneous. Central Iran, with fewer than 20 rainfall events per year, and the Caspian Sea coast, with nearly 450 events per year, represent two contrasting rainfall regimes. Both the Caspian coast and the Zagros highlands receive slightly less than 10 mm of rainfall per event, while the mean duration of rainfall events in these regions is approximately 10 hours. On the Caspian coast, moisture originating from the Caspian Sea, and over the Zagros Mountains, moisture originating from the Arabian Sea, contribute to the observed rainfall duration and amount per event. Rainfall event intensity in the western Zagros and along the entire southern coast of Iran is considerably higher than in other parts of the country. The transport of moisture from the Arabian Sea into Iran contributes to the high intensity of rainfall events in these regions. Rainfall erosivity is very low across most parts of Iran; however, the Zagros Mountains, the Caspian coast, and some mountainous areas exhibit relatively high levels of erosivity. The mean annual rainfall erosivity over Iran is approximately 80  MJ.mm.ha^-1.h^-1.y^-1.