Analyzing cumulative environmental effects of agricultural land drainage in southern Ontario, Canada

Abstract

The temporal and spatial attributes of agricultural land drainage suggest that drainage is a potential source of cumulative environmental change. This paper applies a cumulative effects model of drainage to the case of southern Ontario, Canada. The application is focused on the cumulative effects of drainage on volume and timing of water flow, content of nitrate-nitrogen and atrazine residue in receiving channels, and area and distribution of wetlands. These are investigated using available information on drainage-environment interactions and geographic information system (GIS) technology. Available data show that the combined effect of two pathways of accumulation in drained areas (i.e. subsurface flow and runoff) increases the content of nitrate in water, but decreases the atrazine content, relative to the single pathway (i.e. runoff) in undrained areas. Drainage alters timing and volume of water flow at the field scale, but evidence of accumulation of these changes at the watershed scale is inconclusive for southern Ontario. This is attributed to counteractive processes and constraints of available data bases. Fragmentary effects are evident in the spatial association between drainage density and change in area and contiguity of wetlands at the subregional and regional scales. Application of GIS demonstrated that 47% of the decline in total wetland area (1312 ha) in Peel Township during 1800–1990 is attributable to subsurface drainage. Average patch size decreased nearly 50%, and the number of large patches (> 50 ha) declined by about 65%. In southern Ontario, counties with the greatest reduction in wetland area are consistently characterized by high drainage density. Collectively, the empirical application demonstrated the utility of the cumulative effects model, and showed that it is possible to conduct a cumulative effects analysis using available information sources.