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
• Watershed:
– The area that contributes to a river or stream
• Watershed Divide:
– The boundary that separates two watersheds
– Usually a ridge or upland area
Watershed Delineation
(based on topographic map)
Subsurface Capture
Georgia’s Major
River Basins
Runoff
Efficiency:
Proportion of
precipitation that ends
up as streamflow
• Water Budget Equation
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–
–
–
Q = P - ET
Q is the mean annual streamflow
P is the mean annual precipitation, and
ET is the mean annual evapotranspiration
Location
Athens, GA
Seattle, WA
Olympic Mts, WA
Tucson, AZ
P
50
40
120
12
ET
35
20
20
35
Q
15
20
100
0
Where do you think the extra water in Tucson is coming from?
• Depth vs. Flow
– We can convert depth per time (inches per year) to a volume per time
(ft3/s), but how?
• By multiplying by the watershed area, Q = A · D
• This is because, if you add 1" to your bathtub, the volume is the area of
the base times the depth
• Think of it as spreading the water out over the watershed
• The depth is over the whole watershed area.
• The base is the watershed area
• Example
– For a mean annual streamflow depth of D = 15"/yr
– For a A = 10-mi2 watershed:
• A = 10-mi2 x 640 acres / mi2 = 6,400 acres
– Using Q = D A
• Q = 96,000 acre-inches per year = 8,000 acre-ft per year
•
Acre-Foot => A volume of water
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–
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–
•
Equal to one foot of water that covers one acre of land
Lake Lanier holds 2,000,000 acre-feet of water
Georgia agriculture uses many times this much in a year
As do the Georgia pulp and paper mills.
Example:
– If I have a 100-acre golf course, and I put on 3" of water, how many acre-feet is this?
• (100 acres) x (3") x (1 ft / 12") = 25 AF
– Let's say I do this every week during the summer (20 weeks)
• (25 AF/wk) x (20 wks/yr) = 500 AF/yr
– How big of a pond do you need if the pond is 10 feet deep?
• 500 acre-feet / 10 ft = 50 acres!
– This only works if there is no inflow to the pond.
•
Changes in Storage, ΔS = I – O
– ΔS is the change in water storage
– I are the hydrologic inputs, such as rain
– O are the hydrologic outputs, such as streamflow and evapotranspiration
•
Example:
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–
–
–
–
I = 20 AF/wk, inflow to pond
O = 25 AF/wk, outflow from pond for irrigation
ΔS = I - O = -5 AF/wk, change in pond storage
For 20 weeks of irrigation, we would only need a pond that held 100 AF
For a 10-foot deep pond this is only 10 acres instead of 50!
• Precipitation
• Percolation, water moving through
the unsaturated zone.
• Canopy Interception, water
collected on plant leaves and
• Recharge, water moving from the
stems
unsaturated zone to the saturated
zone across the water table
• Throughfall, precipitation
minus interception
• Exfiltration, water moving from
below the soil surface to the
• Overland flow, also called
surface
surface runoff
• Infiltration, water moving from
above the soil surface, into the
soil.
Infiltration &
Hillslope Flow
• Reason why infiltration decreases during a
rainstorm:
– Soil wets up, filling all empty pores
– Low permeability (restricting) layer below surface
– When soil is bare, pores become clogged with
eroded clay particles
Infiltration Capacity
(Runoff happens when rainfall intensity
exceeds the infiltration capacity)
Methods for Increasing Infiltration
• Surface mulching
– Protects soil surface during rainstorm
– Ponded water moves more slowly downslope
– Soil humus increases aggregate formation - peds
• Depression storage
– Increases depth of ponding so higher gradient
– Contour tilling decreasing downslope velocity
• Soil liming, CaOH, CaCO3, CaSO4
– Increases aggregate formation - flocculation
– Also increases base saturation
– Can improve soil pH
• No-till agriculture, planting w/o plowing
– Maintains and improves soil structure
– Increases soil organic matter
Where Does Water in Rivers and
Streams Come From?
a. Pushed up from the center of the earth by pressure
b. Pushed up through the earth by the winds on the
oceans
c. The earth eats salt water and uses the energy of
the salt to pump water to springs
d. Mostly overland flow from rainfall
e. None of the above
Flow Components
Urban: Overland flow
Rural: Subsurface flow
Map of saturated areas showing expansion during a rainstorm.
• Precipitation on channels, ponds, lakes:
– The area covered by water in some watersheds is large, perhaps up to 20%
• Precipitation on saturated areas near channels:
– Following prolonged rainfall, the areas near streams become wet, and act just
like the channel
• Overland flow:
– Also called sheet flow and surface runoff, it is water on the surface, flowing
downhill, that is not in a channel
• Subsurface flow:
– Shallow and deep subsurface flow through soil and aquifers, usually discharging
into or near channels
• Ground Water Hydrology
– Ground water is the water held in pores in the
subsurface
– Ground water supplies the baseflow (flow during dry
periods) to streams.
