Hydrological Properties and Flow Paths Change with 4.1 Million Years of Soil Development in the Hawaiian Islands

A well-characterized chronosequence of soils in the Hawaiian archipeligo provides an excellent opportunity to examine the effects of time on hydrological properties and flow paths in a humid tropical environment. Detailed hydrological studies were conducted at the extreme ends of a chronosequence of soils in the Hawaiian Islands as a part of a larger study evaluating hydrologic losses of nitrogen (N) under elevated N supply. Specifically, we determined in-situ soil-water retention, soil hydraulic conductivity, and flow path characteristics on a 300 year old Andisol and 4.1 million year old Oxisol both supporting native montane tropical forest. We found that surface and subsurface soils drained rapidly at the young site but observed significant differences between surface and subsurface soil-water retention and hydraulic conductivity characteristics at the old site. An artificial rainfall experiment with deuterium isotope tracer showed that water was dominantly downward advecting at the young site. At the old site, water moved fast as by-pass flow through the near-surface soils to an impeding subsurface clay layer and then moved both laterally along the clay contact and slowly downward as piston flow. Across the soil age gradient, soil anisotropy and the probability of lateral flow increased as saturated hydraulic conductivity in subsurface soils declined. Findings from this study demonstrate that soil development with time can have a profound effect on the rate and direction of water flow which have important implications for the rate and trajectory of soil and ecosystem formation, nutrient cycling and storm runoff mechanisms.