Susceptibility to sheet and rill erosion

Susceptibility to Sheet and Rill Erosion

The susceptibility of land to sheet and rill erosion is governed largely by the topsoil texture, slope of the land and length of slope. Other factors include hydrophobicity, percentage stone cover, tendency for aggregates to slake and disperse, size and weight of surface particles or aggregates and the probability of intense summer rainfalls.

Soil loss from sheet and rill erosion is difficult to assess because of variability in soil loss within an area and the problem of measuring something that is not there. Sheet and rill erosion greatly reduces productivity, particularly in the case of texture contrast soils. The topsoil or A horizon is where most nutrients, organic matter, seed and macroporosity so desirable for a seedbed exists. If this is stripped away through soil loss the fertility of the soil is lost and productivity reduced.

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The land characteristics and management factors involved in sheet and rill erosion are described in Table 5.

Table 5 Land characteristics and management factors involved in sheet and rill erosion

Processes	Land features affecting processes	Factors affected by land characteristics	Management factors that modify land characteristics
Sheet and rill erosion occur when the forces due to rainfall, flowing water and gravity overcome the cohesion and weight of the soil particles/aggregates	Vegetation structure, percent surface cover (including litter) leaf area, rooting depth and perenniality	Exposure of surface soil Intensity of raindrop impact Infiltration/run-off ratio Velocity of surface flow Transpiration and hence infiltration rate and volume of surface flow	All aspects of the vegetation are affected by selection of species and control of biomass by practices such as cultivating, clearing, trafficking, fertilising, grazing, trampling, harvesting, burning
Processes involved are: detachment of exposed soil by - raindrop impact - surface flow	Climate rainfall intensity/duration seasonal rainfall/ evapotranspiration regime	Intensity of raindrop impact Volume of water exceeding infiltration rate and hence volume of surface flow Soil water content and hence infiltration rate and volume of surface flow
Processes involved are: detachment of exposed soil by - raindrop impact - surface flow	Geology permeability of rock or unconsolidated sediments	Soil water content and hence infiltration rate and volume of surface flow
deposition Surface flow occurs on any sloping surface when the rainfall rate exceeds the infiltration rate	Topography microrelief slope degree and length slope and landform shape position in landscape	Infiltration/runoff ratio Velocity of surface flow Volume and velocity of surface flow Tendency to concentrate surface flow Volume of runon	Contour cultivating, contour banking and strip cropping reduce slope length and affect microrelief
Off-site effects include increased sedimentation and runon in streams and on lower lands	Soil profile permeability depth and water-holding capacity size/weight of surface particles/aggregates cohesion of surface particles/aggregates, including tendency to slake and disperse tendency to surface seal and hydrophobicity percent stone cover	Infiltration rate and hence volume of surface flow Infiltration/runoff ratio Detachment and transport Detachment Infiltration rate and hence volume of surface flow Infiltration/runoff ratio and velocity of surface flow	The above management practices controlling biomass affect soil organic matter content, which in turn affects all listed soil characteristics except surface rock Direct soil compaction and disruption by trampling, trafficking and cultivating affect soil permeability, waterholding capacity and size/weight and cohesion of aggregates

Source: Aldrick, et. al. (1988)

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