Soils

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Soils Hydrology II

• Soil Water
• Precipitation and Evaporation
• Infiltration, Streamflow, and Groundwater
• Hydrologic Statistics and Hydraulics
• Erosion and Sedimentation
• Soils for Environmental Quality and Waste
  Disposal
• Issues in Water Quality

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Soil Erosion

• The biggest threat to agricultural and forestry
  production worldwide.
• Soil is the basis of much of the wealth on this
  planet
• if we don't take care of it - treat it as a
  renewable resource, rather than use it up as we
  are doing now - there may be difficult problems
  with soil productivity in the future.

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Soil Loss in the United States
Each dot represents 250,000 tons. Total US soil
loss in 1997 was 2 billion tons. The worst
erosion occurs in the Mississippi Valley and the
Midwestern corn belt. These areas have silty
soils, rolling topography, and intensive farming.
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Plato on Soil Erosion - 400 BC

• The soil which kept breaking away from the
  highlands keeps continually sliding away and
  disappearing into the sea. What now remains,
  compared with what existed earlier, is like the
  skeleton of a sick man, all the fat and soft
  earth having wasted away and only the base
  framework of the land being left.
• What are now mountains were lofty soil-clad
  hills the stony plains of the present day were
  full of rich soil, the mountains were heavily
  wooded - a fact of which there are still visible
  traces. There are mountains in Attica which can
  support nothing but bees but which once were
  clothed, not so very long ago, with fine trees
  suitable for roofing the largest buildings - and
  roofs hewn from the timber are still in
  existence. The country produced boundless
  pastures for cattle.
• The annual supply of rainfall was not lost, as
  it is at present, through being allowed to flow
  over the denuded surface into the sea, but was
  received by the country, into her bosom, where
  she stored it in her impervious clay and so was
  able to discharge the drainage of the heights
  into the hollows in the form of springs and
  rivers with an abundant volume and a wide
  territorial distribution. The shrines that
  survive to the present day on the sites of
  extinct water supplies are evidence for the
  correctness of my present hypothesis.

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• Piedmont streams have not always run red from
  clay.
• The Southeast suffered tremendous erosion losses
  during the cotton era (1830-1930).
• Up to 12 was lost, especially in the Piedmont.
• Much of this soil ended up in the streams,
  rivers, and valley bottoms of the Piedmont.
• The effects of this sediment in the river systems
  are still evident today.

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Average annual loads of suspended sediment
carried by rivers of Atlantic drainage of the
United States during years near 1910 and 1970
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Soil Erosion in the Southeastern Piedmont
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Level of Protection

• High lt 25 NTU (mg/L)
• Moderate 25 - 80
• Low 80 - 200
• Very Low gt 200

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Relationship Between Soil Erosion and Crop
Productivity
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• Georgia Soil and Water Conservation Commission
• Formed to protect, conserve and improve the soil
  and water resources of the State of Georgia. The
  Commission's goal is to make Georgia a better
  place for its citizens through the wise use and
  protection of basic soil and water resources and
  to achieve practical water quality goals.
• Georgia Forestry Commission
• Provides leadership, service, and education in
  protection, management, and wise use of Georgia's
  forest resources.
• U.S. Natural Resources Conservation Service
• Provides leadership in a partnership effort to
  help people conserve, maintain, and improve our
  natural resources and environment.
• U.N. Food and Agriculture Organization
• Has a mandate to raise levels of nutrition and
  standards of living, to improve agricultural
  productivity, and to better the condition of
  rural populations.

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Suspended sediment in three Georgia rivers
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• Point-Source Discharge
• Water discharged into a stream from a pipe or
  structure, usually associated with a city or
  industry.
• Nonpoint-Source Discharge
• Water discharged over a wide area, not coming
  from a pipe, usually associated with farms,
  homes, forests, etc.
• Detachment
• The removal of fine particles from aggregates.
  This is a necessary step in erosion because the
  aggregates are too big to move.
• Transport
• After detachment has occurred, transport is the
  movement of detached particles off the source
  area (field, construction project, bare-soiled
  clearcut) and, eventually to surface waters.

