In 1975, ASTM Committee D18 on Soil and Rock published the first version of , titled Standard Test Method for Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer . The method was deceptively simple: drive two concentric metal rings into the ground, fill both with water, and measure how fast the water level drops in the inner ring. The outer ring created a buffer zone so water flowed vertically from the inner ring, not horizontally. This gave a true infiltration rate—how fast rainwater, irrigation, or runoff enters the soil.