ERT 468 Surface Water Management Soil Erosion by Water ERT 468 Surface Water Management MDM AIMI ATHIRAH AZNAN LECTURER BIOSYSTEMS ENGINEERING PROGRAM SCHOOL OF BIOPROCESS ENGINEERING, UniMAP

Introduction Geological Erosion Factors Affecting Erosion Natural Natural soil erosion by water is often referred to as geological erosion. This includes soil-forming as well as soil-eroding processes that maintain the soil in a favorable balance suitable for the growth of most plants. This long-time eroding process caused most of the present topographic features, such as canyons, stream channels, and valleys. Introduction Geological Erosion Soil erosion by water can be a natural process or caused by human disturbance.

Introduction Accelerated Erosion Factors Affecting Erosion Human Induced Factors Affecting Erosion In contrast, accelerated erosion associated with human disturbances is one of the most important agricultural and natural resource management problems in the world. Human disturbances include agricultural, mining, forestry, and construction activities. Disturbances due to human or animal influences can reduce vegetative cover and compact soil. Following disturbances, runoff and soil erosion rates increase well above geological levels. Accelerated erosion can lead to a loss of soil productivity and adversely affect surface water quality and flood flows. Introduction Accelerated Erosion

Soil erosion by water is the dominant geomorphic process for much of Earth’s land surface (Terrence et al., 2001). Water erosion also contributes to the detachment, transport and deposition of soil particles. When runoff occurs, water will accumulate as it flows downstream, resulting in a larger volume of water traveling at incremental velocity. Soil erosion by water is a major worldwide problem as compared to wind erosion (Singer and Munns, 1999). By selectively removing organic matter and clay, water erosion not only removes but also may reduce the soil chemical capacity to retain added nutrients.

Factors Affecting Soil Erosion by Water Climate Vegetation Topography Soil Disturbances

CLIMATE Precipitation Temperature Wind Humidity Solar Radiation Temperature and wind are most evident through their effects on evaporation and transpiration. However, wind also changes raindrop velocities and the angle of impact. Humidity and solar radiation are somewhat less directly involved in that they are associated with temperature and rate of soil water depletion.

The relationships among precipitation characteristics, runoff, and soil loss are complex. Rainfall amount, intensity, and energy all impact erosion rates. Studies on individual erosion processes have found that erosion due to raindrop splash and shallow overland flow varies with the rainfall intensity to a power varying from 1.56 to 2.09 (Watson and Laflen, 1986), with the power of 2 generally being accepted. Such erosion may also be affected by raindrop energy, drop diameter, and runoff rates, particularly on permeable soils. Concentrated flow erosion is a function of runoff rate, which depends on both rainfall intensity and soil infiltration rates. Gullying and channel erosion processes are also dominated by runoff rates.

Gullying Erosion Channel Erosion

SOIL Physical properties of soil affect the infiltration capacity and the extent to which particles can be detached and transported. Example: clay particles are more difficult to detach than sand, but clay is more easily transported. Soil detachability increases as the size of the soil particles or aggregates increase Soil transportability increases with a decrease in the particle or aggregate size

Soil Chemical and Biological Characteristics Properties that Influence Erosion Soil Structure Soil Texture Organic Matter Water Content Clay Mineralogy Density Soil Chemical and Biological Characteristics

The Major Effects of Vegetation in Reducing Erosion Retardation of erosion by resisting erosive forces Interception of rainfall by absorbing the energy of the raindrops on plant canopy and surface residue, reducing surface sealing and runoff Physical restraint of soil movement The Major Effects of Vegetation in Reducing Erosion Transpiration, which decreases soil water, resulting in increased storage capacity and less runoff Improvement of aggregation and porosity of the soil by roots and plant residue Increased biological activity in the soil

Benefits of Crop Residues from Vegetation When crop residues are left in the field after harvest, or when natural vegetation senesces and drops leaves and needles, the potential for erosion is decreased. Residue and tillage management practices can have a dramatic effect on soil erosion, usually greater than soil properties. Any practice that maintains a residue cover on the soil surface decreases erosion potential, while any practice that removes, burns, or buries vegetative residue increases the potential for soil erosion. Conventional tillage practices generally bury all residue. Reduced or minimum tillage practices, with fewer operations or less aggressive machines, leave more residue on the soil surface, decreasing runoff and erosion rates. The residue itself will decay with time, ranging from under a year for residues like soybeans to several years for herbaceous residue and several decades for wood and woody debris.

Topographic features that influence erosion are: TOPOGRAPHY Topographic features that influence erosion are: Slope length and steepness Shape (including concave, uniform, or convex) Size and shape of the watershed. On steep slopes, soil is more easily detached and transported downslope. On longer slopes, an increased accumulation of overland flow tends to increase concentrated flow erosion. Concave slopes, with flatter slopes at the foot of the hill, deliver less sediment than convex slopes.

