Indicator of risk of water contamination by P for Soil Landscape of Canada polygons

The indicator of risk of water contamination (IROWC) is a component of the Agriculture and Agri-Food Canada Agri-Environmental Indicator project. The IROWC measures progress in reducing the risk of water contamination from agricultural activities, focusing on N and P. The objective of this study was to propose a methodology for an IROWC-P applicable at the Soil Landscape of Canada (SLC) polygon level (1:1 000 000 map scale) using an indexing approach. The sources of data included Census of Agriculture, SLC and soil survey databases and provincial soil test data. The IROWC-P considers the following site characteristics: soil erosion and potential for overland flow, annual P balance (crop residues, manure and inorganic fertilizer), soil test P (STP) and degree of soil P saturation (DSPS). IROWC-P classifies polygons for their potential risk of P transfer to surface waters according to five vulnerability classes (i.e., very low, low, medium, high and very high). The methodology was tested on a pilot basis for selected SLC polygons in the province of Quebec using 1981 and 1991 census data. Preliminary results indicated that the proposed methodology showed some sensitivity to changes in agricultural practices between 1981 and 1991 and reflected differences in risk of P contamination from areas of intensive compared to areas of extensive agriculture. The difference between the selected areas was mainly attributed to the STP, DSPS, manure and inorganic fertilizer P polygon characteristics. The temporal variations in the IROWC-P ratings were attributed mainly to the manure and inorganic fertilizer P polygon characteristics.