Estimating Streamflow Channel Losses with the Green-Ampt Model

1
Estimating Streamflow Channel Losses with the
Green-Ampt Model

• Neil Hutten
• Ag Eng 558
• April 20, 2001

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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

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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

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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.

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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

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Green-Ampt Overview

• Ponded or unponded infiltration
• Deep homogeneous soil
• Water infiltrates as piston flow
• Sharply defined wetting front

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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.

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Green-Ampt Model
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Green-Ampt Parameters

• Effective Suction at Wetting Front
• Effective Hydraulic Conductivity
• Soil Porosity

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Green-Ampt Depth to Wetting Front
11
Green-Ampt Infiltration Rate
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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

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Wetting Front Suction
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Wetting Front Suction with Texture
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Porosity with Texture
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Hydraulic Conductivity with Texture
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Green-Ampt Parameters asDetermined by
Rawls/Brakensiek
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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

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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.

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Measured Channel Loss

• Stream gaging station 11
• Bennetts measured loss 2 cfs/mile
• Channel Width varies from 40 to 60 feet
• 40 feet was used
• Measured infiltration rate 1.04 cm/hr.

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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

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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)

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Depth to Wetting Front and Infiltration Rate for
Sandy Loam
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Depth to Wetting Front and Infiltration Rate for
Sand
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Ten-hour Wetting Depth and Infiltration Rates
with Green-Ampt Parameters
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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.

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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.