Evaluating the Relative Importance of Shallow Subsurface Flow in a Prairie Landscape

Abstract

Core Ideas Contrary to established theories, vadose water contributes considerably to Canadian Prairie stormflow. Travel velocities of subsurface flow may exceed surface flow in some cases. Traqnsmissivity feedback and macropore flow promote “old” water deliveryto Prairie streams. Hydrologic literature on the Northern Great Plains, including the Canadian Prairies, has mainly focused on Hortonian overland flow as the dominant runoff generation mechanism. This study focused on subsurface water movement and its contribution to streamflow in high‐latitude, subhumid, prairie landscapes with flat topography. Three “riparian‐to‐stream” sites were monitored in southeastern Manitoba, Canada, and four hydrologic events (snowmelt triggered and rainfall triggered) were selected for analysis. A dual water sourcing strategy, including conservative and non‐conservative tracers, was applied to distinguish the relative importance of event (“new”) and pre‐event (“old”) runoff sources. Surrogate measures of antecedent wetness conditions (AWCs: based on rainfall, snow water equivalent, and soil saturation) data and streamflow and water table fluctuation data were used to identify factors influencing the contributions of different runoff sources. Results showed significant old water contributions during snowmelt‐ and rainfall‐triggered events, especially during the summer season. At an engineered grassland site, decreases in snow cover extent were accompanied by “flashy,” high‐magnitude stream hydrographs. Larger peak discharge was observed coincident to elevated AWCs at all sites. Travel velocities of surface‐applied dye were faster than those of subsurface‐applied dye, except for one rainfall‐triggered event. Rapid delivery of old water to streams was observed and hypothesized to be the result of macropore flow, piston flow, or transmissivity feedback depending on the season and water table position. Further investigations are recommended to increase our understanding of subsurface runoff generation mechanisms that promote the rapid movement of old water from riparian areas to streams in Canadian Prairie landscapes.