Geography K–10 · Years 7–8
Ring infiltrometer: how fast water soaks into three surfaces
Water in the world (ACARA Year 7 sub-strand); NSW Stage 4 focus area Water in the World (2015 syllabus) and Water in the world (2024 syllabus)
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The idea
Infiltration rate falls as the soil wets up and differs by surface, which decides how much rain becomes runoff instead of soil water.
Safety card
Hazards
- mallet strike
- sharp pipe edge
- sun
- buried irrigation pipes or cables
Controls
- adult drives the rings; edges filed and taped
- gloves
- hats and shade breaks
- sites checked with the school's general assistant before any ring is driven
Note
No chemicals or heat are used. The activity is run under the school risk-assessment procedure; RiskAssess (riskassess.com.au) holds templates for school practicals and fieldwork. The NSW Department of Education Chemical Safety in Schools package is not triggered.
What you need
- 3 rings cut from 100 mm PVC stormwater pipe, 150 mm long, one end edge filed smooth
- Rubber mallet and a wooden board to drive each ring in as far as the soil allows, 100 mm where it will go (leaving 50 mm above ground for the 25 mm pond) and never less than 50 mm; the depth driven is recorded for each site, because a shallow ring loses more water sideways
- 1 L measuring jug, 200 mL measuring cylinder, cling film
- Ruler with mm marks taped inside each ring, zero at the soil surface
- Stopwatch; data sheet with columns time (s) and water level (mm)
- Three sites: mown lawn, compacted path or oval goal-square, garden bed with mulch removed
How to do it
- At each site drive a ring in with the board on top so it stays vertical, to 100 mm where the soil allows and at least 50 mm in hard ground, and record the depth driven. Press soil against the inside edge to stop leaks.
- Measure the ring's inside diameter and compute the volume for a 25 mm depth from that measurement, not from the pipe's nominal size: nominal 100 mm stormwater pipe bores several millimetres under 100 mm, and an inside diameter of 96.6 mm gives π × 4.83 cm × 4.83 cm × 2.5 cm = 183 mL, where a true 100 mm bore would give 196 mL, a 7 per cent difference. Line the ring with cling film, pour in that volume, then pull the film away so the water starts on the soil at time zero.
- Read the water level every 30 s until the surface is dry or 10 minutes have passed; if it empties early, refill to 25 mm at once and keep timing.
- Convert each 30 s drop to a rate in mm/h (drop in mm × 120) and plot rate against time for the site.
- Repeat the fill twice more at the same ring (the third run is closest to the steady rate).
- Compare the three sites' steady rates and the time each took to accept the first 25 mm.
What you should see
Rate is highest in the first 30 s and falls towards a steady value. The compacted path is expected to be slowest and may not accept 25 mm in 10 minutes, and the garden bed fastest. The FAO permeability classes for agriculture and conservation give a scale to report against (Table 15 on the FAO page gives cm/hour, which × 10 is mm/h: very slow below 1.3, slow 1.3 to 3, moderately slow 5 to 20, moderate 20 to 63, moderately rapid 63 to 127, rapid 127 to 250, very rapid above 250 mm/h; that column leaves 3 to 5 mm/h unclassed, and the table's cm/day column puts slow at 3 to 12 cm/day, 1.25 to 5 mm/h); the table is for saturated samples, so the field steady rate is compared with it as a guide only. A single ring also loses water sideways beneath its rim rather than straight down, so the rate it reports is larger than one-dimensional infiltration, commonly by a factor of two for a ring this small and this shallowly seated; that is why field practice uses double-ring or buffered infiltrometers. The class reports its rate as an upper estimate, states the depth each ring was driven, and notes that a factor of two spans two or three of the FAO classes. The learner reports the steady rate in mm/h for each site from the third run. It worked if the third run is slower than the first at every site (the soil is wetting up).
What changes
- What you change
- surface (lawn, compacted path, garden bed)
- What you measure
- infiltration rate (mm/h) over time
- What you keep the same
- ring size and depth driven
- initial water depth 25 mm
- reading interval
- three fills per site
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Soil soaks up water at the same rate all through a storm.
- A green lawn on compacted soil absorbs rain like a garden bed.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE4-2GE4-7
- Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE4-PRI-01GE4-TAP-01
- Australian Curriculum v9AC9HG7K01AC9HG7S02AC9HG7S03
Sources
The pages the author read to write this activity.