Jean-Thomas Denault

Development of a method for estimating the likelihood of crack flow in Canadian agricultural soils at the landscape scale

Indicators of risk of water contamination by agricultural pollutants are developed in Canada to assess sustainability of agriculture. Crack flow (CF), a key pathway for sub-surface contaminant transport, is part of the transport-hydrology algorithm used in two of these risk indicators. The objective was to develop a methodology for predicting the likelihood of CF in Canadian agricultural soils at the landscape scale. The algorithm considers soil clay content, crack development followed by a runoff event based on water budget, tile drainage, and crops. More than 40% of Canadian farmlands had moderate to very high likelihood of CF, mainly in Manitoba, Ontario and Quebec, due to frequent runoffs on cracked clay soils potentially contributing to groundwater contamination. In Ontario and Quebec, farmlands with high CF likelihood correspond to regions under intensive tile drainage, which increases the risk of lateral translocation of contaminants to surface water bodies. Besides being a component of risk indica...

Preferential pathways of phosphorus movement from agricultural land to water bodies in the Canadian Great Lakes basin: A predictive tool

Allaire, S. E., van Bochove, E., Denault, J.-T., Dadfar, H., Thériault, G., Charles, A. and De Jong, R. 2011. Preferential pathways of phosphorus movement from agricultural land to water bodies in the Canadian Great Lakes basin: A predictive tool. Can. J. Soil Sci. 91: 361-374. Preferential flow processes, such as crack flow (CF), burrow flow (BF), finger flow (FF) and lateral flow (LF) are known as factors enhancing phosphorus (P) transport from agricultural soils to water bodies. The objective of this study was to develop a methodology for predicting the likelihood of preferential flow processes in agricultural soils at the landscape scale and their potential occurrence around the Canadian Great Lakes. The methodology considered climate, soil and crop parameters and a water budget that calculated surface runoff and drainage. Crack flow largely depended upon soil clay content, BF on soil texture and climate, FF on layering in sandy soils and LF on the presence of trees, slope and soil restricting layers. Crack flow had a high likelihood to occur southern Lake Ontario and all around Lake Erie. A high likelihood of FF could be found in the area where CF was low (i.e., in the sandy soils north of Lake Huron and Lake Ontario). Burrow flow had a medium likelihood to occur on Manitoulin Island and close to the shoreline north of Lake Ontario. Medium to high likelihood of lateral flow might occur in the area south of Lake Ontario, west of Toronto in a narrow band towards Lake Huron, and to a lesser extend in a large area northeast of Lake Huron. Lateral flow may transport soluble P in areas where P was previously carried downward by FF from inland (in soils) to surface water bodies. In several areas, tile drainage may transport all forms of P carried downward from the soil surface to the subsurface by CF and BF to lake tributaries. Preferential flow distribution maps could be used as tools for supporting the identification of agricultural lands where management might enhance subsurface processes of P transport toward groundwater or surface water bodies.

Temporal trends of risk of water contamination by phosphorus from agricultural land in the Great Lakes Watersheds of Canada

van Bochove, E., Denault, J.-T., Leclerc, M.-L., Thériault, G., Dechmi, F., Allaire, S. E., Rousseau, A. N. and Drury, C. F. 2011. Temporal trends of risk of water contamination by phosphorus from agricultural land in the Great Lakes Watersheds of Canada. Can. J. Soil Sci. 91: 443-453. The indicator of risk of water contamination by phosphorus (IROWC_P) was designed to estimate the level of risk of P contamination in water and how the level of risk has changed over 25 yr (1981-2006) in agricultural watersheds of Canada. IROWC_P allows for a qualitative assessment of this risk in comparison with other regions of eastern and western Canada, and the identification of high to very high risk watersheds may require on-site assessment and the development of remedial action plans. This study presents an in-depth analysis of IROWC_P results in the major Great Lakes watersheds of Canada. The risk of water contamination by P remains acceptable (very low to moderate) in most Great Lakes watersheds, but better management practices (e.g., reduced fertilization and manure application rates) and improved control of surface runoff may be required in watersheds which are at increased risk. The Canadian watersheds of the Great Lakes basin showed a 39% reduction in their P applications in excess of crop requirements between 1981 and 2006 bringing the Ontario provincial P balance close to equilibrium in 2006. Vulnerable areas were found south of Kitchener in the Lower Grand River watershed and east of Lake Simcoe.

Risk of phosphorus desorption from Canadian agricultural land: 25-year temporal trend.

Phosphorus (P) use in excess of crop needs may impact surface water quality and contribute to eutrophication. However, P loss from agricultural land to water has never been estimated at the Canadian national scale. In this paper, the risk of P desorption from Canadian agricultural land is assessed by the source component of the indicator of risk of water contamination by P (IROWC-P). The IROWC-P source component (P_source) characterized the mobilization potential of soluble P and integrated four models of P desorption by water for dominant agricultural soil series of Canada on the soil landscape of Canada polygon scale (1:1,000,000). The objective of our study was to describe and evaluate a standardized method for deriving the P_source component. The P_source was assessed over 5-yr intervals from 1981 to 2006 for scientifically based knowledge by relating annual P balance values, soil test P (STP) analyses, soil P saturation index, and Self-Davis water extractable P extraction values. Results show trends of soil P enrichment for most Canadian provinces over the 25-yr period but also an increased percentage of farmland classified above the water extractable soil P environmental threshold of 4 mg P kg. The Canadian Prairies and Ontario showed small P_source values and almost no farmland above the environmental threshold. Quebec and the Atlantic Provinces had P_source values that exceeded the environmental threshold in 2006; more than 33% of farmland is classified above the environmental threshold value.