FLOW AND SOLUTE TRANSPORT THROUGH THE SOIL MATRIX AND MACROPORES OF AHILLSLOPE SEGMENT

Subsurface flow from various portions of a soil profile on a steep, fo rested hillslope was evaluated by two sets of step-change miscible dis placement tests at different application rates and antecedent hydrolog ic conditions. Solutions of NaCl (1000 mg L-1 Cl-) were applied at ste ady state rates (equivalent to 20 and 30 mm h-1 of standing water over the entire plot area) using a line irrigation source located 1.5 m up slope (lateral distance) from an excavated soil pit. Subsurface flow a nd tracer breakthrough from five portions (the organic-rich soil layer including macropores, the mineral soil matrix, and three groups of ma cropores in the mineral soil layer) of the soil profile were individua lly 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 drainag e, macropores (including organic-rich soil) contributed proportionally more flow than during periods when upslope drainage was minimal. Outf low from macropores during the test with wet antecedent conditions had lower Cl- concentrations than drainage from the soil matrix, suggesti ng dilution in macropores from upslope drainage. Effective pore volume s calculated for the flow-averaged breakthrough data from the entire p rofile were much less (< 40%) than the estimates (measured by tensiome ters) of total volume of pore water, suggesting that preferential flow significantly contributed to subsurface transport of tracer. The pore volume for the entire profile increased only slightly with increasing application rate; however, the relative proportions of pore volumes c alculated for individual portions varied proportionally to antecedent hydrologic conditions. These changes are attributed to the expansion o f individual macropores with surrounding soil and the lateral extensio n of macropore networks during wetter conditions.