A simple infiltration test for recharge estimation

A simple infiltration test gives a practical first measure of how quickly soil accepts water. It is useful for preliminary recharge potential estimation, but it does not directly reveal how much groundwater an aquifer receives. That distinction matters in Australia, where coastal sands, compacted urban clay and intense summer storms can produce very different outcomes.

This guide explains a field method using basic equipment, careful measurements and sound site selection. It covers the calculation of results, their interpretation in Australian conditions and their place within a wider groundwater assessment. The basic version suits homeowners, land managers and community groups; a double-ring arrangement provides a useful upgrade when greater reliability is needed.

What infiltration testing reveals

Ponded water moves downward through soil under gravity, and a controlled test records that movement. After the initial wetting and filling of small pores, the intake rate becomes broadly indicative of the soil’s saturated hydraulic conductivity under the test conditions. It describes how readily the surface transmits water, not how much usable groundwater the aquifer beneath can supply.

Groundwater recharge depends on effective rainfall, soil-moisture storage, vegetation, runoff, depth to water and aquifer properties. A high rate can mean rapid drainage rather than dependable storage. A low rate can coexist with regional groundwater movement into the area. The field result therefore belongs beside rainfall records, soil information and bore-level data, rather than replacing them.

India’s National Project on Aquifer Management provides aquifer mapping resources that illustrate how regional hydrogeological information can support groundwater management. This is a useful international reference, while Australian decisions should rely on state and local authorities and observations from the site being assessed.

Choosing a safe and representative site

Choose an open, reasonably level patch that represents the ground you want to assess. Avoid hollows, recently filled soil, irrigated garden beds, vehicle tracks, hardstanding and ground beside buildings. Photograph the site and record its coordinates, soil texture, vegetation, slope, cracks, stones and animal activity. The conditions matter: a lawn being checked for rainfall absorption is a different question from a site assessed after a dry summer.

Obtain the owner’s permission and check council or public-land requirements. Before disturbing the soil, identify buried electricity, gas, water and telecommunications services and follow the relevant locating procedure. A lawn test differs from constructing a bore or pumping groundwater, but licensing and approval requirements still vary. For example, the Water Management Act 2010 in New South Wales and the Water Act 1989 in Victoria control specified groundwater activities. Confirm local rules before digging or installing irrigation, and avoid contaminated sites and septic fields.

Equipment for the field test

Use an open-ended metal or rigid plastic ring, ideally 200 mm in internal diameter, with straight walls and enough height to hold 50 mm of water. A 150 mm ring also works if its area is measured accurately. Gather a 2-litre graduated jug, a ruler, a stopwatch or phone timer, tape measure, gloves, notebook, timber block and mallet. A camera and an optional 300–400 mm outer ring are useful.

Australian garden and hardware retailers commonly stock pond liners, irrigation fittings and similar rings. Choose a ring with an open base, not a sealed floor, and check its internal diameter rather than relying on the nominal size. Online soil-science suppliers also sell field infiltrometers, but a purpose-built instrument is not essential for screening. Keep children and pets away, wear gloves and never use the ponded water for drinking.

Running the test step by step

Clear surface litter gently without tilling or pulverising the soil. Drive the ring about 20–30 mm into the ground using a timber block so that it sits securely and level. Avoid striking services, stones or roots, and close obvious gaps by firming the surrounding soil lightly. Measure the internal diameter at two positions and record the soil condition before adding water.

Fill the ring to 50 mm above the soil surface and allow for the initial rapid intake before timing the measured phase. Maintain that depth by adding known volumes as the level falls. Record elapsed time and every addition: check every five minutes during the first 15 minutes, then every 15–30 minutes. Continue until the calculated rates are close across at least three successive readings, commonly over one to two hours. Clay may need longer.

Choose a dry window and stop if rain, irrigation or strong evaporation compromises the run. With an outer ring, note whether it collects escaping water and keep a clear gap between rings. Repeat the procedure in at least three separate, similar patches, spaced far enough apart not to disturb one another. Keep the ring fixed during each run, and calculate each location separately rather than pooling every early reading into one average.

Calculating the infiltration rate

Calculate the rate for each interval from the volume added and elapsed time. With area in square centimetres, infiltration rate (mm/h) = water added (mL) ÷ [ring area (cm²) × time (hours)]. One millilitre over one square centimetre equals one millimetre of water depth, so no further conversion is needed.

For a 200 mm ring, the area is approximately 314 cm². An addition of 500 mL over 30 minutes gives about 3.2 mm/h. Once the rate stabilises, use the mean of the final three consistent intervals and check whether their variation suits the purpose. Repeat the calculation for each location, then report the median, range and test conditions rather than a single polished figure.

If a later 500 mL addition takes one hour, the same ring gives about 1.6 mm/h. The difference shows why the initial fast intake should not be treated as final soil capacity. Account for evaporation and lateral losses where possible, and end the run if cracking, slumping or leakage changes the conditions. Keep the original readings so the calculation can be checked.

Reading results in Australian conditions

Interpret the result with the landscape, not in isolation. Perth’s coastal sands can accept water rapidly but may store little and transmit contaminants quickly; fast intake is not a guarantee of a sustainable supply. Melbourne’s heavy clay and compacted fill can produce much slower rates, yet groundwater conditions vary across suburbs and geological settings. Soil infiltration, aquifer recharge and bore yield are separate questions.

Brisbane’s intense summer storms can deliver more rain than heavy soil can absorb, turning part of a storm into runoff and flooding. That does not show that no recharge occurred. The portion that infiltrates may still replenish soil moisture and deeper aquifers. Compare measured intake with local rainfall intensity, the depth to the water table and observed water-level changes before drawing conclusions about a storm event.

Local records can refine the picture: Bureau of Meteorology rainfall, soil and land-use maps, nearby bore hydrographs and state groundwater reports all help. Repeat the test after meaningful rain and across seasons when the purpose is long-term management. A defensible recharge assessment combines infiltration with rainfall, soil-moisture balance, vegetation and aquifer data.

Turning measurements into a local water plan

A basic result is strongest as screening evidence. If measured intake exceeds rainfall intensity, the surface may accept much of that rainfall; if it is lower, additional water may run off or pond. Neither relationship determines recharge by itself, because soil storage and the unsaturated zone may absorb water first. Compare any numerical estimate with groundwater-level responses, soil-moisture measurements or modelled percolation before assigning a recharge volume to a property or catchment.

Use the test to choose where more detailed work is worthwhile, such as monitoring a bore, assessing a proposed rainwater-harvesting area or designing a recharge structure. Treat unusually high or low results as a prompt to repeat the measurement and investigate heterogeneity. Professional interpretation is appropriate before major landscaping, bore construction or changes to groundwater extraction.

Keep a dated record of the location, weather, preceding rainfall, soil description, ring dimensions, added volumes, calculated rates and anomalies. Before changing drainage, irrigation or bore works, share the results with your council or a qualified hydrogeologist. Start with three replicate runs in a safe area, keep the method consistent when conditions change, and use the evidence to build a realistic local groundwater plan.