4 Minute Read

Test procedure and significance

The infiltration rate is the velocity or 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. 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 soil texture (the size of the soil particles) and soil structure and is a useful way of categorizing soils from an irrigation point of view. The most common method to measure the infiltration rate is by a field test using a cylinder or ring infiltrometer. The infiltration curve should be determined for normal soil moisture conditions before irrigation takes place, i.e. usually when the top soil is dry.

Equipment needed: Shovel/hoe, Hammer (2 kg), Watch or clock

Step 1: Hammer the 30 cm diameter ring at least 15 cm into the soil. Use the timber to protect the ring from damage during hammering. Keep the side of the ring vertical and drive the measuring rod into the soil so that approximately 12 cm is left above the ground.

Step 2: Hammer the 60 cm ring into the soil or construct an earth bund around the 30 cm ring to the same height as the ring and place the hessian inside the infiltrometer to protect the soil surface when pouring in the water.

Step 3: Start the test by pouring water into the ring until the depth is approximately 70-100 mm. At the same time, add water to the space between the two rings or the ring and the bund to the same depth. The water in the bund or within the two rings is to prevent a lateral spread of water from the infiltrometer.

Step 4: Record the clock time when the test begins and note the water level on the measuring rod.

Step 5: After 1-2 minutes, record the drop in water level in the inner ring on the measuring rod and add water to bring the level back to approximately the original level at the start of the test. Record the water level. Maintain the water level outside the ring similar to that inside.

Step 6: Continue the test until the drop in water level is the same over the same time interval. Take readings frequently (e.g. every 1-2 minutes) at the beginning of the test, but extend the interval between readings as the time goes on (e.g. every 20-30 minutes). Note that at least two infiltration tests should be carried out at a site to make sure that the correct results are obtained.

The figures below indicates the infiltration test performed with a single ring. Accordingly the ring is inserted into the soil and water is subsequently poured. The observations are recorded for stipulated durations.

Sample infiltration curve is provided here for reference.