Estimating Streamflow Channel Losses with the Green-Ampt

Model

Neil Hutten

Ag Eng 558

April 20, 2001

Presentation Outline

• Introduction and Motivation

• Channel Loss Estimation Methods

• Rawls & Brakensiek (1983) Determinations of Green-Ampt Parameters

• Application to a stream site

• Uncertainties and Conclusions

Motivation

• RCRA requires TSDs to determine whether they are located in 100-year floodplains

• Additional engineering studies are required if TSD is located in a floodplain ($$)

• Floodplain extent is influenced by losses• Stream “loss” is groundwater “gain”• Stream channel losses can be pathways for

subsurface contaminant transport

Commonly-used methods to estimate stream channel losses

• Assume infiltration losses are balanced by local precipitation gains

• Representative Reach Loss– Stream Gage A minus Stream Gage B

• Adjust/extrapolate a “known” rate• Adjust peak flows from regression equations

to equal peak flows obtained from flood frequency analyses of gaged data.

Limitations of stream loss methods

• No data at ungaged sites

• Ephemeral or intermittent streams

• Extent of groundwater contributions

• Evapotranspiration vs. Infiltration

• Channel wetting and drying cycles

• Single value for an entire stream reach

Green-Ampt Overview

•Ponded or unponded infiltration

•Deep homogeneous soil

•Water infiltrates as “piston flow”

•Sharply defined wetting front

• Green-Ampt Overview

• Rate Form (f) of G-A Equation assumes a ponded surface so the infiltration rate equals infiltration capacity of the media.

• Depth of Ponding can be neglected.

Green-Ampt Model

•Green-Ampt Parameters

• Effective Suction at Wetting Front

• Effective Hydraulic Conductivity

• Soil Porosity

Green-Ampt Depth to Wetting Front

Porosity

timeSuctionCapCondHydLf

N

tSfKLf

)(*).(*.).(2

2

Green-Ampt Infiltration Rate

FrontWettingtoDepth

FrontWettingtoDepthSuctionCapCondHydf

Lf

LfSfKf

.*.

)(*

•Rawls and Brakensiek (1980s)

• Determined ranges of values for: – Wetting Front Suction – Hydraulic Conductivity – Soil Porosity

• For eleven USDA Soil Textures

• 1200 Soils, 5000 Horizons, 34 States

• Methods described in standard references

Wetting Front Suction

Porosity

ClayPercentC

SandPercentS

:Where

)])(S(000799.0

))(C(00348.0)C)(S(0000136.0))(C(0016.0))(S(0016.0

))(S(04989.0)C)(S(000344.0)(809.3)C(00158.0)(326.753.6exp[Sf

2

222222

22

Wetting Front Suction with Texture

Porosity with Texture

Hydraulic Conductivity with Texture

Green-Ampt Parameters asDetermined by Rawls/Brakensiek

Meanwhile, back at the river…

• Streamflow Losses on Big Lost River were determined from stream gage station data by Bennett (1990)

• Average annual streamflow: 1965 to 1987

• Sixteen (16) streamflow measuring sites and stations

Big Lost River Losses

• 1.5 cfs/mi: west bndry, INEEL to div. dam• 2.5 cfs/mi: div. dam to Hwy 26• 5 cfs/mi: Hwy 26 to Lincoln Blvd (ICPP)• 1 cfs/mi: Lincoln Blvd (ICPP) to Lincoln Blvd

(NRF)• 4 cfs/mi: Lincoln Blvd (NRF) to BLR Sinks• 2 cfs/mi: above BLR Sinks• 18 cfs/mi: in the Big Lost River Sinks.

Measured Channel Loss

• Stream gaging station 11

• Bennett’s measured loss = 2 cfs/mile

• Channel Width varies from 40 to 60 feet; – 40 feet was used

• Measured infiltration rate = 1.04 cm/hr.

Specifics at Station 11

• Coarse pebble to cobble gravel above gaging station 11

• Sediment grades to sand and “sandy silt” below station 11

• Sandy Loam set of Green-Ampt parameters was used for “sandy silt”

Selection of G-A Parameters

• Sandy Loam (R-B) Parameters

• Porosity (phi) = 0.453cc/cc

• Wetting front Suction head Sf = 11.01 cm

• Hydraulic conductivity Ks = 1.09 cm/hr

• “Modelled” Sandy Loam Infiltration Rate after ten hours was 1.61 cm/hr

• Measured rate was 1.04 cm/hr (2 cfs/mi)

Depth to Wetting Front and Infiltration Rate for Sandy Loam

0

5

10

15

20

25

30

35

1 3 5 7 9 11 13 15 17 19

Depth toWetting (cm)

Infiltration Ratecm/hour

Depth to Wetting Front and Infiltration Rate for Sand

0

10

20

30

40

50

60

70

80

1 3 5 7 9 11 13 15 17 19

Depth toWetting (cm)

Infiltration Ratecm/hour

Ten-hour Wetting Depth and Infiltration Rates with Green-Ampt Parameters

After Ten (10) HoursWet Front Hydraulic Wet. Dept. Infiltr. Rate

Porosity Suction Conduct. (cm) (cm/hr)

Sand 0.437 4.95 11.78 51.66 12.91Loamy Sand 0.437 6.13 2.99 28.90 3.62Sandy Loam 0.453 11.01 1.09 23.02 1.61Loam 0.463 8.89 0.66 15.92 1.03Silt Loam 0.501 16.68 0.34 15.05 0.72Sa Cl Loam 0.398 21.85 0.15 12.83 0.41Clay Loam 0.464 20.88 0.10 9.49 0.32Si Cl Loam 0.471 27.30 0.10 10.77 0.35Sandy Clay 0.43 23.90 0.06 8.17 0.24

Uncertainties

• Pre-existing initial moisture contents were not considered

• R-B Porosity, Wetting Front Suction, and Hydraulic Conductivities for media larger than sand not available

• Infiltration characteristics of ephemeral channel bottoms compared to infiltration of upland soil sites

• Layering, textural changes, surface crusts, etc.

Conclusions

• Green-Ampt parameters developed by Rawls and Brakensiek may be a useful tool to determine stream channel infiltration loss rates.

• Ten-hour Modeled Infiltration Rate (1.6 cm/hr) approximated the measured infiltration rate (1.0 cm/hr)

• Time frame of measured infiltration rate was not specified.

• Compare field samples with R-K parameters

• This is a research area worth further investigation.