Principles of Soil Infiltration - University of Minnesota Extension ...

Stormwater U Education Program:
Bioretention Systems: The “Dirt” on
Soils, Water, and Infiltration
Infiltration Measurement in Stormwater BMPs
Farzana Ahmed
Research Assistant
University of Minnesota
August 9, 2011
Minnetonka Community Center
Minnetonka, MN
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What is soil infiltration?
Process by which water on the ground surface enters
the soil

Why is it important to measure the
infiltration rate?
To predict the efficiency of the planned infiltration
practice.

Why is it important to measure the
infiltration rate?
To determine performance and schedule
maintenance, because:
Accumulation of soil particle at the down
slope reduce the infiltration rate of soil.

What affects the infiltration rate?
• Soil properties (i.e. hydraulic conductivity, porosity)
• Soil compaction during construction of BMPs

Techniques to measure infiltration
rate
• Double Ring Infiltrometer
Constant head test.
Inner ring dia 20cm and
outer ring 40cm.
Cannot capture the spatial
variation of infiltration

Spatial variation of saturated
hydraulic conductivity of soil
Images courtesy of N. Olson

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)
• 42 cm long - 10 cm inner
diameter cylinder - two parts.
• Clear acrylic pipe and finished
steel.
• A metric measuring tape -
adhered outside of the pipe
• Stopwatch
• A manual and a software

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)
Pound the MPD into the soil

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)
Fill the MPD with water and record the
waterlevel with time

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)
Measuring initial and final moisture
content:
• Guide for estimating soil water moisture
content based on soil feel and appearance for
several soil textures (Handout, page#3)

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)
MPD spreadsheet
Input data:
Initial moisture content
Final moisture content
Head vs time data
Output:
Saturated Hydraulic
conductivity of
soil(K sat)
Soil Suction
MPD has been purchased by City of St. Cloud
and City of Edmonton

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)

Modified Philip Dunne Infiltrometer
(MPD Infiltrometer)

Infiltration rate measurement
F(
t)
F( t)
K satt
ln1
f ( t ) K sat
1
F ( t )
Where,
F(t) = Cumulative infiltration;
Ψ = Soil suction;
∆θ = Difference between saturated moisture content and initial moisture content;
f(t) = Infiltration rate

Comparison with MPD Infiltrometer
and Double Ring Infiltrometer
Ksat (cm/hr)
30
25
20
15
10
5
0
Saturated Hydraulic Conductivity obtained from the
device
MPD Infiltrometer DRI Infiltrometer

Comparison with MPD Infiltrometer
and Double Ring Infiltrometer
Time(hr)
4
3.5
3
2.5
2
1.5
1
0.5
0
Time required to perform the test
MPD Infiltrometer DRI Infiltrometer

Comparison with MPD Infiltrometer
and Double Ring Infiltrometer
Volume of water(L)
30
25
20
15
10
5
0
Water required to perform the test
MPD Infiltrometer DRI Infiltrometer

Take home message
• Measurement of infiltration rate is important to
determine the efficiency of prospective practice, to
determine performance and schedule
maintenance of existing practice.
• We need to know the spatial distribution of
infiltration rate to estimate infiltration rate.
• MPD infiltrometer is designed for spatial
distribution
has been used at up to 20 locations simultaneously,
allowing for up to 40 measurements per day, with a
three-person team.

Thank you!!
Questions
&
Comments??