Sloping Soil Surface Research Articles

Particle travel rates and the enrichment of carbon in the sediment discharged by rain-impacted flows

Soil carbon losses from sheet and interrill erosion areas have an impact on the global carbon cycle. Sediment discharged in many experiments where artificial rainfall has been applied to sloping soil surfaces show greater proportions of carbon in the sediment than found in the cohesive soil source. In many cases, conclusions drawn from these experiments fail to recognise that the enrichment of carbon in the sediment discharge is a result of unsteady conditions that occur before the steady state develops. It is known that particles of sand of different sizes move at different rates when raindrop induced salation occurs with the result that initially the particle size distribution is finer than the source but tends towards that of the source as time progresses. The presence of carbon in aggregates influences the density of aggregates. Here a qualitative model of sediment transport of sand and coal particles by raindrop induced saltation is used to demonstrate time varying enrichment of coal resulting from the effect of carbon on particle settling velocity. The model also demonstrates how the layer of loose particles that develops on top of the cohesive soil surface influences the relative amounts of carbon and sand in the sediment discharges. The steady state is not achieved until the slowest moving particle detached at the farthest distance from the downslope boundary is discharged. That may not be achieved in many of the experiments reported in the literature.

Effect of Slope on Infiltration Capacity and Erosion of Mount Merapi Slope Materials

Infiltration which occurs on slope has a specific behavior that can be parameterized. One of the reasons is because the slope generates less ponding on the sloping soil surface. This condition affects infiltration rate and surface runoff proportion of water from any kind of rainfall distribution in time. Since surface runoff tends to be higher, erosion rate is also to be higher on slope. The slope steepness is the most important parameter of a slope. In this study, the effect of slope steepness on infiltration capacity and erosion rate of Mount Merapi bare slope material were tested in a laboratory using rainfall simulator. Three values of slope steepness of 36%, 47%, and 58% were used. The rainfall intensity was set constant and was found has rate of 116,312 mm/hour. The infiltration rate was measured by volumetric balance principle and the erosion rates were measured by collecting the eroded grains at the downstream end tank. Infiltration rate was evaluated by using Horton method and the average erosion were analyzed from the eroded grain data for each test. After fitting the recorded infiltration rate data to the Horton equation, the infiltration capacity was obtained. The results were the relationship between slope steepness and the affected the infiltration capacity and erosion for each test. Infiltration capacity is found to increase and the decrease with the increase of slope steepness while erosion rate is found to increase on the steeper slope. The increase of erosion range is between 15% to 33% for each 1o increase of slope steepness. In addition, polynomial and exponential equations were developed to express the relationship between slope steepness and infiltration capacity and also the erosion rate.

A laboratory experimental system for infiltration studies

The investigation of a few hydrological processes under natural conditions can be distorted by their interactions. In this context, a laboratory system that allows a few mechanisms of the infiltration process to be studied univocally is presented. The core component of the system is a physical model consisting of a soil tank with slope angle, γ, adjustable from 1 ° to 15 °. A generator of artificial rainfall can produce rainfall rates up to 50 mm h−1. Surface runoff and deep flow, Qd, are continuously monitored. An overall analysis of three previous investigations performed by the physical system and directed to clarify the infiltration process is also briefly reported. These investigations, that concerned the validation of a local conceptual model for erratic rainfalls, the role of run-on and the effects of sloping soil surfaces, were all carried out by using different configurations of the system. Great slope effects in bare soils were observed. For example, under steady conditions, a ratio Qd(γ = 1°) / Qd(γ = 10°) equal to about 4 was observed in a loam soil. Finally, on the basis of the acquired knowledge, further investigations to be realized with the same basic elements are proposed to derive a conceptual model that describes the soil surface gradient effects.

Determination of Hydraulic Properties in Sloping Landscapes from Tension and Double-Ring Infiltrometers

The majority of landscapes, natural or cultivated, are nonlevel. However, specifically designed instruments are not available for estimation of soil hydraulic properties in sloping landscapes. The objective of this study is to examine if tension and double-ring infiltrometers are suitable for determination of soil hydraulic properties on sloping soil surfaces. A field experiment was conducted in a silt loam soil (Typic Haplustolls) in Saskatchewan, Canada to explore the usefulness of tension and double-ring infiltrometers for the determination of soil hydraulic properties in sloping landscapes. Soil surfaces were created to represent four treatments, 0 (level), 7, 15, and 20% slopes. For each treatment, water infiltration rates were measured using a double-ring infiltrometer and a tension infiltrometer at −3, −6, −10, −13, −17, and −22 cm water pressure heads. In addition, three-dimensional computer simulation studies were performed for a tension infiltrometer with various disc diameters and water pressure heads for different surface slopes. Steady-state infiltration rate, field-saturated hydraulic conductivity, unsaturated hydraulic conductivity as a function of water pressure head, macroscopic capillary length parameter, and water-conducting macro- and mesoporosity were compared for different surface slopes. These parameters were not significantly different (p < 0.05) between level and sloping lands. Experimental and numerical results of this study suggest that both tension and double-ring infiltrometers are suitable for characterization of surface soil hydraulic properties in landscapes with slopes up to 20%.

