Physical and chemical controls of preferred path flow through a forested hillslope

Abstract

Investigations of the physical and chemical characteristics of macropores and mesopores at two forested sites established for subsurface transport research are summarized. The hydrologically active macroporosity (pores larger than 1 mm diameter) is a very small fraction of the total soil porosity but is sufficient to conduct a large proportion of ponded infiltration. Mesopores (pores less than 1 mm diameter that are generally drained at field capacity) are sufficient to conduct total infiltration during the majority of rain events and have a much higher surface area than macropores. The cation adsorption coefficient of macropore walls was similar to bulk soil for mineral coatings but was higher for organic linings derived from roots. The subsurface outflow from a 0.46 ha subwatershed generated by two precipitation events displayed two contrasting patterns of chemical concentration. At the time of peak discharge, the concentration of Na, K, Mg, Ca and S was close to maximum (high supply); and for Al, Fe and Mn, chemical concentration was highest prior to peak discharge (low supply). A pathlength-supply hypothesis is proposed, based on (1) changing proportions of mesopore flow path lengths with stage of the subsurface hydrograph and (2) the ability of mesopore surfaces and adjacent micropores to continue supplying chemicals to percolating soil water. Macropores are viewed as being important physical conduits in convergent flow zones of watersheds but as having little influence on water quality.