Ali Reza Vaezi; Mahdi Ebadi
Abstract
Introduction: Soil water erosion on the slope lands involves detachment, transport and deposition of soil materials due to erosive forces of raindrops and surface runoff. Surface runoff can produce relatively high soil loss and is often the dominant hillslope erosion process. The rate of surface runoff ...
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Introduction: Soil water erosion on the slope lands involves detachment, transport and deposition of soil materials due to erosive forces of raindrops and surface runoff. Surface runoff can produce relatively high soil loss and is often the dominant hillslope erosion process. The rate of surface runoff which controls surface erosion on the uniform areas in the hillsolpes, is dependent on rainfall intensity and slope steepness. In various studies, the relationships between rainfall characteristics and surface runoff were well known. Many studies have been performed on the relationship between runoff and rainfall characteristics and soil loss.The effects of slope steepness on the surface runoff and soil loss were also investigated by many researchers. In a few studies, the transportation of soil particles has been studied. For examples, some studies showed that the soil particles have different susceptibility to transport by surface flow . However, limited information is available on the effects of rainfall intensity and slope steepness on the transportability of soil particles by surface runoff in the semi-arid areas. Therefore, the objective of this study was to investigate the effects of rainfall intensity and slope steepness on the transport rate of soil particles by surface runoff in a medium soil texture in semi-arid region.
Materials and Methods: A clay loam soil with similar particle size distribution (33.15% sand, 33.22% silt and 33.63% clay) was provided to study the detachability of soil particles by surface runoff. Soil loss and particle size distribution of eroded material were determined in the soil under zero, 10%, 20%, 30% and 40% slope steepness using simulated rainfall with 10, 20, 30, 40, 50, 60, 70, 80 and 90 mm h-1 in intensity. Soil samples were filled to 32 cm 50 cm flumes with 7 cm depth and exposed to simulated rainfalls. Surface runoff, surface soil erosion and particle size distribution (PSD) of eroded material were determined in the slopes under simulated rainfalls. A total of 135 trials were carried out on 45 soil samples using the factorial completely randomized
design with three replications. Data of surface soil erosion and transportation of soil particles were compared using the Duncan's test among the rainfall intensities and slope steepness.
Results and Discussion: No surface runoff and surface soil erosion were observed in 10 mm h-1 rainfall intensity. Rainfall intensity of 20 mm h-1 appeared to be the threshold rainfall intensity to make surface runoff and surface soil erosion. Based on the results, surface runoff, surface erosion and kind of eroded soil particles were significantly affected by rainfall intensity (P< 0.001). Significant relationships were found between rainfall intensity and surface runoff (R2= 0.98) and surface erosion (R2= 0.99). Surface runoff increased strongly with increasing rainfall intensity. Increases in the rainfall intensity caused more runoff production as well as more detachment of soil surface particles . Surface runoff and surface erosion were affected strongly by the slope steepness. With an increase in the slope steepness, more surface runoff was produced and in consequence, surface soil erosion was considerably increased. Significant differences were found in the PSD of eroded material among the diffrent rainfall intensities (P< 0.001) and the slope steepness (P< 0.001). Silt showed to be the sensitive soil particles to surface erosion in rainfall intensities and slope steepness. Silt included about 66% and 74% of eroded soil particles in the rainfall intensities and the slope steepness, respectively. Sand fractions (very coarse sand, coarse sand, medium sand, fine sand, very fine sand) were the resistant soil particles to surface erosion in the rainfall intensities and the surface slopes. In higher rainfall intensities and slope steepness, more surface soil erosion was produced which was associated with the more transport of silt.
Conclusion: Rainfall intensity was the more important factor than the slope steepness in the soil loss and transportation rate of soil particles by surface runoff. Silt was the most susceptible soil particle to erosion by surface runoff in the rainfall intensities and the slope steepness. The transportation of very coarse sand and clay didn’t appear significant differences for both the rainfall intensities and the slope steepness. Protection of soil surface from raindrop impact is essential for prevention of runoff and soil loss in steep slopes especially for intensive rainfalls.