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Wind Erosion

• Suspension
• When very fine particles, silt and clay, are
  picked up by the wind and carried in the
  atmosphere.
• These particles essentially float on the wind and
  are carried high in the atmosphere.
• They may be deposited hundreds and even thousands
  of miles away from where they were picked up.
• Deposition areas of wind blown soils may
  eventually build up layers of loess soils.

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• Saltation
• The bouncing of medium and fine sand over the
  ground surface, usually about 0.5 to 3 feet in
  the air.
• When the particles fall back to the ground, their
  impact lifts other particles which begin to
  saltate.
• Because of these chain reactions, saltation
  becomes more severe the longer the high winds
  blow.
• If you have ever been to the beach on a day with
  strong winds, you have probably experienced
  saltation of stinging sand.
• Creep
• The rolling of coarse sands along the ground
  surface.
• Creep is responsible for the formation and
  movement of sand dunes in bare deserts.
• Creeping soils can be trapped with soil fences,
  and the fences at the beach are meant to hold
  sand on the dunes.

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Downwind Effect of a Windbreak
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Fluvial Erosion

• Raindrop Impact
• Causes detachment of fine particles from soil
  aggregates, and it is also the initiator of
  transport.
• Most energy is transferred rapidly to soil
  particles when the raindrop crashes into the
  ground.
• Raindrop impact is the major detaching mechanism
  on bare soils.

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• Overland Flow
• If the rainfall rate exceeds the infiltration
  rate, surface flow will commence over the soil
  surface.
• This runoff collects in micro-depressions and
  forms channels.
• These small channels then merge into larger
  channels which causes both detachment and
  transport of soil particles.
• Sheet Erosion
• Movement of the soil surface that does not
  involve channel flow.
• Sheet erosion mostly consists of soil detachment
  from raindrop impact.
• Subsequent transport is caused by raindrop splash
  and a very thin layer of overland flow.
• Sheet erosion uniformly removes soil from a
  planar area, and it causes relatively low rates
  of erosion.
• The thin film of flow delivers sediment to rills.
  

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• Rill Erosion
• When the contributing area becomes large enough,
  the thin layer of overland flow starts to cut
  small channels (1-6" deep), called rills or
  rilles, into the soil surface.
• Rills are formed when the velocity of the flow on
  the soil is large enough to create shear stresses
  sufficient to detach and entrain soil particles.
• Rills transport the sediment dislodged by sheet
  erosion and carry it off the eroding surface.
• Rills also pick up and transport additional
  sediment from the walls and bottoms of the rills
  themselves.

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• Gully Erosion
• Rill erosion on a larger scale, gullies can
  become enormous - an example is Providence Canyon
  in Southwest Georgia.
• The basic definition of a gully is a rill that is
  too deep to cross with farm machinery.
• One way they form is when rills come together and
  concentrate even more flow
• A second way is when ground water seeps out near
  a spring and washes out a channel below the
  spring. Streambanks and stream bottoms also erode
  during high flows due to the shear stress of fast
  moving water.

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Channel Erosion
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• Erosion rates are usually expressed as inches of
  topsoil per year or tons per acre per year.
• An acre-furrow slice weighs two million pounds if
  the soil bulk density is 1.4 kg/L.
• Erosion rates of 10-50 t/ac/yr are common on
  steep, cleared lands, and this translates into a
  loss of 1 inch of topsoil in 3-15 years.
• In other words, the field loses the Ap layer in
  20-100 years.
• A tolerable rate of erosion, according to the
  NRCS, is 3-5 t/ac/yr, which is the approximate
  rate of new topsoil formation (B horizon turning
  into A with humus addition ).

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Universal Soil Loss Equation

• A R K LS C P
• R Rainfall Erosivity Factor
• A combination measure of climate factors such as
  typical rainfall intensities, probability of
  extended periods of wet weather, and types of
  precipitation (convective, cyclonic, snow, etc.).
  The USDA developed maps of R values around the
  country.

