Assessment of the Effect of Particle Size Distribution and Rock Fragment on Water Infiltration Variables in Some Soils of a Semi-Arid Region

Document Type : Research Paper

Authors

1 Department of Soil Science and Engineering, Faculty of Agriculture, University of Zanjan, Zanjan, Iran

2 Department of Soil Science and Engineering, Faculty of Agriculture

Abstract

Water infiltration into soil is one of the most important physical properties for effective water and soil resource management, and it can be influenced by soil particle size distribution. This study was conducted to investigate the role of particle size distribution and gravel content on water infiltration indices in selected soils of a semi-arid region in Zanjan Province, Iran. Water infiltration was measured at 68 locations using the double-ring infiltrometer using the double-ring infiltrometer method. The measured infiltration indices included cumulative infiltration, effective infiltration depth, initial infiltration rate, final infiltration rate and mean infiltration rate, along with particle size distribution and several other physical properties. The results of the correlation analysis indicated that effective infiltration depth exhibited a significant negative correlation with silt content (p< 0.01), whereas initial infiltration and initial infiltration rate were positively and significantly correlated with silt content (p< 0.05). Bulk density also showed significant correlations with the different fractions of primary soil particles (p< 0.05). In contrast, bulk density was negatively and significantly correlated with all infiltration indices (p< 0.05). These findings suggest that particle size distribution plays a crucial role in the infiltration process not only through direct effects, but also indirectly by influencing bulk density. Furthermore, analysis of different gravel content classes (0–20, 20–40, and > 40%) demonstrated that increasing gravel content led to an improvement in infiltration indices, such that cumulative infiltration and mean infiltration rate were significantly correlated with total gravel content (p< 0.05). Overall, the physical structure of soil acts as a key determinant of the water infiltration pattern, and understanding the complex interrelationships among soil properties requires integrated approaches and advanced statistical analyses.

Keywords

Main Subjects

Background and purpose:

Soil water infiltration is one of the fundamental components of the hydrological cycle and plays a critical role in regulating surface runoff, groundwater recharge, soil moisture retention and the functioning of terrestrial ecosystems (Zhang et al., 2016; Hillel, 1998). Infiltration is defined as the movement of water from the soil surface into a porous medium under the combined influence of gravitational forces and matric suction and it is considered a key indicator for describing soil hydraulic behavior (Zewide, 2021). Despite extensive research, significant knowledge gaps remain regarding the precise mechanisms through which primary particle size distribution and the coarse fragment fraction influence soil water infiltration across different regions of Iran. The present study was therefore conducted to investigate the effects of particle size distribution on soil water infiltration in Zanjan Province, where a substantial proportion of agricultural lands are characterized by light to medium soil textures and variable gravel contents. The specific objective of this study was to evaluate the effects of particle size distribution, as well as gravel content and size, on selected soil water infiltration indices under the climatic conditions of Zanjan.

Materials and methods:

