Improving infiltration modelling for crusting soils

Infiltration and surface runoff modelling is important in many disciplines including environmental engineering, mine site rehabilitation, ecology and agronomy. Major errors in modelling can occur when the throttling effect of soil surface crusts, which reduce infiltration and subsequently increase surface runoff, are not considered. The aim of this project was to improve the modelling of infiltration on crusting soils by measuring the density of the soil surface crust. A rainfall simulator was used to create a surface crust on soils (Sodosol and Chromosol) susceptible to surface crusting. The surface crusts resulted in greater than 90 per cent of applied rainfall becoming runoff. Several methods of measuring the crust density were trialled, with X-ray micro Computed Tomography (X-ray CT), when combined with the traditional soil core method, found to be the most accurate and reliable. The HYDRUS-1D software application was used to model infiltration. Measured soil parameters, including crust density, and applied rainfall rates were used as model inputs. The inclusion of crust density into HYDRUS-1D resulted in insignificant improvements to modelling accuracy. Inverse modelling identified that this was as a result of HYDRUS-1D predicted saturated hydraulic conductivity values being three to four orders of magnitude larger than obtained from the inverse solution. The application of average crust hydraulic parameters, obtained from the set of inverse solutions, was found to provide a close approximation of infiltration rates— once surface runoff had commenced — and cumulative infiltration. The findings of this project indicate that the use of average surface crust hydraulic parameters could provide major improvements in infiltration modelling accuracy using HYDRUS-1D without the requirement for additional sampling and analysis during field surveys. Further experimentation on a broader range of soils under differing vegetation regimes is required to validate the conclusions of this project