9: SOIL EROSION

Soil erosion

• The wearing away of the land surface by running water, wind, ice, or other geological agents, including such processes as gravitational creep and landslide.

• Loss of soil through the action of water, wind, or gravity.

• Identified by World Bank (1989) as the Philippines’ worst environmental problem.

ON-SITE IMPACTS OF SOIL EROSION

• Reduction in soil depth

• Structure deterioration (Reduction in water infiltration and increase of water runoff)

• Nutrient loss

• Decline in soil fertility

• Loss of production

• Decline in soil biota

OFF-SITE IMPACTS OF SOIL EROSION

• Flooding (Sfeir-Younis and Dragun 1993)

• Sedimentation of the irrigation system (PCARRD 1991b) and water reservoir (Cruz, W. et al 1988b)

• Reduced hydroelectric output, reduction of productive life of hydroelectric dams

• Destruction of coral reefs (Kummer 1992a; PCARRD 1991b; World Bank 1989)

• Geological erosion

• Accelerated erosion

GENERAL TYPES OF SOIL EROSION (2)

GEOLOGICAL EROSION

Wearing away of the earth’s surface by water or other natural agents under natural environmental conditions of climate, vegetation, and so on, undisturbed by man.

ACCELERATED EROSION

Erosion much more rapid than normal, natural, geological erosion; primarily a result of the activities of humans or, in some cases, of animals. This is often 10 to 1,000 times as destructive as geological erosion, especially on sloping lands in regions of high rainfall.

1. DETACHMENT OF SOIL PARTICLES FROM THE SOIL MASS

2. TRANSPORT/ENTRAINMENT OF THE DETACHED PARTICLES DOWNHILL BY FLOATING, ROLLING, DRAGGING, AND SPLASHING

3. DEPOSITION OF THE TRANSPORTED PARTICLES AT SOME PLACE LOWER IN ELEVATION

Process of soil erosion: (3)

≈6-8 inches

• The most versatile part of the soil

• An inch of soil requires thousands of years to develop but can easily be lost overnight

DETACHMENT OF SOIL PARTICLES FROM THE SOIL MASS

• Rainfall – most important detachment agent

• Detachment-limited erosion – eroding agents have the capacity to transport more material than is supplied by detachment

TRANSPORT/ENTRAINMENT OF THE DETACHED PARTICLES DOWNHILL BY FLOATING, ROLLING, DRAGGING, AND SPLASHING

• Transport-limited erosion – more material is supplied than can be transported

DEPOSITION OF THE TRANSPORTED PARTICLES AT SOME PLACE LOWER IN ELEVATION

Occurs when sufficient energy is no longer available to transport the particles.

1. It detaches soil.

2. It destroys granulation.

3. Its splash, under certain conditions, causes an appreciable transportation of soil.

RAINDROP IMPACT EXERTS THREE IMPORTANT EFFECTS:

Influence of raindrops

• When raindrops impact on a wet soil surface, they detach soil particles and send them flying in all directions.

• A very heavy rain may splash as much as 225 Mg/ha of soil.

• Some of the particles splash as much as 0.7 m vertically and 2 m horizontally.

Influence of raindrops

• IMPACT OF RAINDROPS AT 30KM/H.

• PARTICLES ARE SEPARATED DUE TO BEATING OF RAINDROPS.

• SURFACE SOIL PORES FILLED WITH SOIL PARTICLES, REDUCING INFILTRATION.

• SURFACE FLOW BEGINS DUE TO LACK OF INFILTRATION.

1. Sheet erosion

2. Rill erosion

3. Gully erosion

SPECIFIC KINDS OF SOIL EROSION (BY WATER) (3)

Sheet erosion

The removal of a fairly uniform layer of soil from the land surface by runoff water.

Rill erosion

• Numerous small channels of only several centimeters in depth are formed.

• Occurs mainly on recently cultivated soils.

Gully erosion

Water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths, ranging from 0.3-0.6 m to as much as 23-30 m.

