Master of Arts (MA)
Geography & Environmental Studies
Faculty of Arts
Hydrological processes occurring within the vadose zone, especially in heterogeneous soils and tills typical of the Canadian shield, are not well understood. This research investigates the importance of the vadose zone in a small headwater basin (Harp 4-21) in the Canadian shield with respect to the generation of stream runoff quantity and quality during episodic rainfall and snowmelt events. The study focused specifically on: ﬁrstly the effect of variable antecedent moisture conditions on water-table and stream response, secondly, the significance of the stored vadose water in water extracted from a rising water-table, and thirdly the significance of preferential flowpathways in the vadose zone as a means of rapidly rotating, stored vadose water to the stream during runoff events. The instrumentation of the Harp 4-21 basin includes three v-notch weirs along the stream, numerous piezometers and wells, several soil lysimeters, and three tensiometer nests. Much of the data used in this study was obtained from five sites located along a hillslope transect. Soil water content at each of the hillslope sites was accurately determined using time domain reflectometry (TDR), whereas water table elevations were measured using both electronic water level tapes and calibrated rod ﬂoats. Preferential flowpath data was derived from ﬂow gauging and chemical analyses of water samples. Results obtained from the near-stream and lower slope reaches of the hillslope transect indicated that soil water content in the vadose zone is maintained at a high level of saturation by shallow water-table depths. Further analysis of simultaneously measured soil water content and depth to water—table data clearly showed that the existence of a thick essentially saturated zone overlying the water table (capillary fringe) was responsible for the large and rapid water-table responses observed during many of the nine studied precipitation events. On a basin scale, the seasonal variability of the area in which the capillary fringe extended to the ground surface was largely responsible for the observed trend in basin yield (runoff volume/ rainfall volume) for the nine runoff events. Chemical tracing results using silica, pH and DOC showed that the large vadose water reservoir in the near-stream and lower slope areas was a dominant component in water extracted from a rising water-table during most precipitation events. Silica results from the near-stream vadose zone showed that only the massive groundwater recharge associated with spring snowmelt could completely replace the vadose water reservoir with shorter residence time snowmelt/rainfall water. The rapid routing of stored near-stream capillary fringe vadose water through preferential pathways (macropores, soil pipes) was a significant source of runoff to the stream during runoff events. The presence of soil pipes, along with the significant vadose water reservoir in near-stream and lower slope areas are sufficient to explain the large phreatic/vadose water component typically identified in two component flow separation models, and constitutes the basis of a physically-based conceptual runoff model for the Harp 4-21 basin.
MacLean, Robert Alex, "The role of the vadose zone in the generation of runoff from a headwater basin in the Canadian shield" (1992). Theses and Dissertations (Comprehensive). 380.