TIA is a joint venture of the University of Tasmania and the Tasmanian Government

1. Error and uncertainty with the determination of the soil water retention function and saturated hydraulic conductivity by • Marcus Hardie, Shaun Lisson, Richard Doyle, Bill Cotching Tasmanian Institute of Agricultural Research, Private Bag 54, University of Tasmania, Hobart, TAS 7001, Australia. marcus.hardie@utas.edu.au Phone: +61-3-62261925 (b). Evaporative Flux Tension Infiltration Volumetric Shrinkage Desorption Hydraulic Conductivity Macropore Flow • Methodology • The soil water characteristic (SWC) was determined by; desorption and evaporative flux from intact cores, and inverse parameterisation of tension infiltration in Hydrus 2D • The saturated hydraulic conductivity (Ksat) was determined by; constant head and evaporative flux approaches on saturated intact cores, tension infiltration, and inverse parameterisation of tension infiltration data in Hydrus 2D • Results • Ksat varied by up to four orders of magnitude depending on methodology and initial soil water content (Fig. 1) • The SWC differed between the laboratory drying and insitu wetting approaches due to hysteresis, air entrapment, clay swelling and water repellence (Fig 2.) • The evaporative flux and inverse approaches were improved by independent determination of the θr. • Introduction • Most soil-water models require knowledge of the soil water characteristic (SWC) and saturated hydraulic conductivity (Ksat) • However in vertic and water repellent soils these properties vary depending on antecedent soil moisture • Soil Type: Bleached Sodic, Natric, Brown, Kurosol with water repellent A1 horizon, and dispersive, vertic B2 horizons TIA is a joint venture of the University of Tasmania and the Tasmanian Government Figure 1. Effect of methodology and initial moisture content on saturated hydraulic conductivity Error bars represent ±1 standard error. Kcore- constant head, Kevap – evaporative flux, Kinverse – inverse solution of tension infiltration with Ksat in the objective function, KRETC extrapolation of K(ψ)n to zero, K-0.13kPa near saturated hydraulic conductivity at -0.13 kPa, dry refers to infiltration at very low antecedent soil moisture content, wet refers to infiltration when soil was near field capacity. • Conclusion • Water repellent and vertic soils are unable to be characterised by assessment at a single antecedent soil moisture content • Laboratory approaches which require soil saturation (desorption & evaporative flux) are unlikely to misrepresent field behaviour of soils at lower soil moisture contents • In non-vertic soils the evaporative flux approach is supported for rapid determination of Ksat and SWC • The inverse tension infiltration approach was able to discern the effects of moisture content on macropore properties (θs, α, and Ksat) • The inverse tension infiltration approach required independent measurement of θr and was not sensitive to flow through mesopores and micropores Infiltration -inverse solution of in situ tension infiltration using Hydrus 2D, dry - infiltration at very low antecedent soil moisture content, wet - infiltration when soil was near field capacity, no Qr - inverse solution in which θr was excluded from the objective function and independently determined by desorption. Figure 2. Effect of methodology and initial moisture content on the soil water characteristic