FLOW MECHANISMS THROUGH CONTINUOUS AND BURIED MACROPORES

Abstract

Tillage destroys stable continuous macropores and at the same time creates unstable tillage pores within the tilled layer. The buried macropores below the tilled layer are not harmed. Flow through continuous pores is usually considered to be laminar. The objectives of this study were to determine the conditions under which water would flow into a buried macropore in a soil column, and to determine the flow regime inside a macropore. Packed columns (457 mm length, 76 mm diameter) were constructed with a sloping base to measure separately outflow from the artificial macropore (6 mm diameter) and the soil matrix. The columns were then tilled artificially by removing the top 12 cm and repacking. In addition, a study was conducted in a sloping box of soil with four macropores beneath a tilled layer. Dye was added to two 76-mm-diameter rings within the box, either immediately or 1 day before ponded infiltration into the rings. Ponded outflow rates from continuous 6-mm macropores ranged from 5 to 11 ml/s, which is equivalent to Reynolds numbers as high as 2370. Outflow also occurred from buried macropores, although the rate was reduced (0.01 to 0.3 ml/s) compared with continuous macropores. If dye was added immediately before infiltration, dye appeared in macropore outflow, but if dye was added 1 day before infiltration, no dye appeared in macropore outflow but only in matrix outflow. This indicated either preferential flow carrying the dye (if no time for binding to soil) or preferential flow of water around the dye. Even buried macropores can function as preferential flow pathways, and turbulent flow might occur through large, continuous macropores.