1. Chapter 11 Infiltration, Streamflow and Groundwater
The area that contributes to a river or stream
The boundary that separates two watersheds
Usually a ridge or upland area
4. Watershed Hydrology
5. Watershed Delineation
8. Subsurface Capture
9. 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
10. Water Budgets - Some US Cities Location P ET Q
Athens, GA 50 35 15
Seattle, WA 40 20 20
Olympic Mts, WA 120 20 100
Tucson, AZ 12 35 0
Where do you think the extra water in Tucson is coming from?
11. Runoff Efficiency
12. 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
13. 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
14. 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
15. 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.
16. 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.
17. 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
18. Pond Storage
19. 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!
20. 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
Water moving from above the soil surface, into the soil.
Water moving downward through the unsaturated zone.
Water moving from the unsaturated zone to the saturated zone
Water moving from below the soil surface to the surface
23. Infiltration Capacity
25. 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
26. Wetting Front Infiltration
27. Infiltration Capacity
29. Methods for Increasing Infiltration Surface mulching
Protects soil surface during rainstorm
Ponded water moves more slowly downslope
Soil humus increases aggregate formation - peds
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
30. 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
35. Map of saturated areas showing expansion during a rainstorm.
38. 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
Also called sheet flow and surface runoff, it is water on the surface, flowing downhill, that is not in a channel
Shallow and deep subsurface flow through soil and aquifers, usually discharging into or near channels
39. 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.
40. Biogeochemical Cycling
41. 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.
42. 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.
43. 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.
44. 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.
45. 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.
46. 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.
47. Subsurface Hydrology
48. 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
51. 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.
53. SRS Aquifer Tests: Southwest Pad
55. We need to use Darcy’s Law:
Q = A K G
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
56. 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.
58. 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
59. Find the ground-water flux:
q = K G
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.
60. 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.
61. Seven Landscape Factors that Drive Channel Morphology Geology and Soils
Vegetation: riparian and upslope
62. 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.
63. 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.
64. 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.
65. 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.
66. 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.
67. 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.
68. 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.
69. Quiz 11 Download from my website:
Soils and Hydrology website