Determination of Hydraulic Properties in Sloping Landscapes from Tension and Double‐Ring Infiltrometers

The majority of landscapes, natural or cultivated, are nonlevel. However, specifically designed instruments are not available for estimation of soil hydraulic properties in sloping landscapes. The objective of this study is to examine if tension and double‐ring infiltrometers are suitable for determination of soil hydraulic properties on sloping soil surfaces. A field experiment was conducted in a silt loam soil (Typic Haplustolls) in Saskatchewan, Canada to explore the usefulness of tension and double‐ring infiltrometers for the determination of soil hydraulic properties in sloping landscapes. Soil surfaces were created to represent four treatments, 0 (level), 7, 15, and 20% slopes. For each treatment, water infiltration rates were measured using a double‐ring infiltrometer and a tension infiltrometer at −3, −6, −10, −13, −17, and −22 cm water pressure heads. In addition, three‐dimensional computer simulation studies were performed for a tension infiltrometer with various disc diameters and water pressure heads for different surface slopes. Steady‐state infiltration rate, field‐saturated hydraulic conductivity, unsaturated hydraulic conductivity as a function of water pressure head, macroscopic capillary length parameter, and water‐conducting macro‐ and mesoporosity were compared for different surface slopes. These parameters were not significantly different (p &lt; 0.05) between level and sloping lands. Experimental and numerical results of this study suggest that both tension and double‐ring infiltrometers are suitable for characterization of surface soil hydraulic properties in landscapes with slopes up to 20%.

Transport of bacteria on sloping soil surfaces by runoff

Pathogenic bacteria exist at soil surfaces as a result of practices as spreading of liquid manure on agricultural lands or use of treated wastewater for irrigation. Rainfall is a major factor affecting vertical and horizontal movement of bacteria in soil. Surface runoff carries bacteria significant distances downstream causing serious threats to ground and surface waters. This study uses a nalidixic acid-resistant Escherichia coli strain as a biotracer monitoring extent of bacterial migration on sloping soil surfaces by runoff action. Two 10×10-m plots in two sites having different slopes were sprayed with water containing biotracer. Soil texture at sites was clay loam. Sixteen days after spraying, two heavy rainfalls that caused runoffs were recorded. First rainfall occurred 2 days after spraying plots. Samples were collected from soil and runoff at different distances downstream of the plots. Biotracer was found in soil and runoff samples some 20 m downstream from center point of plot having the milder slope. Biotracer was found in soil and runoff samples further downstream of the second plot with the steeper slope reaching a 35- and 30-m distance respectively. Most soil and runoff samples collected after the second rainfall, occurring 15 days after inoculation, contained no biotracer except small numbers found in soil samples taken from center point of each plot 5 m downstream. Results confirm the important role of runoff in bacterial transport on soil surfaces. They show E. coli survives in semiarid areas for a long time and increases potential of contamination. © 2000 John Wiley & Sons, Inc. Environ Toxicol 15: 149–153, 2000

Transport of bacteria on sloping soil surfaces by runoff

Transport of bacteria on sloping soil surfaces by runoff

A simple, effective method for determining the bulk density of stony soils

Abstract A new technique of determining bulk density based upon the excavation method is especially useful in stony soils and surface mine spoils. The technique involves determining the volume of an excavated hole using an expanding urethane foam which solidifies to form a perfect cast of the hole. Volume of the cast is determined by water displacement. Comparison of this method with the sand method of volume determination in a surface mine spoil showed no difference in calculated values of bulk density. However, bulk densities determined by driving a steel core of known volume into the stony surface mine spoil were consistently 12–17% lower than those determined by excavation. The new technique may be applied to sloping soil surfaces and may be used for bulk density determinations at depth in the exposed vertical face of a soil pit.

Water Flow Through Layered Anisotropic Bedded Soil with Subsurface Drains

AbstractResults for steady‐state water flow in layered bedded soils with subsurface drains are presented. The soil was assumed to consist of one or more layers bounded below by an impervious barrier and above by a soil surface of uniform slope. Each soil layer was considered water‐saturated and uniformly isotropic or anisotropic within the layer. Anisotropy was defined such that ζ ≠ κ where ζ and κ are the horizontal and vertical hydraulic conductivities, respectively. The finite element method was used in the numerical solution for steady and saturated flow. Although the numerical techniques presented can handle any number of layers, results are only presented for one‐ and two‐layered profiles. The effect of the degree of anisotropy (R) and equivalent hydraulic conductivity (K) for individual layers (where R = [ζ/κ]1/2 and K = [ζ κ]1/2) on drain outflow, water seepage along the soil surface, and the relative flow within each layer was investigated. The presence of subsurface drains increased the total flow into the bedded soil surface and decreased (often eliminated) water seepage outflow along the sloping soil surface. The influence of the depth of subsurface drains below the ground surface was dependent upon both K and R of the soil layers. In general, higher K and higher R in the layer containing the subsurface drain resulted in higher drain flows.

