NEED FOR INFILTROMETERS

Jan 8, 1997

Robert M. Dixon, Ph.D

There is a great need for simple field measurement methods to provide realistic infiltration rates for input to hydrologic models. This paper describes the use of newly developed closed and open top infiltrometers for obtaining input data. The authors and their coworkers have shown that two physical properties of the soil surface regulate water infiltration into dry soils and the upward counterflow of displaced soil air. These relationships are embodied in the Air-Earth Interface Concept which states that surface microroughness and macroporosity control infiltration into initially dry soils by regulating the exchange of water and displaced soil air across the air-earth interface. The microrough and macroporous surface has very high infiltration rates because fluid exchange is relatively unimpeded, whereas the microsmooth and microporous surface has very low infiltration rates due to severe impediment of fluid exchange. Almost all infiltrometers and rainfall simulators greatly over estimate infiltration because of unimpeded lateral flow of displaced soil air beneath the wetting front. One exception is the closed-top infiltrometer which operates at a negative surface head, thereby inducing fluid counterflow across the air-earth interface as often occurs during natural infiltration events. Newly developed closed and open top infiltrometers provide a method for determining the range of infiltration rates which could occur under natural conditions. Under some conditions, rates measured with open top infiltrometers are an order of magnitude greater than those measured with a closed top infiltrometer. The greatest differences occur in soils which have hydrophobic properties, capillary discontinuities, and well developed networks of macropores. These infiltrometers used side-by-side can provide the hydrologic modeller with a realistic range of infiltration inputs.

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