Soils & Hydrology II

9. Soil Water

10. Precipitation and Evaporation

11. Infiltration, Streamflow, and Groundwater

12. Hydrologic Statistics and Hydraulics

13. Erosion and Sedimentation

14. Soils for Environmental Quality and Waste Disposal

15. 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 – Q = P - ET– Q is the mean annual streamflow– P is the mean annual precipitation, and – ET is the mean annual evapotranspiration

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?

• 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 – 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:– 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• Canopy Interception, water

collected on plant leaves and stems

• Throughfall, precipitation minus interception

• Overland flow, also called surface runoff

• Infiltration, water moving from above the soil surface, into the soil.

Infiltration & Hillslope Flow

• Percolation, water moving through the unsaturated zone.

• Recharge, water moving from the unsaturated zone to the saturated zone across the water table

• Exfiltration, water moving from below the soil surface to the surface

• 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 waterAn 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 = A K G

• 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.

Snoqualmie River

• 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

• 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.

• 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.

Climate

Geology

Topography/Topology

Soils Vegetation

Flows Sediment Loading

Woody Debris

Loading

Channel Habitat Structure, Chemistry, Physical Conditions, & Biota

Ecosystem Engineers:

Humans & Beavers

Largely unmodified by watershed activities.

Time since disturbance*

*Major disturbances include fires, hurricanes, glaciation, epidemics, keystone episodes, etc.

Landscape factors that determine stream morphology, habitat, & biota (Jackson 2008)

Biogeographic Setting

• 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– 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.

• 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.