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
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• What is the significance of understanding streamflow?
• Why are we concerned with how it relates to
Landscapes?
• Streamflow is important because it is related to:
– Construction of houses, bridges, spillways, and culverts
– Surface Runoff over landscapes, including flooding
– The associated processes of Erosion, Transport, and
Deposition.
– Drinking and Irrigation water supplies, especially during
droughts
– Recreational activities, such as boating and fishing
– Navigation of commercial shipping and transport
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Hydrograph:
– Plots precipitation and runoff over time.
– Runoff can be discharge, flow, or stage
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Storm Hydrograph
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Storm Hydrograph
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Storm Hydrograph
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Storm Hydrograph
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Storm Hydrograph
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Lag
Time
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Flow behavior for different streams
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Hydrograph Behavior
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Hydrograph Behavior:
Related to channel size
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Hydrograph
for 1997
Homecoming
Weekend
Storm
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Hydrograph Behavior:
Also related to channel patterns
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Measurement Units
cfs: cubic feet per second
gpm: gallons per minute
mgd: million gallons per day
AF/day: Acre-Feet per day
cumec: cubic meters per second
Lps: liters per second
Lpm: liters per minute
• 1 cfs 
– 2 AF/day
– 450 gpm
– 28.3 Lps
• 1 m3/s = 35.28 cfs
• 1 mgd  1.5 cfs
• 1 gpm = 3.785 Lpm
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WEIR: Used to provide accurate flow measurements
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Weir Types
Circular opening:
Q = c  r2 h1/2
Rectangular:
Q = c W h3/2
Triangular:
Q = c h5/2
where
Q is flow, cfs
c are weir coefficients
h is stage, ft
r is the pipe diameter, ft
W is the weir width, ft
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Coweeta
Hydrologic
Station
Rectangular Weir
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V-Notch (Triangular) Weir
Field Velocity Measurements
Flow Equation:
Q=vA
where
Q is the discharge, cfs
v is the water velocity, ft/s
A is the flow cross-sectional
area, ft2
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Discharge Measurements
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Manning's Equation
• When flow velocity measurements are not available
• v = (1.49/n) R2/3 S1/2
• where
– v is the water velocity, ft/s
– n is the Manning's hydraulic roughness factor
– R = A / P is the hydraulic radius, ft
• A is the channel cross-sectional area, ft2
• P is the channel wetted perimeter, ft
– S is the water energy slope, ft/ft
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• Hydrologic Statistics:
– Trying to understand and predict streamflow
• Peak Streamflow Prediction:
– Our effort to predict catastrophic floods
• Recurrence Intervals:
– Used to assign probability to floods
• 100-yr flood:
– A flood with a 1 chance in 100 years, or a flood
with a probability of 1% in a year.
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Return Period
• Tr = 1 / P
– Tr is the average recurrence interval, years
– P is exceedence probability, 1/years
• Recurrence Interval Formulas:
– Tr = (N+1) / m
– Gringarten Formula: Tr = (N+1-2a) / (m-a)
• where
– N is number of years of record,
– a = 0.44 is a statistical coefficient
– m is rank of flow (m=1 is biggest)
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River Stage:
The elevation of the
water surface
Flood Stage
The elevation when the
river overtops the
natural channel
banks.
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Bankfull Discharge
Qbkf = 150 A0.63
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• Rating Curve
– The relationship between river stage and discharge
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Peak Flows in Ungaged Streams
• Qn = a Ax Pn
• where
– A is the drainage area, and
– Pn is the n-year precipitation depth
– Qn is the n-year flood flow
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•
Q2 = 182 A0.622
Q10 = 411 A0.613
Q25 = 552 A0.610
Q100 = 794 A0.605
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Channel flooding vs upland flooding
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Curve Number Method
• Most common method used in the U.S. for predicting
stormflow peaks, volumes, and hydrographs.
• Useful for designing ditches, culverts, detention ponds, and
water quality treatment facilities.
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• P = Precipitation, usually rainfall
– Heavy precipitation causes more runoff than light precipitation
• S = Storage Capacity
– Soils with high storage produce less runoff than soils with little storage.
• F = Current Storage
– Dry soils produce less runoff than wet soils
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• r = Runoff Ratio => how much of the rain runs
off?
