WATER RESOURCES RESEARCH, VOL. 30, NO. 4, PAGES879-890, APRIL 1994 Flow and solutetransport through the soil matrix and macropores of a hillslopesegment Yoshio Tsuboyama Forestry and Forest Products Research Institute, Tsukuba Norin Kenkyu Danchi-nai, Ibaraki, Japan Roy C. Sidle Forest Service Intermountain Research Station, U.S. Department of Agriculture, Logan, Utah ShojiNoguchi and Ikuhiro Hosoda Forestry andForestProducts Research Institute, Tsukuba Nofin KenkyuDanchi-nai, Ibaraki,Japan Abstract. Subsurface flow from various portions of a soilprofile on a steep, forested hillslope was evaluated by two sets of step-change miscible displacement tests at different app!•ication rates and antecedent hydrologic conditions. Solutions of NaC1 (1000 mg L -• C1-) were applied at steady state rates (equivalent to 20 and 30 mmh-t of standing water over the entire plot area) usinga line irrigation sourcelocated 1.5 m upslope (lateral distance) from an excavated soilpit. Subsurface flow and tracer breakthrough from five portions(the organic-rich soil layer including macropores, the mineral soil matrix, and three groups of macropores in the mineralsoil layer) of the soil profile were individually measured and analyzed using a convective-dispersive model. Matrix flow dominated discharge from the soil pit during tracer tests(70-93% of total discharge). However, during wet periods with upslope drainage,macropores (including organic-rich soil) contributed proportionally more flow than duringperiods when upslope drainage was minimal. Outflow from macropores duringthe test with wet antecedent conditions had lower C1- concentrations than drainage from the soil matrix, suggesting dilution in macropores from upslopedrainage. Effective pore volumes calculated for the flow-averaged breakthrough data from the entire profile were much less (<40%) than the estimates (measured by tensiometers) of total volumeof pore water, suggesting that preferential flow significantly contributedto subsurface transport of tracer. The pore volume for the entire profileincreased only slightly with increasing application rate; however, the relative proportions of pore volumes calculated for individual portions varied proportionallyto antecedent hydrologic conditions. These changes are attributedto the expansion of individual macropores with surrounding soil and the lateral extensionof macroporenetworksduring wetter conditions. Introduction Subsurface flow in forest soils affects water delivery to streams, water quality, and pore water pressure distribution within hillslope hollows. Recently, the role of subsurface flow in the generation of stream drainage has received much attention [Tanaka et al., 1988; Pearce, 1990; Wilson et al., 1990; McDonnell et al., 1990]. Although stable isotope studies have been usefulin determining the relative propor- tions of "old" and "new" water that contribute to stream- flow [Sktash et al., 1986; Stewart and McDonnell, 1991], important questions remain as to specific mechanisms and pathways of subsurface flow, particularly in regardto the influence of macropores and the interaction among various hydrologiccomponents within the soil. Because of the naturalvariability in stable isotope concentrations between soilwater and groundwater [DeWalle et al., 1988] as well as within and among storm events [McDonnell et al., 1990], other techniques must be utilized to elucidate pathways, Copyright 1994 by the American Geophysical Union. Paper number 93WR03245. 0043-1397/94/93 WR-03245$05.00 travel distances,and quantitative partitioning of subsurface flow within hillslope soils. Vertical movement of chemicals has been extensively studied in agricultural [Misra et al., 1974; Gaudet et aI., 1977; Ju•?,, and Sposito, 1985] and forest •oils [Sidle et al., 1977; Sidle and Kardos, !979; Jardine et al., 1990]. Both the convective-dispersive equation [e.g., Nielsen and Biggar, 1962; Rose and Passioura, 1971; van Genuchten and Wier- renga, 1986] and stochastic-convection models [e.g., Jury, 1982; White et al., 1986; Hornberger et al., 1990] have been used to simulate the movement of solutes in field soils. Soil structure and the presence of macropore networks are known to influence the transport of chemicals to groundwa- ter [Edwards et al., 1988; Brusseau and Rao, 1990]. How- ever, on steep forested slopes underlain by a hydrologic barrier, these morphologic features can also facilitate lateral subsurface transport [Luxmoore et al., 1990; Wilson et al., !991a, b]. This lateral movement is important for cycling of nutrients and transport of potential contaminants such as herbicides, pesticides,and fertilizers to headwater streams. The distribution of pore water pressure within soils of steep hillslope drainages is greatly affected by subsurface 879