S. F. Eslami; A. R. Vaezi
Abstract
Introduction: Soil erosion by water is the most serious form of land degradation throughout the world, particularly in arid and semi-arid regions. In these areas, soils are weakly structured and are easily disrupted by raindrop impacts. Soil erosion is strongly affected by different factors such as rainfall ...
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Introduction: Soil erosion by water is the most serious form of land degradation throughout the world, particularly in arid and semi-arid regions. In these areas, soils are weakly structured and are easily disrupted by raindrop impacts. Soil erosion is strongly affected by different factors such as rainfall characteristics, slope properties, vegetation cover, conservation practices, and soil erodibility. Different physicochemical soil properties such texture, structure, infiltration rate, organic matter, lime and exchangeable sodium percentage can affect the soil erodibility as well as soil erosion. Soil structure is one of the most important properties influencing runoff and soil loss because it determines the susceptibility of the aggregates to detach by either raindrop impacts or runoff shear stress. Many soil properties such as particle size distribution, organic matter, lime, gypsum, and exchangeable sodium percentage (ESP) can affect the soil aggregation and the stability. Aggregates size distribution and their stability can be changed considerably because of agricultural practices. Information about variations of runoff and sediment in the rainfall events can be effective in modeling runoff as well as sediment. Thus, the study was conducted to determine runoff and sediment production of different aggregate sizes in the rainfall event scales.
Materials and Methods: Toward the objective of the study, five aggregate classes consist of 0.25-2, 2-4.75, 4.75-5.6, 5.6-9.75, and 9.75-12.7 mm were collected from an agricultural sandy clay loam (0-30 cm) using the related sieves in the field. Physicochemical soil analyses were performed in the aggregate samples using conventional methods in the lab. The aggregate samples were separately filed into fifteen flumes with a dimension of 50 cm × 100 cm and 15-cm in depth. The aggregate flumes were fixed on a steel plate with 9% slope and were exposed to the simulated rainfalls for investigating runoff and soil loss (sediment). Ten same rainfall events with 60 mm h-1 in intensity for 30 min were applied using a designed rainfall simulator in the lab. The rainfall simulator had a rainfall plate with a dimension of 100 cm × 120 cm which has been fixed on a metal frame with 3m height from the ground surface. Runoff and sediment samples were collected using a plastic container placed the out-let of the flumes. Runoff generation of each flume was determined based on multiplying total content volume of the tank by volume proportion of water in the sample. Soil loss for each event was determined using multiply the container volume and sediment concentration of the uniform sample. Initial soil moisture was measured in the aggregate samples before each rainfall event in order to investigate its effect on the runoff and sediment variations in the event scales. Runoff, soil loss and initial soil moisture data were evaluated for normality before any statistical analysis using SPSS version 18 software. Differences of runoff and soil loss among different rainfall events were analyzed using the Duncan's test.
Results and Discussion: Based on the results, the soil was calcareous having 16% equivalent calcium carbonate. Low amount of organic matter (0.6%). The measured aggregate stability showed to be very low, indicating high susceptibility of the aggregates to water erosion processes. Significant differences were found among the rainfall events in runoff (p< 0.05), sediment (p< 0.001) and sediment concentration (p< 0.001) which were associated with aggregate breakdown by raindrop impacts in the rainfall events. Runoff and sediment were strongly increased from each event to other event. Significant relationship was found between sediment and runoff in the events (R2= 0.89, p< 0.001). However, sediment showed to have higher increasing trend as compared to runoff variation pattern in the event scale. Sediment value was very low in the first rainfall event due to high portions of the water-stable aggregates and low level of soil moisture. Difference in runoff from each event to other event was directly related to variation of infiltration rate. In the final events, aggregate disruption was strongly enhanced and remarkably decreased the soil infiltration rate so runoff and sediment significantly increased. After seventh rainfall event, sediment production was observed to be higher (2.93 times) as compared with runoff production and in consequence sediment concentration strongly increased. The difference in the infiltration rate among the rainfall events was attributed with differences in initial soil moisture and macropores.
