Soils & Hydrology II
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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
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.
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.
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.”
• 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.
Average annual loads of
suspended sediment carried
by rivers of Atlantic drainage
of the United States during
years near 1910 and 1970
Soil Erosion in the Southeastern Piedmont
Level of Protection
High
Moderate
Low
Very Low
< 25 NTU (mg/L)
25 - 80
80 - 200
> 200
Relationship Between Soil
Erosion and Crop Productivity
• 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.
Suspended sediment in three Georgia rivers
• 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.
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.
• 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.
Downwind Effect of a Windbreak
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.
• 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.
• 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.
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.
Channel Erosion
• 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 ).
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.
Rainfall Factor
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.
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
• 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.
• 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).
• 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.
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).
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
Rising
Stage
Sampler
DH-48
Beadload Sampler
Coshocton
Sampler
Preventing Soil Erosion
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Vegetative cover
Surface stabilization
Velocity reduction
Peak flow reduction
Inspection and maintenance
General
Terrace
Design
Types of Graded Terraces
Forest Management