For this study, the sampling sites were selected based on criteria including land use type, vegetation cover, soil texture and available information from soil survey maps. Soil water infiltration was measured at 68 selected locations across Zanjan Province using the double-ring infiltrometer method. Cumulative water infiltration was recorded at time intervals of 0, 0.5, 1, 2, 3, 5, 10, 15, 20, 30, 45, 60, 80, and 90 minutes and measurements were continued until the infiltration rate reached a steady state. Infiltration indices, including cumulative infiltration (CI), effective infiltration depth (EID), initial infiltration rate (IIR), final infiltration rate (FIR), and mean infiltration rate (MIR), were derived from the measured infiltration data. Soil samples required for the determination of physical and chemical properties were collected as disturbed samples from the 0–60 cm soil depth and as undisturbed samples from the 0 – 20 cm soil depth. Primary soil particle size distribution was determined using the hydrometer method (Yavari et al., 2021), and five sand size fractions were separated using the sieve method. In addition, the properties of soil particles larger than 2 mm were analyzed following standard procedures described in Methods of Soil Analysis (Klute, 1986). Particles larger than 2 mm were classified into four categories: (1) gravel (2–64 mm), (2) cobbles (64–256 mm), (3) stones (> 256 mm) and (4) total coarse fragments (> 2 mm). Bulk density was determined using the metal cylinder (core) method (Culley, 1993). Equivalent calcium carbonate content (Page et al., 1982) and soil organic matter (Walkley and Black, 1934) were also measured to assess their influence as aggregating agents on soil primary particles. Effective infiltration depth (EID) was estimated based on the principle of mass conservation as described by Hillel (1982). A set of descriptive statistics, including minimum, maximum, mean, median and standard deviation, was calculated for EID, MIR, IIR, FIR, II and CI. The normality of data distributions was evaluated using the Kolmogorov–Smirnov test. To analyze the effects of independent variables on soil water infiltration characteristics: (1) Mean comparison analyses were conducted using one-way analysis of variance (One-Way ANOVA) and error indices to examine statistically significant differences among different soil particle size distribution classes and gravel content categories in relation to soil water infiltration indices. (2) Pearson and Spearman correlation analyses were applied to evaluate the strength and direction of relationships between quantitative variables with different data distributions. Data analysis, graphical representation, and mean comparisons were performed using Excel (2016), Python programming language (v3.x), SPSS (2022) and R within the Posit Cloud environment.

Results:

The results of the correlation analysis indicated that effective infiltration depth exhibited a significant negative correlation with silt content (p< 0.01), whereas initial infiltration and initial infiltration rate were positively and significantly correlated with silt content (p< 0.05). Bulk density also showed significant correlations with the different fractions of primary soil particles (p< 0.05). In contrast, bulk density was negatively and significantly correlated with all infiltration indices (p< 0.05). These findings suggest that particle size distribution plays a crucial role in the infiltration process not only through direct effects, but also indirectly by influencing bulk density. Furthermore, analysis of different gravel content classes (0–20, 20–40, and > 40%) demonstrated that increasing gravel content led to an improvement in infiltration indices, such that cumulative infiltration and mean infiltration rate were significantly correlated with total gravel content (p< 0.05). The results showed that soil organic matter, as a cementing agent of primary soil particles, is a highly effective factor in the infiltration process. Accordingly, all soil infiltrability indices exhibited positive and highly significant correlations with soil organic matter (p< 0.01). In addition, the results of one-way analysis of variance (ANOVA) indicated that all organic matter levels (<1, 1–2, 2–3, and >3%) had a highly significant effect on cumulative infiltration (p< 0.01). In contrast, calcium carbonate did not show a significant effect on soil water infiltration.

Conclusion:

Overall, the results of this study indicate that soil water infiltration is a process strongly dependent on soil physical structure. The findings demonstrate that soil textural components exert significant effects on infiltration indices; in this regard, coarser particles predominantly enhance soil infiltrability by improving soil porosity and pore connectivity. The results further emphasize that the influence of particle size distribution on water infiltration is not solely direct, but may also be exerted indirectly through alterations in soil structure, bulk density, and consequently the soil pore system. An increase in gravel content resulted in improvements across all infiltration indices, highlighting the imp÷ortance of coarse fragments in regulating soil hydraulic behavior. Among the soil properties examined, soil organic matter was identified as a key factor promoting water infiltration, whereas the effect of calcium carbonate under the studied conditions was negligible. Overall, given that the relationships between soil physical properties and water infiltration indices are often nonlinear and multifactorial, the application of multivariate analytical methods and machine learning algorithms appears essential for a more accurate characterization and interpretation of these complex interactions.

Author Contributions

All authors contributed equally to the conceptualization of the article and writing of the original and subsequent drafts.

Data Availability Statement

Data available on request from the authors.

Acknowledgements

This study was supported by the University of Zanjan and is acknowledged. The authors thank the Soil Science Department of the University of Zanjan for different soil physicochemical analysis.

thical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

The author declares no conflict of interest.

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Iranian Journal of Soil and Water Research
Volume 57, Issue 2
May 2026
Pages 513-529

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