Postgraduate Profiles

Gavan McGrath

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Thesis: An exploration of the rainfall controls on pesticide transport via fast flow pathways

Pesticides are often transported to streams and/or groundwater as a result of the occurrence of rainfall events which trigger surface runoff or rapid preferential flow through the unsaturated zone. Much of the theory of solute transport has been derived assuming steady state or slowly varying upper boundary conditions and continuous, averaged process descriptions. However, a review of pesticide dynamics reveals that pesticides are episodically transported, predominantly through discrete flow pathways and this transport is often initiated as a near surface process, driven by naturally variable and intermittent rainfall intensities. The objective of the thesis is to better understand how the structure of natural variability of rainfall intensities impacts upon pesticide transport by these fast flow processes.

Why my research is important

While climate has been recognised as an important control on pesticide transport to surface water and groundwater, little is still understood about how to use this qualitative information to improve risk assessments by water quality managers. Regulators are currently relying on modelling by highly parameterised complex water flow and transport models as risk assessment tools. However these types of models suffer from the problem of equifinality and hence high predictive uncertainty. In addition climate is often relegated as an historical artifact for model input. Similarly desktop based risk indices often neglect important transport processes such as surface runoff and preferential flow as well as the variability of climate. This research sits nicely in between the two current approaches in terms of complexity, and captures explicity the empirically observed influence of climate allowing the development of a climate based risk assessment.