• A water table:
– Separates the ground water under positive pressure (saturated
zone or phreatic zone) from the water under negative pressure
(unsaturated zone or vadose zone)
• Above the water table is an unsaturated zone
– Water pressures are negative
– Soils hold water due to capillary forces.
• Below the water table is the saturated zone
– Water pressures are positive
– Water flows freely into wells
– A well or piezometer can be used to measure the location of the
water table.
– The water table is generally smooth, just like the land surface
– Water tables rise in wet periods, fall in dry periods
Perched aquifer:
A zone of saturation above an aquitard that prevents the water
from moving downward.
Unconfined (or water table) aquifer:
A zone of saturation below the regional water table.
An aquifer:
Moves significant quantities of water to a well
An aquitard:
Has some, but not much, ability to move water
An aquiclude:
Is almost impermeable
Confined Aquifers
• A confined aquifer is isolated from above
and below by aquitards.
– Most of its flow comes from recharge at
outcrops in the updip direction.
– Confined aquifers have a potentiometric surface
instead of a water table.
– Sometimes the potentiometric surface rises
above the ground surface, in this case the wells
flow naturally and are called artesian.
Savannah River Site
• Find the flow to Savannah
– Use Darcy’s Law:
– Q=AKG
• Assume a hydraulic
conductivity
– K = 0.003 ft/s
• Using the contour lines, we
estimate a hydraulic
gradient:
– Between the 100 ft and 60 ft
contours, the head drop is 40
feet
– The distance between these
contour lines is approximately
six miles or 32,000 feet (using
the map scale).
– The hydraulic gradient is G =
40 ft / 32,000 ft = 0.00125.
• To get the area, consider points A and B on the potentiometric map.
– They are on the same contour, so water is flowing perpendicular to the line
between A and B in a southeasterly direction.
– The aquifer thickness is b = 600 feet
– The aquifer width is w = 25 miles between points A and B
– The area is A = b w = (600 ft) x (132,000 ft)
• The flow through the aquifer is:
– Q = K A G = (0.003 ft/s) x (600 ft) x (132,000 ft) x (0.00125)
– Q = 300 ft3/s = 192 mgd
– This is enough water to provide domestic supply for
approximately 1.3 million people (assuming a per capita use of
150 gallons/day).
– If you go back to the schematic for hydraulic conductivities
shown in Chapter 9, you can see that the range of conductivities
for carbonate rocks is huge.
– The flow estimated above could easily be 10 times greater.
• Find the hydraulic gradient
– G = Δh / L
– Δh is the water surface
change between contours
• We can use the 80 and 60 foot
contours to get a change in
head of 20 feet
– L is the distance between
contour lines.
• The average distance between
these two contours is
approximately 1/2 mile (from
the scale at the bottom) or 2640
feet.
• Therefore the gradient is:
– G = Δh / L = 20 ft / 2640 ft
– G = 0.00758 ft/ft
Snoqualmie River
• Find the ground-water flux:
– q=KG
– q = (1.5 x 10-3 ft/s) x (0.00758 ft/ft)
– q = 1 ft/day
• This has the units of a velocity
– Flux is often called the darcian velocity.
– It is equivalent to the average velocity calculated as if water moved through
the entire aquifer
– Rather than just through the pores of the aquifer as it actually moves.
• Find the total flow, Q = q A
– q is from the previous step
– A is the cross-sectional area of flow
• equal to the length of the valley between A and B (approximately 2.5 miles or
13200 feet) times the average depth of the aquifer (100 feet).
– Q = (0.0015 ft/s) x (0.00758) x (1,320,000 ft2)
• Q = 15 ft3/s
– This aquifer flow is discharging to the river.
• Therefore, flow in the river next to point B should be at least 15 cfs greater than
adjacent to point A
• Keep in mind that aquifer water is entering the river from the other side as well.
Relationship of hillslope flow processes
with land management concerns
•
Floods and Baseflows
– Soil and vegetative conditions determine how rainfall moves to streams and thus
dictate baseflows and flood peaks and volumes.
– Land managers want to maximize infiltration and minimize overland flow to minimize
flooding and maximize baseflows.
•
Stormwater Management
– The magnitude of hydrologic alteration caused by development depends on the degree
to which soils are disturbed, vegetation is altered, and land is covered with pavement.
– Appropriate design of stormwater management and treatment facilities depends on the
ability to predict this change.
•
Hillslope Stability
– The location and timing of landslides is largely driven by subsurface flow conditions.
– For example, seepage areas on steep hillslopes are high landslide danger areas.
•
Site Productivity and Irrigation Needs
– Soil moisture is a limiting factor for tree and crop growth in much of the U.S.