–r=Q/P
• r = 0 means that little runs off
• r = 1 means that everything runs off
–r=Q/P=F/S
• r = 0 means that the bucket is empty
• r = 1 means that the bucket is full
• F=P-Q
or
r = Q / P = (P - Q) / S
– the soil fills up as it rains
• Solving for Q yields:
– Q = P2 / (P + S)
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• S is maximum available soil moisture
– S = (1000 / CN) - 10
– CN = 100 means S = 0 inches
– CN = 50 means S = 10 inches
• F is actual soil moisture content
– F / S = 1 means that F = S, the soil is full
– F / S = 0 means that F = 0, the soil is empty
Land Use
Wooded areas
Cropland
Landscaped areas
Roads
CN
25 - 83
62 - 71
72 - 92
92 - 98
S, inches
2 - 30
4 - 14
0.8 - 4
0.2 - 0.8
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Curve Number Procedure
• First we subtract the initial abstraction, Ia, from the observed
precipitation, P
– Adjusted Rainfall: Pa = P - Ia
– No runoff is produced until rainfall exceeds the initial abstraction.
– Ia accounts for interception and the water needed to wet the organic layer
and the soil surface.
– The initial abstraction is usually taken to be equal to 20% of the maximum
soil moisture storage, S, => Ia = S / 5
• The runoff depth, Q, is calculated from the adjusted rainfall, Pa , and
the maximum soil moisture storage, S, using:
– Q = Pa2 / (Pa + S)
– or use the graph and the curve number
• We get the maximum soil moisture storage, S, from the Curve
Number, CN:
– S = 1000 / CN - 10
– CN = 1000 / (S + 10)
• We get the Curve Number from a Table.
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Example
• A typical curve number for forest lands is CN = 70, so the maximum soil
storage is: S = 1000 / 70 - 10 = 4.29"
• A typical curve number for a landscaped lawn is 86, and so
– S = 1000 / 86 - 10 = 1.63”
• A curve number for a paved road is 98, so S = 0.20”
• Why isn’t the storage equal to zero for a paved surface?
– The roughness, cracks, and puddles on a paved surface allow for a small amount
of storage.
– The Curve Number method predicts that Ia = S / 5 = 0.04 inches of rain must fall
before a paved surface produces runoff.
• For a CN = 66, how much rain must fall before any runoff occurs?
– Determine the maximum potential storage, S = 1000 / 66 - 10 = 5.15"
– Determine the initial abstraction, Ia = S / 5 = 5.15” / 5 = 1.03"
– It must rain 1.03 inches before runoff begins.
• If it rains 3 inches, what is the total runoff volume?
– Determine the effective rainfall, Pa = P - Ia = 3" - 1.03" = 1.97"
– Determine the total runoff volume, Q = 1.972 / (1.97 + 5.15) = 0.545"
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Unit Hydrographs
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Unit Hydrograph
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Unit Area Hydrographs
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Unit Hydrograph Example
• A unit hydrograph has been developed for a watershed
– The peak flow rate is 67 L/s for 1 mm of runoff and an area of 100 ha
– What is the peak flow rate for this same watershed if a storm produces 3 mm
of runoff?
• The unit hydrograph method assumes that the hydrograph can be scaled linearly
by the amount of runoff and by the basin area.
• In this case, the watershed area does not change, but the amount of runoff is three
times greater than the unit runoff.
• Therefore, the peak flow rate for this storm is three times greater than it is for the
unit runoff hydrograph, or 3 x 67 L/s = 201 L/s.
• What would be the peak flow rate for a nearby 50-ha watershed for
a 5-mm storm?
– Peak Flow: Qp = Qo (A / Ao ) (R / Ro )
where
• Qp is the peak flow rate and Qo is for a reference watershed,
• A is the area of watershed and Ap is the area of reference watershed.
– Q = (67 L/s) (50 ha / 100 ha) (5 mm / 1 mm) = 168 L/s
– In this case, the peak runoff rate was scaled by both the watershed area
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the runoff amount.
Flood Routing
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• Streamflow and Land Management
– BMPs improve soil and water quality
• Most of our attention is placed on preventing pollution,
decreasing stormwater, and improving low flows.