DISTURBANCES Disturbances can be natural, such as prolonged periods of wet weather or particularly severe storms, or human induced, such as construction or tillage. In many climates, erosion occurs as the result of only a few critical runoff events each decade, which overshadow year-to-year small, but frequent, erosion events.

Types of Soil Erosion by Water

Mechanics of Soil Erosion Three steps to accelerated erosion by water: Detachment or loosening of soil particles caused by flowing water, freezing and thawing of the topsoil, and/or the impact of falling raindrops. Transportation of soil particles by floating, rolling, dragging, and/or splashing. Deposition of transported particles at some place lower in elevation.

Raindrop Erosion Impact of water drops directly on soil particles Soil detachment and transport The relationship between erosion, rainfall momentum, and energy is determined by raindrop mass, size distribution, shape, velocity, and direction. Factors affecting the direction and distance of soil splash are: Slope Wind surface condition impediments to splash such as vegetative cover and mulches.

Rain enhances the translocation of soil through the process of splashing. Individual raindrops detach soil aggregates and redeposit particles. The dispersed particles may then plug soil pores, reducing water intake. Once the soil dries, these particles develop into a crust at the soil surface and runoff is further increased.

Sheet Erosion Sheet erosion is the uniform removal of soil in thin layers by the forces of raindrops and overland flow.  Recognized by either soil deposition at the bottom of a slope, or by the presence of light - colored subsoil appearing on the surface.

Sheet Erosion Rarely occurs because rills form almost simultaneously with the first detachment and movement of soil particles. The beating action of raindrops combined with surface flow causes initial rilling. Raindrops detach the soil particles, and the detached sediment can reduce the infiltration rate by sealing the soil pores. Gradually remove the nutrients and organic matter which are important to agriculture and eventually lead to unproductive soil. 

Rill Erosion An erosion process on sloping fields where water concentrates in small channels, called rills, as it runs off.

Rill Erosion Rill erosion is the detachment and transport of soil by a concentrated flow of water. Rills are eroded channels that are small enough to be removed by normal tillage operations. Rill erosion is the predominant form of surface erosion under most conditions.

Gully Erosion Water accumulates and flows rapidly, creating deep channels called gullies. Gullies are too large to be removed by tillage and their depth can range from 0.5 m to as much as 25 or 30 m.

Gully Erosion Produces channels larger than rills. Distinguished from rills in that gullies cannot be obliterated by tillage. Develops by processes either simultaneously or during different periods of its growth. These processes are: waterfall erosion or headcutting at the gully head erosion caused by water flowing through the gully or by raindrop splash on exposed gully sides alternate freezing and thawing of the exposed soil banks slides or mass movement of soil into the gully. Runoff then removes loose soil from the gully floor.

Models to Predict Water Erosion

Models to Predict Water Erosion The Universal Soil Loss Equation (USLE) was originally developed to estimate sheet and rill erosion losses from cultivated fields in the United States. It is now also applied to regions outside the U.S., and to range lands and forest lands. The equation is used to show how different soil and management factors influence soil erosion. Newer versions of the USLE, (RUSLE (Revised Universal Soil Loss Equation) has been developed. Major changes to the USLE incorporated into RUSLE include: new and improved isoerodent maps and erodibility index (EI) distributions for some areas new soil erodibility factors which reflect freeze-thaw in some geographic areas new equations to account for slope length and steepness additional sub-factors for evaluating the cover and management factor for cropland and rangeland includes new conservation practice values for cropland and rangeland.

A = R * K * LS * C * P Developed by MSU Predicted soil loss due to water erosion Results in t/acre/year or t/ha/yr R Erosivity factor Quantifies the erosive force of rainfall and runoff. It takes into account both total amount of rainfall and its intensity. K Soil erodibility factor Represents the ease with which a soil is eroded. It quantifies the cohesiveness of a soil and its resistance to detachment and transport. LS Slope length and steepness factor It is the ratio between the plot in question and a standard unit plot. Steeper slopes lead to higher flow velocities; longer plots accumulate runoff from larger areas and thus also result in higher flow velocities. C Vegetative cover and management factor Considers the type and density of vegetative cover as well as all related management practices such as tillage, fertilization and irrigation. This is the most complicated factor to calculate. P Erosion control practices factor Influence of conservation practices such as contour planting, strip cropping, grassed waterways, and terracing relative to the erosion potential of simple up-down slope cultivation. Developed by MSU Online Soil Erosion Assessment Tool: http://www.iwr.msu.edu/rusle/

Glossary Retardation: the action of delaying or slowing the progress or development of something. Restraint: a measure or condition that keeps someone or something under control or within limits. Aggregation: the formation of a number of things into a cluster. Senesces: (of a living organism) deteriorate with age.