Design and Use of Closed‐top Infiltrometers

Several similar closed‐top infiltrometers were designed and used to simulate effective surface heads ranging from −3 to +1 cm of water. Design was based on the principle that natural positive soil air pressure may be simulated by creating an equivalent negative air pressure above ponded surface water. Effective surface head h s , defined as the difference between the ponded water depth h w and either the actual or simulated soil air pressure head h a , is negative when h a is greater than h w . Under natural field conditions, negative h s often causes counterflow of soil air during water infiltration. Narrow ranges in h s surrounding zero greatly affect infiltration by controlling air and water flow in soil macropores and weakly hydrophilic micropores. Thirty‐minute cumulative infiltration increased 19% per centimeter increase in h s for one soil and 33% per centimeter for another soil. Closed‐top infiltrometers make possible realistic infiltration measurements under the negative h s commonly produced by rain and irrigation waters as they infiltrate natural soils. These devices may also be useful in studying the infiltration effects of several interacting parameters that are related to h s including soil surface macroporosity and roughness, macropore space extent and continuity, antecedent soil air and water, surface wettability and water repellency, wetted surface area and shape, and surface crust and slope. The infiltration response to h s in the range −3 to +1 cm is of practical significance since such a range results from common soil and water management practices.