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Rainfall Factor
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• K Soil Erodibility Factor
• Accounts for factors such as texture, organic
  content, and aggregate stability. The Soil Survey
  maps list the K factors for each soil.
• LS Length-Slope Factor
• Accounts for both the length and steepness of the
  slopes. Erosion increases as the slope length
  increases because the depth and velocity of water
  increases. Erosion also increases as the slope
  gradient (steepness increases) because overland
  flow moves faster on steeper slopes.
• C Cropping Factor
• Accounts for the type of vegetative cover. C
  factors are very low for forests and very high
  for bare soils.
• P Conservation Practice Factor
• Accounts for any soil conservation measures
  applied to the land to reduce erosion rates.

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Types of Sediment Measurements

• Turbidity
• A measure of the clarity of the water sample.
• Increasing turbidity is an indication of
  dissolved or suspended solids present in the
  water column.
• Substances which increase turbidity include
  particles of suspended sand, silt or clay,
  organic substances, coagulated organic colloids
  containing iron and aluminum hydroxides, and
  microorganisms including phytoplankton and
  zooplankton.

• NTU vs. JTU
• Before the advent of modern light scattering
  devices, turbidity was measured using the Jackson
  candle turbidimeter in Jackson Turbidity Units
  (JTU).
• The NTU measure is not exactly equivalent to the
  JTU, but is approximately the same, i.e., 40 NTU
  ? 40 JTU.

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• Secchi disk
• Used to determine the optical clarity of deep
  water bodies such as lakes, reservoirs, estuaries
  and oceans.
• A standard disk, generally 20 cm in diameter, is
  lowered by rope to a depth where it is no longer
  visible and raised until the disk is discernable.

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• Suspended Solids Concentration
• Suspended solids are mineral and organic
  particles supported by turbulence within the
  fluid column.
• The total suspended solids concentration is
  determined by extracting and weighing the
  suspended solids, reported in units of mass per
  unit volume, typically milligrams per liter
  (mg/L).

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• Hydrometer
• Used to measure the density of the sediment
  solution.
• The density, or specific weight, increases as the
  sediment concentration increases.
• Because the larger particles settle very quickly,
  only the smaller particle classes can be
  successfully determined.
• An additional problem with the hydrometer method
  results assuming the particle density for the
  suspended sediment fraction.

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• Bedload Transport
• Bedload solids are those sediments that are
  transported along or near the bed of a stream.
• These sediments are generally larger than
  suspended solids and either roll or bounce along
  the stream bed.
• Bedload solids may comprise the bulk of the total
  load transported by the stream because of their
  high concentrations (generally higher than 10
  g/L, and frequently higher than 100 mg/L).

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Measuring Water Erosion

• Sampling method
• grab samples
• automated samplers
• Effect of location
• depth, bank, bend
• Effect of stage
• low vs high
• Comparing turbidity to suspended solids

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Rising Stage Sampler
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DH-48
Beadload Sampler
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Coshocton Sampler
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Preventing Soil Erosion

• Vegetative cover
• Surface stabilization
• Velocity reduction
• Peak flow reduction
• Inspection and maintenance

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General Terrace Design
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Types of Graded Terraces
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Forest Management
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Chap 13 Quiz

• 1. The streams most commonly degraded in Georgia
  by sediment today are (choose one)
• a. agriculture b. forestry c. urban d.
  mountain
• 2. Why does Georgia have such high erosion?
  (choose any/all/none)
• a. steep slopes b. erodible soils c. intense
  rainfall d. land-disturbing activities
• 3. Match
• a. Suspended Solids _____ Clarity of lake water
• b. Turbidity _____ Sands on streambed
• c. Bedload _____ Filterable solids
• d. Secchi Depth _____Clarity of river water
• 4. Give two reasons why we are concerned about
  erosion
• 5. (True/False) Soil mulch and seeding is less
  effective than silt fences and hay bales for
  preventing erosion.