– Some parts of the landscape grow trees or crop better because topographic and
geologic conditions cause water to accumulate in those areas.
– At the extreme, subsurface flow conditions may make an area too wet to grow
many commercially valuable crops.
•
Stream, Slope, and Wetland Geomorphology
– Geologic conditions are a dominant control of hydrologic processes, but runoff
patterns and characteristics in turn alter the landscape.
– Landscapes are never in equilibrium, although some landscapes change much
more rapidly than others.
– Runoff patterns and groundwater flow in a basin determine the number and
distribution of streams and wetlands as well as other landscape features.
•
Water Chemistry
– Interaction, or lack thereof, between water and soils has a strong influence on the
chemical composition of water entering streams and wetlands.
– For instance, most microbial activity, nutrient cycling, and plant uptake occur in
shallow soils.
– The longer flow spends in this zone, the purer the water that leaves the hillslope.
– It also influences the suitability of groundwater as a supply of drinking water.
Landscape factors that determine stream morphology, habitat, & biota (Jackson 2008)
Geology
Climate
Topography/Topology
Soils
Flows
Largely
unmodified by
watershed
activities.
Vegetation
Sediment
Loading
Woody
Debris
Loading
Channel Habitat Structure, Chemistry,
Physical Conditions, & Biota
Time since
disturbance*
Biogeographic
Setting
Ecosystem Engineers:
Humans & Beavers
*Major disturbances include fires, hurricanes, glaciation,
epidemics, keystone episodes, etc.
•
Geology
– The parent geology of a basin determines the type of sediment available to the channel
system.
– Highly weathered granite produces poor gravel
– Channels in weathered granite tend to be sandy.
– Young basalt produces highly resistant, long-lasting gravel.
– Geology also affects the stability of slopes and resulting sediment delivery.
•
Topography
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Channel slope (along with flow) drives the sediment transport capacity of a stream.
Steep channels tend to have rocky and coarse substrate.
Flat channels tend to have sand and fine sediment substrates.
Valley side slopes affect sediment production from upslope activities and can also affect
woody debris recruitment to the channel system.
– Valley confinement controls the amount of energy in the channel versus energy expended on
the floodplain during high flows.
•
Climate
– The characteristics and amount of rainfall in a basin, as well as the potential
evapotranspiration in a basin, determine the amount of flow in a stream per unit area.
– They also affect the stream density in a basin.
– The intensity and depth of rainstorms affects erosion, slope stability, and sediment delivery
to streams.
– Climate drives the type of vegetation that can grow in a basin
•
Soils
– Soil layering, hydrologic characteristics of soil horizons, and depths of soil
horizons are strong controls on the runoff generating processes in a basin.
– Bare soil – Horton overland flow and erosion.
– Mulched soil – higher infiltration rates, less overland flow.
– Affects the type and productivity of vegetation.
•
Vegetation: riparian and upslope
– The quantity and type of vegetation on the uplands determines the amount of
surface runoff and erosion from the hillsides.
– It also affects the actual evapotranspiration with consequences for stream
baseflows.
– Riparian vegetation provides bank stability, shade, and organic debris inputs to the
channel.
•
Flows
– The temporal characteristics of flows and the total volume of flow, along with
channel slope, are the dominant drivers of sediment movement, channel scour, and
woody debris transport.
– They also affect the survival of fish during the low flow period, the flushing of fish
from the channel during high flows, and the scour and transport of fish eggs.
– The amount and velocity of flow affects dissolved oxygen (DO) concentrations and
water temperatures during the summer.
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•
•
Sediment Loading
– The amount of sediment introduced to the stream affects whether a channel is
aggrading, incising, or maintaining a constant level.
– The amount and type of sediment affects the occurrence of pool habitat and the
amount of interstitial habitat in the channel bed material.
Woody Debris
– Woody debris acts as "scour elements" in channels, meaning that pools tend to
form around large woody debris during high flow events.
– During baseflows, these pools are important habitat features for fish. Woody debris
also provides cover for fish, and provides substrate for the growth of
macroinvertebrates (fish food).
– Art Benke, an aquatic entomologist, has determined that woody debris is
responsible for over half the macroinvertebrate production in blackwater rivers.
Ecosystem Engineers (Humans and Beavers)
– Big dams and small dams alter flow hydrographs, geochemistry, sediment routing,
and more.
– Levees.
– Streambank engineering (rip rap, auto bodies, concrete).
– Humans alter soils and vegetation with resulting effects on streams.
•
Time Since Disturbance
– Some disturbances are large and infrequent (e.g. hurricanes, large wildfires, the
cotton farming era), and channels take time to recover from these disturbances.
– Our streams in the GA Piedmont are still recovering from cotton farming that
ended 75 years ago.