Wind Erosion in Western Queensland Australia

Chapter 4 –Modelling Soil Erodibility Dynamicschanges in the time it takes for a soil to reach a condition of maximum erodibility defined byb max . Parameterisations from (iv) to (vi) demonstrate the effects of varying the modelcomponents M and T that define the time of maximum growth.In reality, temporal changes in soil erodibility are driven by dynamic variations in r, M and T.Therefore, a soil will not move up and down a single growth curve in response to rainfall anddisturbance conditions. Rather, variations in the growth rate and disturbance intensities willinduce an irregular/fluctuating growth pattern, for example (vii). Temporary increases in soilmoisture and changes in aggregation and crust strength due to small rainfall events willinduce additional variations in the growth curve. These effects will be further moderated bysoil properties like organic matter content and climatic factors such as solar radiationintensity and evaporation rates. Soil erodibility dynamics could be modelled using staticgrowth rates based on soil type dependence on drought to erode (e.g. after Gillette, 1978).However, the model output would not display realistic temporal patterns unless the effects ofall dominant controls can be included in the growth rate formulation. The strength of theframework lies in this potential for incorporating a dynamic growth rate model to account forvariations in soil responses to rainfall, drought and disturbance mechanisms.Figure 4.6 demonstrates the model sensitivity to changes in the rainfall thresholds that drivedecreases in erodibility. A hypothetical simulation was run with model parameters set for r =-0.15, M = 0.5, T = 0.5 (Equation 4.11). Rainfall thresholds (Equation 4.15) were set for:(a) ∑r < 3, α = 1; 3 < ∑r < 10, α = -20; 10 < ∑r < 30, α = -40; ∑r > 30, α = -60(b) ∑r < 5, α = 1; 5 < ∑r < 10, α = -20; 10 < ∑r < 30, α = -40; ∑r > 30, α = -60(c) ∑r < 10, α = 1; 10 < ∑r < 20, α = -20; 10 < ∑r < 40, α = -40; ∑r > 40, α = -60Decreasing the model sensitivity to rainfall, from (a) through to (c), results in increases in themagnitude of output erodibility values and in the time for which a soil may have an elevatederodibility. Accurately defining both the model growth rates and rainfall sensitivitythresholds will therefore be essential if the conceptual framework is to be applied to modelsoil erodibility dynamics.118