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Infiltrometer is a device used to measure the rate of water infiltration into soil or other porous media. Commonly used infiltrometers are single ring or double ring infiltrometer, and also disc permeameter.

Single ring infiltrometer

The single ring involves driving a ring into the soil and supplying water in the ring either at constant head or falling head condition. Constant head refers to condition where the amount of water in the ring is always held constant. Because infiltration capacity is the maximum infiltration rate, and if infiltration rate exceeds the infiltration capacity, runoff will be the consequence, therefore maintaining constant head means the rate of water supplied corresponds to the infiltration capacity. The supplying of water is done with a Mariotte's bottle. Falling head refers to condition where water is supplied in the ring, and the water is allowed to drop with time. The operator records how much water goes into the soil for a given time period. The rate of which water goes into the soil is related to the soil's hydraulic conductivity.

Double ring infiltrometer

Double ring infiltrometer requires two rings: an inner and outer ring. The purpose is to create a one-dimensional flow of water from the inner ring, as the analysis of data is simplified. If water is flowing in one-dimension at steady state condition, and a unit gradient is present in the underlying soil, the infiltration rate is approximately equal to the saturated hydraulic conductivity. An inner ring is driven into the ground, and a second bigger ring around that to help control the flow of water through the first ring. Water is supplied either with a constant or falling head condition, and the operator records how much water infiltrates from the inner ring into the soil over a given time period.

There are three main problems related to the use of infiltrometers: 1. The pounding of the infiltrometer into the ground deforms the soil causing cracks and increasing the measured infiltration capacity. 2. Natural rainfall reaches terminal velocity. Also natural droplet sizes differ with different types of storms. Pouring water from a measuring cup however loses this momentum and variance. 3. With single ring infiltrometers, water spreads laterally as well as vertically and the analysis is more difficult.