1.3 Infiltration Rate

Knowing how easily water infiltrates into your soil helps you understand how well it is 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 your top soil structure is good. The top soil has stable aggregates, around which water can percolate downwards into the soil profile.

If your infiltration rate is slow, it indicates your top soil structure is poor. The top soil has unstable aggregates and is compact, so it blocks water from percolating downwards. Instead water runs off taking soil and nutrients with it!

The testing area should not be saturated so if it has rained heavily wait until the area is a bit drier.

Here are instructions on how to make an infiltration rate tube.

What to record


  • Time it takes for water to infiltrate into the soil (minutes, seconds)
  • Photos

Equipment


  •  150mm x 150mm metal/plastic tubes (with 85mm depth marked)
  • Water bottle with 450ml marked on it (equates to 1″ depth of water when in tube)
  • Water (4L or so per field)
  • Mallet – for driving tube into soil (optional)
  • Wood block – to protect from damage when hammering in (optional)
  • Stopwatch (on phone)

How to do the test

1. Locate your chosen sample site – you can use the GPS mapping feature on the app to do this if you’ve recorded the sample locations previously.

2. It’s very important to remove all debris, weeds and vegetation ~ 150mm x 150mm square – either trim these down or cut through with a knife

3. Insert the cylinder/tin into the bare ground, to 85mm depth

4. Firm soil gently on the inside of the tube

5. Measure out exactly 450ml of water from your pre-marked water bottle

6. Pour water into cylinder and start timer

7. Stop timer when all the water has disappeared into the ground and the surface is just glistening. Record the time in the app.

8. Repeat steps 5-7. Record this second time in the app. We do this because often the second time gives a better indication of the real infiltration rate that is less dependent on recent weather, as the first test just wets the soil. If the soil is already quite saturated you may find there is very little difference in the first and second times.

9. You can stop here, or if you want to be very confident in your infiltration rate reading or if it has been particularly dry, then it’s worth repeating steps 5-7 again a couple more times to ensure you are getting a representative infiltration rate.

If it’s happening very slowly, carry-out other tests nearby and check back on it every few minutes.

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.

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

Read this article to find out how many times to do the infiltration rate test on each field.

Resources

USDA Infiltration Rate

NRCS Soil Water Infiltration

How to make an infiltration rate tube