Disentangling the Effects of Agriculture Land Use on the Export of Nutrients and Carbon During Flooding Events

Abstract:

Floods are the most widespread of all weather-related natural disasters with considerable social, economic and environmental consequences. Climate change is expected to alter the magnitude and frequency of flooding, and together with land use change (urbanization, agriculture), flooding effects on the environment are expected to be intensified. As part of Canada’s national network, FloodNet, and using data from Ontario’s Provincial Water Quality Monitoring Network, we are investigating changes in nutrient and carbon export from terrestrial to aquatic ecosystems in catchments with various amounts of agriculture land use during a range of hydrological events. In these catchments we compared concentration-discharge (C-Q) relationships and hydrological indicators (e.g., runoff ratio, peak over threshold) with fluxes and export trends of several solutes (total phosphorus, total nitrogen, nitrates, phosphates and dissolved organic carbon), between 108 streams, over 50 years, separated by season (e.g., spring vs summer). The number of flooding events has increased in Canada in the past 2 decades. The general trends are for total phosphorus and nitrogen concentration to be decreasing through time across the catchments regardless of land use. However, nitrates and dissolved organic carbon are increasing through time and with more wet conditions in low agriculture catchments, with a pronounced effect in the past 2 decades. Our results on changing (C-Q) and fluxes can be used to better understand flood risks especially how flooding may exacerbate eutrophication in downstream ecosystems in light of agriculture intensification and increased urbanization.