Evaluation of different methods of soil structural stability determination for predicting splash erosion

 Soil erosion is one of the major obstacles to sustainable development. A large part of Iran has an arid and semi-arid climate, without vegetation with suitable density or even completely without vegetation. Therefore, many parts of the country face high erosion and soil losses. Previous studies showed an increased trend of soil erosion in Iran. Because in situ measurement of soil erosion at the farm or watershed scale is expensive and time-consuming, estimation of soil erosion from easy and ready parameters can be useful. It is well-known that aggregate stability can affect soil erosion. There are many methods developed to measure soil aggregate stability, but there is no specific method that can be used for a wide range of soil types under different land uses. This study was done to compare different methods of aggregate stability determination (i.e., splash rate measurement, shear strength measured with fall-cone penetrometer and wet sieving). Twenty-eight soil samples with different textures, equivalent calcium carbonate, and organic matter were collected from surface soil layers in Isfahan and Chaharmahal-va-Bakhtiari provinces. Particles size distribution of studied the soil was measured. Very coarse sand (VCS), coarse sand (CS), medium sand (MS), fine sand (FS) and very fine sand (VFS) were measured according to ASTM sieves. Also, four components of silt (0.035-0.05, 0.02-0.035, 0.01-0.02 and 0.002-0.01 mm) were measured according to Stock's law by the pipette method. Geometric mean diameter and geometric standard deviation of particles were calculated by Shirazi and Boeresma (1984) relations. Soil splash rate (S) was measured with rainfall simulator, near-saturated soil shear strength (τ) was determined using the fall-cone penetrometer, and mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates were measured by the wet sieving. n The results of this study showed that the sand, silt and clay contents were, respectively, in the ranges of 1.5-51%, 34-73% and 11-35% in the studied soils. Most of the sand particles belonged to the FS and VFS (0.05-0.25 mm) fractions and most of the silt fraction was in the very fine silt (0.002-0.01 mm) fraction. The range of organic matter was 0.08 to 8.8% and calcium carbonate equivalent varied in the range between 10% and 63%. Generally, soil aggregate stability was low and splash erosion was high in the studied soils. The results showed that S showed significant correlations with sand, silt, and geometric mean diameter and geometric standard deviation calculated using all particle fractions, VCS, CS, MS, FS, fine silt and very fine silt. Soil shear strength (τ) had significant correlations with silt, very fine silt, geometric mean diameter and geometric standard deviation. The GMD and MWD had significant correlations with soil organic carbon. The results showed that S had significant and negative correlations with τ and GMD, and there were significant and positive correlations between τ with GMD and MWD. The S was mainly dependent on particle size distribution, while GMD and MWD mainly depended on soil organic carbon. However, both particle size distribution and soil organic carbon would affect τ. This finding might be justified by differences between mechanisms which are responsible for particles detachment. The energies induced by raindrop impact and slaking are the main forces and mechanisms responsible for detachment of particles in splash erosion and wet sieving tests, respectively while the cohesive forces between particles mainly govern soil strength in the fall-cone penetrometer test. The studied soils were clustered based on intrinsic soil properties (i.e., texture, CaCO3 and organic carbon) by using K-means method in MATLAB software, in order to evaluate the capability of different methods in different soil groups. The least significant difference (LSD) test was used in a completely randomized design for mean’ comparisons between the clusters. The mean comparison results showed that the three methods similarly predicted the variation of aggregate stability in different soil clusters. The results of clustering showed that the soil cluster with high organic matter, silt and clay contents and low sand content was more stable than other clusters. Three methods similarly predicted the variation of aggregate stability in different soil groups; therefore, the methods might be used alternatively for aggregate stability determination. Fall-cone penetrometer can be introduced as an in situ method for evaluation of aggregate stability against splash erosion.