How fast does water flow in an unsaturated macropore? Evidence from field and lab experiments

A wide range of available field and lab evidence can lead to useful generalizations about the speed of macropore flow, which often dominates the transport of water and contaminants. In 36 published field tests, the values of maximum transport speed in macropores and other preferential channels vary surprisingly little. The available tests vary widely in type of medium, including fractured rock and various soil textures; in length scale of the test, ranging from 1 to 1000 m and more; in type of tracer used; and in direction of flow. One factor that does significantly affect transport speeds is the supply of water that generates the flow. A sporadic supply of water at the land surface, as from natural rainfall, causes markedly slower preferential flow than a continuous supply, as from steady irrigation. For continuously supplied water, nearly all observations of maximum transport speed fall between 1 and 100 m/d, suggesting that an average value in that range could serve as a guideline for expected transport speed under comparable conditions. Lab experiments in published studies help to explain and support these field results. Visualization studies show that unsaturated macropore flow occurs in four distinct modes. Descriptive names for these are film flow, continuous rivulet, snapping rivulet, and pulsating blob.