Indirect Methods to Elucidate Water Flows and Contaminant Transfer Pathways through Meso-scale Catchments - a Review

We provide a critical review of indirect methods to elucidate water flows and contaminant transfer pathways through meso-scale catchments, as the proliferation of such methods in recent years has made it very difficult for potential users to evaluate their relative merits. We focus on agricultural contaminants such as nitrogen, phosphorus, faecal bacteria and sediment, as there is a substantial risk for them to contaminate waterways wherever agriculture exists. While direct measurements may be feasible at the plot/farm scale, their high resource demands make them prohibitive at the catchment scale. Water flows converging at the catchment outlet offer the opportunity to employ indirect methods to interpret hydrological and/or chemical data observed there as integrative signatures reflecting the various pathways through a catchment. We focus on meso-scale catchments (approximate to 10-10(3) km(2)) as the surface water monitoring data required for indirect methods is typically available at this scale. We reviewed the literature to provide an overview of the numerous methods used (e.g., hydrograph separation, concentration-discharge analysis, pollutograph/loadograph analysis, end-member mixing analysis). Particular attention is given to the spatial scale the methods have been applied to, and their data needs (type of data, required temporal and spatial resolution). Advantages and disadvantages in terms of data availability and underlying assumptions are highlighted to facilitate the selection of a suitable method. While no single indirect method will provide all the answers, well-informed selection of one or more has the potential to greatly advance our understanding of water flows and contaminant transfers at the catchment scale.