1. Erosivity

2. Erodibility

3. Slope

4. Plant cover

5. Human activities

FACTORS INFLUENCING SOIL EROSION (5)

Erosivity

FOR RAINFALL, IT IS A FUNCTION OF:

• ➥ Intensity

• ➥ Duration

• ➥ Mass of raindrop

• ➥ Diameter of raindrop

• ➥ Velocity of raindrop

Erodibility

• Large particles are resistant to transport because of the greater force required to entrain/transport them

• ➥ Fine particles are resistant to detachment because of their cohesiveness

  • ↝ Silts and fine sands – least resistant particles

  • ↝ Soils with 40-60% silt content are the most susceptible

Slope

• Function of slope length and slope steepness (angle/gradient)

  • Slope length – the distance from the point of flow to the point of runoff

• For short slopes, the rate increase in soil loss rises rapidly, but for long slopes, the rate of increase is very small. There is a limit to the length of a slope that will influence soil loss

• As slope steepness increases, soil loss rate also rises at an increasing rate.

• When soil slope exceeds a critical steepness, rill erosion begins, which causes total soil loss to increase rapidly.

Plant cover

• The greatest deterrent to soil erosion is cover.

• The major role of vegetation in reducing erosion is in the interception of the raindrops so that their kinetic energy is dissipated rather than imparted to the soil.

• Under certain conditions, a plant cover can exacerbate erosion. Raindrops intercepted by the canopy may coalesce on the leaves to form larger drops which are more erosive

• Overall, forests are the most efficient (usually with dense ground litter), but a dense growth of grass maybe almost as efficient.

Human activities

Reduction of vegetation cover

• ↝ Tillage (for crop production)

• ↝ Burning

• ↝ Overgrazing

• ↝ Mining

• ↝ Logging

• ↝ Road construction

A = 0.224.R.K.L.S.C.P

The most widely used tool to estimate erosion is the Universal Soil Loss Equation (USLE)

A

ESTIMATED ANNUAL SOIL LOSS PER UNIT AREA (T/HA/YR)

R

THE RAINFALL EROSIVITY FACTOR

K

THE SOIL ERODIBILITY FACTOR

L

THE SLOPE LENGTH FACTOR

S

THE SLOPE GRADIENT FACTOR

C

THE CROPPING MANAGEMENT FACTOR

P

THE EROSION CONTROL PRACTICE FACTOR

USLE

• Developed at the USDA National Runoff and Soil Loss Data Center at Purdue University in a national effort led by Walter H. Wischmeier and Dwight D. Smith.

• Hailed as one of the most significant developments in soil and water conservation in the 20th century

Limitations of USLE

• It does not explicitly represent hydrologic and erosion processes, i.e. runoff. There is considerable interdependence between variables.

• Only predicts the amount of soil loss that results from sheet or rill erosion on a single slope and does not account for additional soil losses that might occur from gully, wind or tillage erosion.

SOIL PROPERTIES THAT TEND TO RESULT IN HIGH K VALUES:

• High contents of silt and very find sand

• Expansive types of clay minerals

• A tendency to form surface crusts

• Presence of impervious soil layers

• Blocky, platy, or massive structure

SOIL PROPERTIES THAT TEND TO MAKE THE SOIL MORE RESISTANT TO EROSION (LOWER K VALUES):

• High SOM content

• Non-expansive types of clay

• Strong granular structure

USLE factors

• A simple method to predict K was presented by Wischmeier et al. which includes the particle size of the soil, organic matter content, soil structure and profile permeability.

• The soil erodibility factor K can be approximated from a nomograph if this information is known.

1. CONTROL OF SOIL EROSION

2. MAINTENANCE OF SOIL FERTILITY AND PRODUCTIVITY

3. AVOIDANCE OF SOIL TOXICITIES

SOIL CONSERVATION (3)

SOIL EROSION CONTROL MEASURES OBJECTIVES

• Dissipation of raindrop impact

• Increase infiltration rates

• Reduction of surface runoff velocities

• Reduction of soil erodibility by enhancing soil properties that resist erosive forces

Surface covers

• The single most effective measure in reducing erosion is to provide a protective cover to the soil surface

• Dissipates energy of raindrops thus eliminating the splash effect

• Impedes water flow

• Increase infiltration

• Reduce runoff

• Minimize erosion

Surface covers

• Help reduce evaporation

• Help reduce soil temperature

• Help reduce weed growth

• Enhance soil fertility

• Suppress weed growth

Cover crops

• Temporary vegetative cover of fast growing annuals

• Grown primarily to protect bare soil

Ground cover

Perennials or regenerating annuals specifically established to protect soil from erosive agents.

