Site Factors, Soil Structure and Preferential Flow and Transport.

Preferential flow in soil macropores facilitates the accelerated leaching of contaminants to groundwater and surface water (via subsurface flow), with attendant risks for human health and ecosystem function. Models that have been developed to deal with this process have mostly been tested ‘piecemeal’ against experimental data gathered at the column or small plot scale. In contrast, few attempts have been made to synthesize our knowledge of how site attributes (e.g. land use and soil management, climatic factors) and fundamental soil properties interact to influence soil structure and preferential flow and transport in macropores at larger scales. This is a critical knowledge gap, since this kind of basic understanding is a necessary pre-requisite to support the development and application of models and decision-support tools at the larger scales (fields, farms, catchments, regions) that are relevant for management.

In this presentation, we summarize some recent work which attempts to bridge the scale gap between flow processes in macropores and the much larger scales of interest for management (e.g. fields, farms, landscapes). We first present results of some recent experiments at both field and small catchment scales, which demonstrate that spatial patterns of preferential solute transport in tilled agricultural topsoil show significant deterministic components related to fundamental soil properties such as texture, bulk density and organic matter content. At the global scale, we then show how literature data can be used to support the development of pedotransfer functions and vulnerability and risk assessment tools that account for the impacts of preferential flow on contaminant transport in the vadose zone.