Infiltration rate

Knowing how easily water infiltrates into your soil tells you how well it’s soaking up rain. You want water to be stored in the soil profile, and not to run off into water courses.

If your infiltration rate is quick, it indicates top soil structure is good, with stable aggregates, around which water can percolate down into the soil profile. If your infiltration rate is slow, it indicates top soil structure is poor, with unstable aggregates and compaction, blocking water from percolating downwards. Instead water runs off taking topsoil and nutrients with it!

The testing area should not be saturated, so if it has rained heavily wait until the area is drier. Learn how to make an infiltration rate tube here.

What to record

The number of minutes it takes for 25mm/1 inch of water to infiltrate into the soil. Record two pours separately – ‘1st inch’ & ‘2nd inch’

Make notes: Anything interesting you’ve noticed!

Photos

Equipment

A smartphone (with Sectormentor downloaded)

150 x 150 mm tubes (with 85 mm depth marked)

Water bottle with 450ml marked on it – (this equates to 1″ depth of water when poured into the cylinder)

Water (4L or so – so you can do a few different samples!)

Mallet – for driving tube into soil (optional)

Wood block – to prevent damage when hammering (optional)

Stopwatch (on phone)

How to do the test

1. Head to the sample location you’re interested in testing, and remove all weeds / debris / vegetation in a 150 x 150 x 150 cm area on the soil.

2. Hammer the infiltration cylinder into the soil, to an 85 mm depth.

3. Gently firm the soil on the inside of the tube, and measure out 450ml of water in your pre-marked bottle.

4. Pour the water into the cylinder and start your timer. Stop the timer when all the water has disappeared into the ground and the surface is just glistening. Record the infiltration time in Sectormentor.

5. Repeat step 4 again in the same location for the second inch – the second time you get a better indication of the actual infiltration rate of your soil (less dependent on recent weather). You may want to keep repeating to see the effect this has on the rate!

6. If you want to work out your infiltration rate in inches/hr. Do 3600/no. of seconds, this gives you the number of inches/hr. If you want to know mm/hr then divide your answer by 25.4.

e.g 20 seconds. Is 3600/20 = 180 inches/hr.

180/25.4 = 7mm/hr

Resources

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052494.pdf

https://www.dropbox.com/s/hykau3m05c1g9yp/NRCS%20Soil%20Water%20Infiltration.pdf?dl=0

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