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Measuring infiltration rate
INFILTRATION RATE
The infiltration rate is the speed at which water enters into the soil. It is usually measured by the depth (in mm) of the water layer that can enter the soil in one hour. For example, an infiltration rate of 15 mm/hour means a water layer of 15 mm on the soil surface will take one hour to infiltrate.
In dry soil, water infiltrates rapidly. This is called the initial infiltration rate. As more water replaces the air in the pores, the water from the soil surface infiltrates more slowly and eventually reaches a steady rate. This is called the basic infiltration rate. The infiltration rate depends on both soil texture and soil structure. Sandy soils, with larger macropores have high infiltration rates while clay soils have much slower rates due to their platy structure and reduced pore size.
BASIC INFILTRATION RATES FOR VARIOUS SOIL TYPES
Soil type Basic infiltration rate (mm/hour)
sand >30
sandy loam 20 – 30
loam 10 – 20
clay loam 5 – 10
clay 1 – 5
Measuring infiltration rates
Materials
infiltration ring (a metal ring 6" in diameter and 8" tall. Metal stovepipe works great)
rubber mallet
10" piece of 2x4
Plastic wrap
stopwatch
ruler or measuring tape
1 gallon of water
Steps
1. Clear any turf or other debris from the area you wish to test.
2. Pound the infiltration ring evenly into the ground using the 2x4 and rubber mallet. Leave 2-3" above ground.
3. Place a sheet of plastic wrap in the cylinder so it covers the bottom and up the sides. The plastic wrap helps keep the soil surface from being disturbed when the water is poured in and allows for even infiltration.
4. Gently pour water into the ring, filling it to the rim. Measure the initial level of water in the ring and record.
5. Pull the plastic wrap out so the water is able to seep into soil.
6. Start the timer as soon as the plastic is removed. At 2 minutes, measure the amount of water left in the ring with the ruler.
7. Refill the ring after the initial measurement and record water level.
8. Repeat steps 6-7 at 1-5 minute* intervals. Record in mm the amount the water level drops and the level of the refilled ring. Make sure you measure at the same location each time.
9. Continue to record measurements until the drop in water level is the same for two consecutive measurements. This will be the basic infiltration rate.
Note: For heavily compacted or clayey soils, measurements may need to be taken at longer intervals.
Infiltration rate example
Clock reading Time Difference Cumulative time Water Level (mm)
Before After
Filling Filling Infiltration Infiltration rate
mm/min mm/hour Cumulative infiltration
2:05:00 0 100
2:07:00 2 2 92 100 8 4.00 240 8
2:10:00 3 5 93 99 7 2.33 140 15
2:15:00 5 10 89 101 10 2.00 120 25
2:25:00 10 20 84 100 17 1.70 102 42
2:30:00 10 30 89 102 11 1.10 66 53
2:45:00 10 40 95 101 7 0.70 42 60
3:05:00 20 60 92 100 9 0.45 27* 69
3:25:00 20 80 91 9 0.45 27* 78
Table from the Food and Agriculture Organization of the United Nations publications website
*basic infiltration rate
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