– Forestry
• Forest streams have less stormflow and total flow, but
more baseflow
• Forest litter (O-Horizon) increases infiltration
• Forest canopies intercept more precipitation (higher
Leaf-Area Indices, LAI)
• Forest have higher evapotranspiration rates
• Forest soils dry faster, have higher total storage
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• Forest Management
– Harvesting
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High-lead yarding on steep slopes reduces soil compaction
Soft tires reduces soil compaction
Water is filtered using vegetated stream buffers (SMZs)
Water temperatures also affected by buffers
– Roads
• Road runoff can be dispersed onto planar and convex slopes
• Broad-based dips can prevent road erosion
– Site Preparation
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Burning a site increases soil erosion and reduces infiltration
Leaving mulch on soils increases infiltration
Piling mulch concentrates nutrients into local "hot spots"
Distributing mulch returns nutrients to soils
Some herbicides cause nitrate increase in streams
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Forestry Compliance in Georgia with Water
Quality Protection Standards (1991-2004)
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• Agricultural Land Management
– Overland flow is a main concern in agriculture
• increases soil erosion, nutrients, and fecal coliform
• increases herbicides, pesticides, rodenticides, fungicides
– Plowing
• exposes the soil surface to rainfall (and wind) forces
• mulching + no-till reduces runoff and increases infiltration
• terracing and contour plowing also helps
– Pastures (livestock grazing)
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increases soil compaction
reduces vegetative plant cover
increases bank erosion
rotate cattle between pastures and fence streams
• Urban Land Management
– Urban lands have more impervious surfaces
• More runoff, less infiltration, recharge, and baseflow
• Very high peak discharges, pollutant loads
• Less soil storage, channels are straightened and piped, no floodplains
– Baseflows are generally lower, except for irrigation water (lawns & septic)
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Benefits of Riparian Buffers
• Bank Stability:
– The roots of streambank trees help hold the banks together.
– When streambank trees are removed, streambanks often collapse,
initiating a cycle of sedimentation and erosion in the channel.
– A buffer needs to be at least 15 feet wide to maintain bank
stability.
• Pollutant Filtration:
– As dispersed overland sheet flow enters a forested streamside
buffer, it encounters organic matter and hydraulic roughness
created by the leaf litter, twigs, sticks, and plant roots.
– The organic matter adsorbs some chemicals, and the hydraulic
roughness slows down the flow.
– The drop in flow velocity allows clay and silt particles to settle out,
along with other chemicals adsorbed to the particles.
– Depending on the gradient and length of adjacent slopes, a buffer
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needs to be 30-60 feet wide to provide adequate filtration.
• Denitrification:
– Shallow groundwater moving through the root zones of floodplains is
subject to significant denitrificiation.
– Removal of floodplain vegetation reduces floodplain denitrification
• Shade:
– Along small and mid-size streams, riparian trees provide significant
shade over the channel, thus reducing the amount of solar radiation
reaching the channel so summer stream temperatures are lower and
potential dissolved oxygen levels are higher.
– Buffers need to be at least 30 feet wide to provide good shade and
microclimate control, but benefits increase up to 100 feet.
• Organic Debris Recruitment:
– River ecosystems are founded upon the leaves, conifer needles, and
twigs that fall into the channel.
– An important function of riparian trees is providing coarse organic
matter to the stream system.
– Buffers only need to encompass half the crown diameter of full-grown
trees to provide this function.
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• Large Woody Debris Recruitment:
– Large woody debris plays many important ecological functions in
stream channels.
– It helps scour pools, a favored habitat for many fish.
– It creates substrate for macroinvertebrate and algae growth, and it
forms cover for fish.
– It also traps and sorts sediment, creating more habitat complexity.
– Woody debris comes from broken limbs and fallen trees.
– The width of a riparian buffer should be equal to half a mature tree
height to provide good woody debris recruitment.
• Wildlife Habitat:
– Many organisms, most prominently certain species of amphibians
and birds use both aquatic and terrestrial habitat in close proximity.
– Maintaining a healthy forested riparian corridor creates important
wildlife habitat.
– The habitat benefits of riparian buffers increase out to 300 feet.
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