Infiltration Rate Calculator

Dividing infiltrated volume by area and time turns a bucket measurement into a flux with the units of a velocity. The area used must be the inner ring only: the outer buffer ring of a double-ring set contributes no water to the tally, it merely floods the surrounding soil so that flow beneath the inner ring stays vertical instead of spreading sideways. In mL per square metre per minute the reading converts to mm/hr by multiplying by 0.06. Readings are repeated until successive intervals stop changing, and that plateau value, the basic intake rate, approximates the saturated hydraulic conductivity of the most restrictive horizon. USDA intake classes place soils at very slow below 1 mm/hr, slow 1 to 5, moderately slow 5 to 20, moderate 20 to 60, moderately rapid 60 to 125, rapid 125 to 250, and very rapid above 250 mm/hr. Many stormwater manuals set a floor near 13 mm/hr (0.5 in/hr) before an infiltration basin or bioretention cell may be permitted, then divide the measured value by a safety factor of 2 to 3 for design. Three limitations recur. Driving the rings shatters soil structure at the wall and can open a leakage path along it. Water viscosity changes with temperature by roughly 2 to 3 percent per degree C, so tests run with warm water overstate cold-season performance. And infiltration is log-normally distributed across a site with coefficients of variation of 50 to 200 percent, so a single test is close to meaningless; three to six replicates summarised by their geometric mean is the usual minimum.

Cylinder infiltrometers grew out of irrigation research in the early twentieth century, when engineers sizing furrow and border-check systems needed to know how fast a field would take water. A. N. Kostiakov proposed in 1932 an empirical power law for cumulative infiltration fitted directly to such tests, and it is still used for irrigation advance calculations. Single rings were soon found to overestimate vertical infiltration because water diverged laterally below the cylinder; a concentric buffer ring was added to suppress that divergence, and the residual errors of both configurations were analysed by Burgy and Luthin in 1956. ASTM codified the double-ring procedure as standard D3385 in the 1970s, fixing the inner ring near 30 cm and the outer near 60 cm and requiring the test to continue until the rate stabilises; it has been revised several times since. Herman Bouwer's chapter on cylinder infiltrometers in the Soil Science Society of America's Methods of Soil Analysis remains the standard critique of the technique's assumptions. Because ponded rings force water through cracks and worm channels that gentle rain would never fill, Perroux and White introduced the tension disc permeameter in 1988, supplying water at controlled suction so that macropore and soil-matrix contributions can be separated; rainfall simulators approach the same problem from the opposite direction. Regulatory demand drove the method's spread: urban stormwater rules from the 1990s onward increasingly require site-specific infiltration testing before an infiltration facility can be approved.