Field measurement of the saturated hydraulic conductivity of a macroporous soil with unstable subsoil structure.

A field method for measuring saturated hydraulic conductivity, K-s, was developed to characterize water now in highly-weathered soils of Sitiung, Indonesia. Soils in this area are known to absorb large volumes of rainwater rapidly. However, K-s data obtained on soil cores do not corroborate field-observed rapid infiltration rates. In the field method, a constant rate irrigation was applied to a field plot, delineated to a depth of 120 cm, and bordered on the surface to contain a depth of ponded water. The rate of irrigation was sufficient to maintain the pending depth at a constant level as well as cause water to overflow from the ponded surface. The difference between the steady-state irrigation and overflow rates was considered to be the instantaneous flux and was assumed applicable to all depths. Simultaneous tensiometric measurements of pressure head as a function of depth provided the hydraulic gradients needed for calculation of K-s using Darcy's law. Hydraulic gradients deviated considerably from unity, and soil saturation did not exceed 92% of porosity. Laboratory-measured RI values for the stable-structured topsoil agreed well with the field data. However, those for the subsoil were 2 to 3 orders of magnitude lower than the field-measured values. The susceptibility of the subsoil to compaction during core extraction and slaking when in contact with free water appeared to be responsible for the highly reduced rates of now in the laboratory samples. The subsoil pore structure was preserved only as long as it was overlain by the stable structured topsoil. Results suggest that measurements of water now on small soil cores may, in some cases, be of questionable value. The field method provided accurate in situ data on plot-size areas. The field plot method used in this study causes minimal disturbance of the soil while the effects of sample confinement and overburden are represented fully in the measurements. (Soil Science 1998;163:841-852).