Variation in matric potential at field capacity in stony soils of fluvial and alluvial fans

Abstract

Field capacity of soil is fundamental for many agronomic management decisions, as well as for hydrological and environmental models. Around the world, field capacity is assumed to occur at defined matric potentials, −10 kPa and –33 kPa being common criteria. However, these criteria have not been tested in fluvial and alluvial fan stony soils (FAFSS). In this project, 57 pits on FAFSS located on the Canterbury Plains were watered to near saturation. After two days of drainage (proxy for field capacity), a 30 × 30 cm pit was excavated in 10 cm increments to a depth of 60 cm for 52 of the pits. At each increment, matric potential was measured. For the remaining 5 pits, matric potential was measured after 4–5 days to a depth of 1.5 m. Matric potential was generally higher than −10 kPa and the matric potential-depth profile of the pits could be characterised by one of five modes. The most common mode was hydrostatic equilibrium, which is normally associated with soils that have a shallow water table (~<2 m). Our results indicate that a capillary break due to a coarse sandy gravel layer at ~ 1 m depth causes a near zero matric potential boundary condition that allows hydrostatic equilibrium to occur regardless of the depth to groundwater. This capillary break was linked to two soil conditions: a layer of open framework gravels or soil fines with specific surface area < 15 m2g−1. Our results provide a basis for better approximations of the matric potential at field capacity, and improved modelling of soil water behaviour in FAFSS.