Chapter 11
Infiltration, Streamflow and Groundwater
• 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 Hydrology
Watershed Delineation
Subsurface Capture
Water Budget Equation
• Q = P - ET
– Q is the mean annual streamflow
– P is the mean annual precipitation, and
– ET is the mean annual evapotranspiration
Water Budgets - Some US Cities
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?
Runoff
Efficiency
Water Budget Example
• Say that we have:
– P = 50" of rain in Athens
– ET = 35" of plant and soil water loss
– The streamflow depth is:
• Q = 50 - 35 = 15” / yr
• We can convert depth per time (15”/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
• 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
– Q = 8,000 acre-ft per year
• Given, 1 cfs (ft3/s)  2 AF/day (acre-feet per day)
– Q = (8,000 AF/yr) / (365 days/yr) = 22 AF/day
– Q = (22 AF/day) / (1 cfs / 2 AF/day) = 11 cfs
Acre-Foot: A volume of water
• Equal to one foot of water that covers one
acre of land
• Lake Lanier holds 2,000,000 acre-feet of
water
• Georgia agriculture easily uses many times
this much in one year
• As do the Georgia pulp and paper mills.
• 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
• S = I - O
– S is the change in water storage
– I are the hydrologic inputs, such as rain
– O are the hydrologic outputs, such as
streamflow and evapotranspiration
Pond Storage
•
•
•
•
I = 20 AF/wk, inflow to pond
O = 25 AF/wk, outflow from pond for irrigation
S = 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!
Infiltration
• Stormwater Budget:
–
–
–
–
–
P=F+I+O
P is the precipitation
F is the infiltration
I is the canopy interception
O is the overland flow
• Infiltration
– Water moving from above the soil surface, into
the soil.
• Percolation
– Water moving downward through the
unsaturated zone.
• Recharge
– Water moving from the unsaturated zone to the
saturated zone
• Exfiltration
– Water moving from below the soil surface to
the surface
Infiltration Capacity
• 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
Wetting Front Infiltration
Infiltration Capacity
Reduction of infiltration
rates with time.
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
Subsurface Flow Components
Map of saturated areas showing expansion during a rainstorm.
Answer
• 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
Relationship of hillslope flow processes
with land management concerns
• 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.
Biogeochemical Cycling
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.
Variation in Site Productivity and
Irrigation Requirements
• 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.
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.
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.
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.
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.
Subsurface Hydrology
• 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:
Managed by:
- U.S. Forest Service
- UGA Savannah River Ecology Laboratory
- U.S. Department of Energy
Contamination problems:
- Chemical wastes
- Radioactive wastes
SRS Aquifer Tests:
Southwest Pad
• We need to use Darcy’s Law:
– Q=AKG
• Assume a hydraulic conductivity of 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.
Groundwater Flow:
Snoqualmie River
• Find the hydraulic gradient, G = h / L
– h 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 = h / L = 20 ft / 2640 ft = 0.00758 ft/ft
• 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.
Seven Landscape Factors that
Drive Channel Morphology
•
•
•
•
•
•
•
Geology and Soils
Topography
Vegetation: riparian and upslope
Climate
Flows
Sediment loading
Woody Debris
Geology and Soils
• 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.
• The parent material also is a factor in soil
conditions.
• Soil layering, hydrologic characteristics of soil
horizons, and depths of soil horizons are strong
controls on the runoff generating processes in a
basin.
Topography
• 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.
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.
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.
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 DO
concentrations and water temperatures during the
summer.
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.
Quiz 11
• Download from my website:
> www.hydrology.uga.edu/rasmussen
> Soils and Hydrology website