Gunnar Kirchhof

Runoff and erosion from very dry clay soil surfaces: does conventional infiltration theory apply?

Accepted conventional infiltration theory states that application of water to dry soil over time follows two phases: first, the sorptive phase where capillarity rapidly fills the pore system, and the steady state phase where the pore system conducts water at the rate of the soil saturated hydraulic conductivity. In most cases, this is also what we observe in the field: water enters the soil quickly and it then slows down to reach a steady rate. This infiltration theory assumes that the wetting angle of the capillaries is 0°; i.e. the capillary has a water film. If the wetting angle is larger than 0°, the capillary potential that drives water intake during the sorptive phase will also be much smaller, resulting in slow sorptivity. On hydrophobic soils, the wetting angle can approach 90° or, in extreme cases become convex.

Applying water to very dry soil, perhaps after a long drought, or soil surfaces that were exposed to bushfires or soils with water repellent organic matter, may not have a capillary water film that allows immediate rapid water intake. Under field conditions this can occasionally be observed when water is applied to very dry soil: it initially ponds on the surface before it then quickly infiltrates. This initial ponding may be the consequence of a larger than 0° wetting angle. The length of initial ponding will affect the onset of runoff and erosion. Although the problem of hydrophobicity is a well-known problem on sandy soils, water repellence on clay soils has been observed but is not well understood.

This project aims to verify if the sorptive phase can be preceded by a ponded phase and under what conditions it can occur. The experimental plan would assess the impact of soil type and texture, soil organic matter, and antecedent soil water content on the formation of an initial potential ponded phase. The research would initially be conducted under laboratory conditions using intact soil cores (St Lucia or Gatton Research Labs) and possibly followed up for in situ field conditions.

The results will have implications on how to prepare and avoid runoff and erosion from very dry soils during a heavy rainfall event.

Quantifying the interaction between soil organic matter, nitrogen fertiliser and hydraulic conductivity on the Hermitage Long term Trials

The Hermitage Long Term trial has been running since 1968. The trial assessed a range of agronomic and soil science responses to conservation agriculture type practices and fertilisation regimes. Much data has been collected over the years with the most recent being submitted for publication by Kuntal Hatia et al. late 2020. This paper provides excellent data for soil aggregation as a consequence of tillage methods, stubble retention, and nitrogen fertilisation. Another important dataset is the impact of these management parameters on saturated hydraulic conductivity and the van Genuchten hydraulic conductivity model. This project aims to measure in situ saturated hydraulic conductivity using rainfall simulation and nitrometers as well as collecting undisturbed cores for Ksat and pF-curve determination to parameterise the van Genuchten model. The aim is to publish a follow-up paper with Kuntal Hatia et al.

Can ground cover-induced concentrated flow increase soil erosion?

Maintaining adequate ground cover is the ‘golden’ rule to reduce soil erosion. Leaf litter or similar absorbs the kinetic energy of rain drops as they hit the soil surface. This reduces soil surface disturbance and prevents aggregate breakdown that can lead to the formation of a surface seal. Under some conditions, however, in particular on steep slopes, adequate ground cover may not protect the soil from erosion even if the soil hydraulic conductivity exceeds rainfall intensity. The mechanism that can lead to runoff and then erosion may be due to the formation of concentrated flow.

Consider a thought experiment: our soil surface has a saturated hydraulic conductivity of 30 mm/h and we have a rainfall intensity of 20 mm/h: we should not get runoff! Now 50% of the soil is covered and hence, only 50% of the soil surface can take in water. This means that the hydraulic conductivity of the uncovered soil must be at least 40 mm/h to prevent runoff (i.e. 20 mm/h from the rainfall plus 20 mm/h that enters the soil as runoff from the groundcover). Therefore we would have runoff as the hydraulic conductivity is only 30 mm/h. Of course, this thought experiment does not apply to the real world, but the principle is still valid.

This project aims to assess the impact of groundcover on the formation of concentrated flow. The experimental plan would assess the effect of soil hydraulic conductivity, slope, and ground cover type and quantity on runoff and erosion. The project will be conducted at the Erosion Processes Laboratory at the St Lucia campus and can be supplemented in-situ using our field rainfall simulator and infiltrometer equipment.

The results will have implications on recommended management practices of sloping soil to prevent soil erosion.

Reducing wind erodibility through increasing aggregate stability through mulch management

The soil property that primarily drives susceptibility to wind erosion is aggregate size distribution. On soils that are susceptible to wind erosion, management practices to reduce wind erosion aim to increase aggregate sizes. This can be achieved by mulch application that leads to an increase in soil organic matter and re-aggregation of soil structure during wetting and drying cycles. How effective mulch application will be is likely to be dependent on the quantity and type or mulch applied. This project aims to assess aggregate size distribution as measured by dry sieving in response to wet-dry cycles and mulch type and quantity. The experiment will be carried out either at Gatton or St Lucia. It will run over two semesters to ensure sufficient breakdown of applied mulch under a number of wet-dry cycles