Mulching

• Practice of spreading plant residues/other organic materials on the ground between crop rows or round tree trunks to protect the bare soil.

• e.g. Day-og (Cordillera forest dwellers)

• Grasses and other plant debris are spread over the area intended for planting, similar to mulching.

Geotextiles

Use of coconets to protect the slopes.

Physical barriers

• Land treatments or physical structures constructed along the contour of the land at defined intervals, to intercept and/or divert runoff.

• Includes bench terracing, contour rockwalls, contour bunds and pole barriers or contour fence.

Bench terracing

• Slope is converted into series of level to nearly level steps running across the slope supported by steep risers with horizontal cultivated area on the step.

• e.g. Banaue rice terraces

• Landscape scale agroforestry farming system. Consists of:

  • Payoh (rice terraces)

  • Pinugo (forest) system

Contour rockwalls

• Fence-like structures made up of the rocks and/or stones piled along the contour of sloping lands.

• e.g. Tuping (Nueva Vizcaya, Cebu, Siguijor)

• Rockwalls reinforced with ipil-ipil, which is regularly trimmed

• Trimmings fed to goat and goat manure used as fertilizer for corn

Contour bounds

• Embankments (riser/humps) of stones, grasses, or compacted soil (or combination) which are constructed along the contour to slow down surface runoff and trap eroded soil

• e.g. Fanya juu in Eastern Africa

• The main purpose is to prevent water and soil loss and to make conditions more suitable for plants to grow.

Pole barriers / contour fences

• Ipil-ipil stems bundled into rows across the slope and staked to form low barriers.

• e.g. Balabag system in Naalad, Naga, Cebu ➥ A 4-5 year tree-crop rotation

• 4-5 years of corn-tobacco cropping and ipil- ipil fallow

Contour hedgerows

• Hedges of leguminous trees or shrubs, or grasses grown on the contour at a defined interval.

• Crops are cultivated on the alleys or spaces between hedgerows.

• Regularly pruned to prevent shading the intercrops

• e.g. Sloping Agricultural Land Technology (SALT)

Natural vegetative strips

• Naturally occurring vegetation like grasses and herbs are left in narrow strips to serve as barriers.

• e.g. NVS in Claveria, Misamis Oriental

• Alleys are planted with corn.

Contour composting

• Banks or barriers trashes and/or earth are constructed across the slope of the land to check surface runoff and soil erosion.

• e.g. Gen-gen (Ikalahans- Southern Cordillera, Caraballo, Sierra Madre)

Diversion canals

Natural/artificial drainage channel along the steepest slope, in the valley or along the boundaries of the upland farm used to accommodate runoff.

Contour canals

• Holes dug intermittently along water channels to:

  • ↝ Intercept water flow and catch the soil particles

  • ↝ Slow down runoff water

  • ↝ Increase infiltration

Grassed waterways

Channel constructed across the slope to catch upslope runoff and divert it safely to a nearby gully, waterway/river.

Check dams

Obstruction walls across the bottom of a gully which reduce the velocity of the runoff and prevent the deepening or widening of the gullies.

Soil traps

Small canals dug along the contour line to:

• ↝ Allow water to pass through

• ↝ Trap soil sediments

Zero tillage

• The practice of sowing crops directly into the residue of the previous crop without cultivation.

• Weeds are usually controlled using herbicides.

Minimum tillage

• The practice of sowing crops on lightly cultivated soil.

• Weeds are usually controlled using herbicides.

• Weeds are retained.

Contour plowing

• The practice of plowing, harrowing, and furrowing along the contour of the land.

Multi-storey agroforestry system

• Multi-layered canopy

• Velocity of rainfall is reduced

• e.g. coconut-based farming systems

1. Cover crops

2. Mulching

3. Ground cover

4. Geotextiles

Surface covers (4)

1. Bench terracing

2. Contour rockwalls

3. Contour bounds

4. Pole barrier / contour fences

Physical barriers (4)

1. Contour hedgerows

2. Natural vegetative strips

3. Contour composting

Vegetative barriers (3)

1. Diversion canals

2. Contour canals

3. Grassed waterways

4. Check dams

5. Soil traps

Canals and soil traps

1. Minimum tillage

2. Zero tillage

3. Contour plowing

Cultural methods (3)