Investigating the particle size distribution of interrill erosion in different soils under land slope gradient

Soil erosion is the most important aspect of land degradation and one of the most important environmental, agricultural and food production problems in the world. More than half of the agricultural lands in semi-arid regions are under rainfed cultivation, and the tensions caused by erosion and destruction of land are important in terms of the effects within the field of erosion, such as soil fertility, and the effects outside the field, such as soil pollution. Information about the size distribution of eroded particles is a substantial step for managing the loss of nutrients and the transfer of pollutants from the soil. For this purpose, this study was conducted to investigate the particle size distribution of sediments resulting from interrill erosion under the influences of slope gradient and soil texture in some soils of the semi-arid region of Zanjan Province.

Toward this, four hillslopes having different soil textures (sandy loam, silty loam, sandy clay loam and clay loam) in four north-south slope percent (five, 10, 15 and 20%) were investigated using a simulated rainfall with an intensity of 60 mmh-1 for 60 minutes. In order to monitor the runoff and sediment samples from the beginning of the runoff until reaching a stable state, were taken at 5-min intervals from the initiation of runoff. The particles size distribution of sediment was determined by the method of particle separation using a series of sieves with diameters of 75, two, one, 0.5, 0.25, 0.1 and 0.05 mm. Also, the percentage of finer particles (silt and clay) was calculated by hydrometer method. Then the sediment particles are divided into eight categories: gravel (1-75 mm), very coarse sand (1-2 mm), coarse sand (0.5-1 mm), medium sand (0.25-0.5 mm), fine sand (0.1-0.25 mm), very fine sand (0.05-0.1 mm), silt (0.002-0.05 mm) and clay (<0.002 mm) were categorized.

The results showed that, with the increase in the slope of the land surface, the size distribution of sediment particles changed in all soils. So that, the transport of particles larger than 0.1 mm (including very coarse sand, coarse sand, medium sand and fine sand) increased with increasing slope. While the percentage of silt particles (0.002-0.05 mm) decreased in all textures. Also, particles with the size class of medium sand and fine sand formed between 40 and 50% of the sediment particles for each texture and did not change significantly with the increase of the slope. The highest ratio of particles in the sediment to the main soil was related to silt particles and in silty loam texture (4.33 times) and the lowest was related to clay particles and in silty loam texture (0.26 times). Sand and silt particles in different textures, had high transmission ratios. Silt particles in three textures of sandy loam, clay loam and silty loam had a ratio of more than one and clay particles less than one. Except in clay loam soil, sand particles changed in the range of 0.83 to 1.24 times compared to the main soil and the overall average ratio of its availability in sediment to original soil was 0.98 times.

In general, this research showed that with increasing slope, the selectivity of fine particles decreases and the share of coarse particles increases. The effects of these changes are more visible in fine-textured soils. Due to the increase in the destruction of aggregates and flow rate with the increase of the surface slope, it is very important to prevent the removal of vegetation and observe the principles of tillage in order to reduce the direct impact of raindrops on the soil surface and reduce the transportability of particles by the flow. Also, the results show that the ratio of particles in the soil texture and the characteristics of soil structure (abundance and stability of aggregates) are the determining factors of particle transport and it is necessary to consider these characteristics in choosing soil conservation methods.