Double ring versus single ring infiltrometer

A double ring infiltrometer has two concentric rings (inner about 30 cm, outer about 60 cm) driven into the soil, with both kept filled to the same water depth. Only the water added to the inner ring is measured.

Advantages over a single ring infiltrometer

1. Reduces lateral spreading: water in the outer ring (buffer) forces the water in the inner ring to infiltrate vertically downward, so the rate measured is true vertical infiltration. In a single ring, water spreads sideways under the ring and gives a value that is too high.
2. More accurate and reliable values of the infiltration capacity and the constant final rate $f_c$.
3. Better field representation because the flow beneath the inner ring is one-dimensional.
4. Repeatable results: the effect of soil non-uniformity near the ring edge is reduced, so tests at different sites are comparable.
5. Usable on all soil types, including very permeable soils where a single ring would show strong lateral flow.

Its disadvantages are that it needs more water and more labour than a single ring, and that driving the rings can disturb the soil.