Infiltration Rate Calculator

Water doesn't soak into soil at a constant rate — it starts fast while the soil is dry and slows as the profile saturates, and Horton's 1940 infiltration equation (R.E. Horton, "An Approach Toward a Physical Interpretation of Infiltration-Capacity," Soil Science Society of America Proceedings) describes that decline as an exponential curve running from an initial rate down to a steady-state final rate. This calculator takes the initial and final rates you measured with a ring infiltrometer, applies a representative exponential decay shape between them over your test duration, and integrates that curve to report the average rate over the whole test and the total depth of water absorbed.

Because a real infiltrometer reading only gives you two rate values and a duration — not a full time series of measurements — the calculator applies a standard decay shape rather than precisely fitting a decay constant to your specific curve; a full lab-quality Horton fit needs several intermediate readings, not just the endpoints. The soil type comparison checks whether your measured steady-state rate falls within the typical range for sand, silt loam, or clay, since a rate well below what's typical for the selected texture usually points to surface sealing, crusting, or subsurface compaction rather than the soil's natural drainage capacity. The runoff potential rating is a simple threshold read on the final rate alone: soils that have slowed to less than roughly a tenth of an inch per hour by the end of the test are flagged as very high runoff risk, since water arriving faster than that steady-state rate has nowhere to go but across the surface.