Introduction: Rill erosion is considered one of the most significant forms of water erosion. Most existing erosion models have been developed to estimate soil loss and formulate conservation measures. For this reason, spatial and temporal variations of erosion at small scales, such as rill erosion, are often simplified in these models. In addition to soil and flow characteristics, soil resistance to erosion is also a function of dynamic external environmental variables. Among these, soil moisture is an external environmental characteristic that can influence soil resistance to erosion. However, limited information exists regarding the precise role of soil moisture content on resistance to water erosion. Therefore, this research aimed to investigate the effect of initial soil moisture and flow characteristics on rill erosion in two erosion-prone soils containing more than 40% silt but with different textures.

Materials and Methods: Soil samples with silt loam and silty clay textures, having approximately similar silt content, were collected from the 0-10 cm of two rain-fed farmlands exhibiting signs of rill erosion in Shirkuh village, Rudbar, Gilan Province. Soil samples did not have considerable differences in other properties such as organic matter content, calcium carbonate equivalent, aggregate stability, and other characteristics. Measurements of rill detachment capacity (Dc) for both soils were conducted under simulated runoff conditions with four flow discharge rates (0.546, 0.986, 1.125, and 1.895 m³h-1) and two slope levels (0.061 and 0.121 m/m), totaling eight combinations of flow discharge and slope, at five initial moisture levels (5, 10, 15, 20, and 30% by mass) with three replications.

Results and Discussion: The results of the analysis of variance showed that the effects of soil type, moisture level, and the interaction effects between soil type and moisture level on Dc were significant at p <0.01. Results indicated that in both silty clay and silt loam soils, Dc decreased with increasing soil initial moisture content. The soil detachment capacity in silt loam soil, after a decreasing pattern, increased from 0.015 kg m-²s-1 at 20% moisture to 0.086 kg m-²s-1 at 30% moisture, indicating the existence of a moisture threshold for changes in susceptibility to rill erosion. whereas in silty clay soil, the trend of detachment capacity continued to decrease with increasing initial moisture content. Analysis of rill erodibility factor at different moisture levels also revealed a pattern similar to the changes in detachment capacity. In the drier moisture range, initial moisture may lead to increased soil strength, and consequently, soil loss decreases until reaching a moisture content corresponding to the plastic limit. Reduced aggregate breakdown and increased cohesion due to the presence of water films likely explain the greater soil resistance with increasing initial moisture. However, there may be an optimal moisture content beyond which the trend of increasing soil resistance with initial moisture begins to decrease or at least does not show incremental changes. This soil moisture threshold depends on the characteristics of the soil under study, such as the plastic limit, which can also be influenced by texture, especially the clay content of the soil. Therefore, the difference in clay content between the two soils probably caused their different behaviors at various moisture levels. Based on the Pearson correlation test results, the correlation of rill detachment capacity in both soils showed the highest correlation coefficient with the stream power parameter (r = 0.76 to 0.83 in silty clay soil and r = 0.76 to 0.94 in silt loam soil). In silty clay soil, increased moisture generally leads to higher correlation, whereas in silty loam soil, a more complex pattern is observed, with Dc at 10% moisture showing the strongest correlation for most parameters. Initial soil moisture had a significant impact on the relationship between detachment capacity and flow characteristics, though this effect varied between the two soil types.

Conclusion: This finding indicates that rill erodibility is not a constant value and varies depending on the interaction effects between initial soil moisture and the texture of the studied soil. Therefore, although some soil properties such as soil texture are considered unchangeable factors over specific time periods (e.g., days, months, or even years) affecting erosion, the manner in which time-dependent parameters, such as initial soil water content (referred to here as initial soil moisture), and its interaction with soil texture influence erosion remains a subject of interest. Results of correlation between soil detachment capacity and flow hydraulic and hydrodynamic properties suggests that stream power may be a good predictor for soil detachment. Overall, these findings suggest that the influence of dynamic external factors, such as initial moisture level, should be considered for more accurate soil erosion modeling.