Doctor of Philosophy (PhD)
Geography & Environmental Studies
Faculty of Arts
This thesis examines phosphorus and nitrate transport in a first-order agricultural catchment in Southern Ontario. Specific areas of concern relate to (1) long- and short-term temporal variability in nutrient export patterns, (2) the role of drainage tiles in annual nutrient export, (3) the effects of antecedent hydrologic conditions (AHC) on nutrient export patterns and (4) temporal variability in the nutrient retention in riparian buffer strips and streams. Temporal variability in hydrochemical export from the study basin over a two-year period is described and quantified and the importance of high magnitude events is highlighted. This is the first comprehensive study to examine the role of such events in annual nutrient export from agricultural catchments (17-58% of annual nutrient export). The significance of winter thaws and in particular major snowmelt events in annual nutrient export from agricultural catchments in Southern Ontario is shown and the need to include the winter period in sampling regimes is demonstrated. The importance of sampling frequency is examined and an improved sampling strategy is suggested for the study basin. The role of drainage tiles in annual hydrochemical export from the basin is quantified and the dominance of drainage tiles as a nutrient source is shown. Tiles account for approximately 42% of annual discharge, but account for the majority of SRP, TP and NO3-export. Tiles within the study basin exhibit spatial and temporal variability in hydrochemical export patterns but the discharge from all tiles within the basin can be predicted from one continuously monitored tile. The effect of antecedent hydrologic conditions in hydrochemical export and the complex effects of successive events on hydrochemical export patterns are also demonstrated. In general, hydrochemical export increases as conditions become successively wetter, although empirical relationships between hydrological variables in the catchment and hydrochemical export are weak. Hydrologic connectivity between surface horizons and drainage tiles by macropores and preferential flowpaths appears to be critical in exporting nutrients from the catchment, and the role of these flowpaths increases as conditions become wetter. The in-stream retention of phosphate during low flows is examined. Stream sediments are unable to retain large pulses of phosphate during very low flow periods due to poor mixing in the water column. The stream is therefore unable to retain pulses of phosphate that may occur following the irrigation of fields receiving liquid manure applications or when fertilizers are directly applied into ditches. These individual studies improve our general understanding of nutrient dynamics and export from agricultural watersheds. This thesis increases our ability to predict nutrient export by (a) suggesting a new sampling strategy to improve the precision of nutrient export estimates in basins; and (b) identifying critical periods of nutrient export and linking these to antecedent hydrologic conditions in the basin.
Macrae, Merrin L., "Temporal variability in nutrient transport in a first-order agricultural basin in southern Ontario" (2004). Theses and Dissertations (Comprehensive). 492.