Abstract: (43 Views)
Introduction and Objective: Investigation of the amount of water infiltration into soil, as one of the key processes of the hydrological cycle, is of special importance in water and soil resources management. Water infiltration into soil determines the amount of surface runoff, groundwater recharge, and the efficiency of irrigation systems, and plays an important role in watershed management. In arid and semi-arid regions such as Baluchestan, where rainfall events are characterized by high intensity and short duration, accurate understanding of infiltration and runoff behavior can help managerial decision-making aimed at reducing soil erosion and increasing water resources efficiency. Previous studies have shown that factors such as land slope, initial soil moisture, soil texture, organic matter content, and rainfall intensity and duration have direct effects on the infiltration process and runoff initiation.
Since investigating these factors at the farm scale is associated with limitations, the use of a rainfall simulator as a laboratory and controllable tool is considered an effective method for analyzing relationships between physical and hydrological parameters. The present research was conducted with the aim of quantitatively and analytically investigating the effects of initial soil moisture, slope, and rainfall intensity on infiltration and surface runoff in loamy sand soil (similar to the agricultural soil of the Baluchestan region) in the Hydraulic Structures Laboratory of the University of Sistan and Baluchestan. This study, while evaluating the physical model (rainfall simulator) and the theoretical model (Green-Ampt equation), attempts to examine the relationship between experimental results and watershed management applications.
Introduction and Objective: This research was carried out by performing 17 independent experiments with four rainfall intensities (1.61, 1.50, 1.40, and 1.30 mm per minute), four rainfall durations (8, 10, 11, and 5 minutes), and three slopes (0, 1.5, and 2.5 percent). The soil used was loamy sand without organic matter in order to reduce the effect of other factors.In each experiment, hydraulic parameters including cumulative runoff depth, runoff coefficient, infiltration rate, and cumulative infiltration were measured, and the obtained results were compared with theoretical data calculated using the Green-Ampt equation. In order to evaluate the agreement between the physical and the theoretical models, statistical indices including mean absolute percentage error, mean absolute error, coefficient of determination, and root mean square error were used. In all experiments, the rainfall height above the soil surface was constant, soil compaction and texture were assumed to be uniform, and the soil surface was assumed to be without vegetation cover.
Results: The results showed that in zero-slope experiments, cumulative infiltration increased uniformly and nonlinearly with time, and the Green-Ampt model showed good agreement with experimental data, especially during the early stages of rainfall. With increasing slope to moderate values, cumulative infiltration was still well simulated, but cumulative runoff depth and runoff coefficient were slightly higher than theoretical values. In experiments with a 2.5% slope, initial infiltration showed good agreement with the model, but with the passage of time and increasing rainfall duration, experimental runoff exceeded the predicted values and infiltration rate showed a faster decrease compared to the model.
In all experiments, cumulative runoff depth showed a nonlinear increasing trend with time, and experimental data were almost always higher than theoretical values. The difference between experimental results and the model increased with increasing slope, rainfall intensity, and initial soil moisture. The runoff coefficient also showed a gradual increase in all experiments, indicating a reduction in infiltration capacity and an increase in the contribution of surface runoff; a trend that was underestimated by the Green-Ampt model.
Infiltration rate in all experiments was maximum at the beginning and then gradually decreased and approached an approximately constant value. The Green-Ampt model simulated the decrease in infiltration rate more gently than the actual data, especially under sloping conditions. Overall, the results showed that although the Green-Ampt model is suitable for loamy sand soil and simple conditions, it needs modification and development under sloping conditions and intense or long-duration rainfall.
Conclusion: Based on the results of this study, infiltration behavior and runoff generation in loamy sand soil are strongly dependent on initial soil conditions and rainfall characteristics, and changes in land slope can play a determining role in weakening or strengthening the infiltration process. The findings showed that as rainfall conditions shift from simple and short-duration events toward more intense, longer, and steeper-slope conditions, the contribution of surface processes in the water cycle increases and the effective infiltration capacity of the soil decreases.
Comparison of experimental results with the Green-Ampt equation indicated that this model has acceptable performance in representing the general trend of infiltration, especially under flat conditions and moderate initial soil moisture; however, under more complex conditions including higher slopes and intense rainfall, due to structural simplifications, it is not capable of accurately simulating the contribution of runoff and the actual decrease in infiltration rate.
Overall, the results of this study can provide a scientific basis for improving runoff estimation, designing soil conservation measures, and managing water resources in arid and semi-arid regions, especially in watersheds similar to the Baluchestan region, and emphasize the importance of simultaneous use of physical experiments and theoretical models in the analysis of hydrological processes.
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