Micro-Topographical Variations of Rainfall Incident on Ridges of a Cultivated Field

ABSTRACT THE actual rainfall intensity incident on sloping soil surfaces differs considerably from standard measurements. Deviations of 60% have been found in this study between opposite-lying sides of furrows, which are equivalent to differences of 4:1. A model is presented, relating the above rainfall variations to the aspect and inclination of the receiving surface relative to the direction of incoming rainfall. Results are relevant to the study of runoff and erosion in cultivated fields. In a broader theoretical context, results of the present study are significant for modeling the role of topographical scale on the distribution of incident rainfall.

A model of the redistribution of disaggregated soil particles by rainsplash

AbstractA model of the dispersion of splash droplets from a single raindrop impact on a sloping soil surface is combined with a theory of the entrainment of mineral particles from a disaggregated mixture in splash droplets to obtain a model of the dispersion of such particles by a raindrop impact. Stochastic modelling techniques extend this further to a model of the spatial redistribution of soil on a plot after a period of rainfall.Since the model is probabilistic and physically based it enables the incorporation of further advances in the understanding of splash erosion at all stages and can simulate the effect of the stochastic nature of rainfall and soil properties on the process. Several different situations are simulated. These include the movement of marked soil particles from point sources and the spatial patterns of erosion on a sloping plot. The model can also simulate the differential erosion of different soil particle size fractions.

A physically‐based model of the dispersion of splash droplets ejected from a water drop impact

AbstractA model has been developed which predicts the dispersion of splash droplets produced by the impact of a water drop on a sloping soil surface.Experimental results of the ejection velocities and ejection angles of the splash droplets are generalized to a planar slope and the resulting splash distances are calculated taking into account the effects of air resistance. The predictions are presented in terms of the numbers of splash droplets from the impact point to surrounding squares arranged in a grid on the slope.The model explains many experimentally observed features of raindrop splash in terms of the mechanics of the processes involved and can make predictions of the effects of slope, wind, raindrop size, and soil properties on droplet dispersion. The component of the raindrop velocity parallel to the surface of the slope is identified as the main factor determining the degree and the direction of the asymmetry in the splash droplet dispersion.By combining the model with a theory of the entrapment of soil in the splash droplets it is possible to extend it to predict the dispersion of soil particles by raindrop impact, which is the basis of a model of soil erosion by rainsplash.

Postemergence effects of different distribution patterns of tri‐allate granules on Avena fatua L. and wheat plants

Summary:The phytotoxicity of 10% w/w tri‐allate granules to emerged plants of Avena fatua L. and wheat was investigated in the glasshouse, and to A. fatua alone under outdoor conditions. Tri‐allate was applied at 1.68 and 3.36 kg a.i./ha using two patterns of granule distribution (1) the granules uniformly distributed over the soil surface and (2) the same weight of granules as in (1) but applied only to a central area of the soil surface. Wheat was very tolerant of high concentrations of tri‐allate. Under optimum conditions of temperature and soil moisture in the glasshouse phytotoxic effects on wild oats were visible within 6 h and coleoptile or primary leaf entrapment was observed within 3 to 4 days of treatment. Phytotoxic effects of tri‐allate were detectable at distances up to 12 cm from the site of restricted‐granule application, although beyond 8 cm the effect was sub‐lethal. On a sloping soil surface (1:8 gradient) the dispersal of tri‐allate was approximately the same upwards or downwards from the site of herbicide application. Under outside conditions where the mean temperature was about 5 C tri‐allate dispersal was sub‐lethal beyond 3 cm although it evidently occurred primarily as vapour.

Mathematical Analysis of Steady Saturated Flow through a Multilayered Soil with a Sloping Surface

AbstractAn analytical solution is presented for a two‐dimensional multilayered hillside seepage problem. The soil is horizontally stratified with each layer having a different permeability. The flow medium is assumed to be water‐saturated to the soil surface, bounded below by an impermeable barrier at a finite depth, and bounded laterally by vertical streamlines. Seepage occurs into the flow medium along the upper part and out along the lower part of the sloping soil surface. Two situations are analyzed, one with a constant slope soil surface and another with an abritrarily shaped soil surface. The potential and stream functions for the problem are developed by starting with a general series solution to Laplace's equation and using a modified Gram‐Schmidt method to determine the series coefficients. Solutions are derived for two‐layered and three‐layered soils. From these derivations, solutions for soils with more than three layers can be readily deduced. The results presented include flow nets, seepage velocities and infiltration rates for two‐layered and three‐layered soils.

A Mathematical Solution to a Hillside Seepage Problem

AbstractA solution is presented for a two‐dimensional hillside seepage problem. The soil is assumed to be water‐saturated to the soil surface where the soil water is at atmospheric pressure. Seepage occurs into the flow medium along the upper part and out along the lower part of the sloping soil surface. The flow medium is infinitely deep and bounded laterally by vertical stream‐lines. Conformal transformations are used to find the hydraulic head and stream functions. Although the solution is analytic, a complex integral must be evaluated numerically for application. Numerical data presented includes flownets, seepage velocities, infiltration angles, and volume of flow as a function of slope.