Predicting the Downstream Impacts of Headwater Burial in Kemptville Creek Watershed (Ontario) Using the SWAT Model

Abstract

Headwater streams can compose up to 80% of the total stream length within a watershed but remain most susceptible to stream burial during land use change or management. Rerouting streams into tile drains, canals, non-perforated pipes and paving over, are all examples of stream burial. This study explores the importance of delineating headwater streams within a watershed hydrological network and the downstream impacts of headwater burial. We used the Soil and Water Assessment Tool (SWAT) model to predict changes in discharge, chlorophyll a, DO and sediment for downstream ecosystems, following headwater stream burial. Within the study area of Kemptville Creek subwatershed of the Rideau River Watershed in Kemptville and surrounding area, Ontario, Canada, 908 headwater stream segments totaling 443km of stream length were delineated from a 10-hectare threshold. Model calibration was significantly improved with the inclusion of delineated headwater streams, increasing R2 by 0.49. Headwater streams were confirmed with field observations in 70% of 30 selected headwater stream sites. Headwater streams were classified based on land use disturbance coefficients and potential downstream organic subsidy contribution. Each headwater class saw 100% stream burial through conversion into irrigation canals using SWAT model. Headwater stream burial was also applied to all headwater streams as one collective group at 25, 50, 75 and 100%. Ontario Benthic Biomonitoring Network (OBBN) samples from Rideau Valley Conservation Authority were assessed using the Family Biotic Index (FBI) OBBN metric and Shannon-Weiner Diversity Index. Predicted discharge, chlorophyll a, dissolved oxygen (DO) and sediment inputs were compared against the OBBN community structure, at 7 sample sites. Headwater stream burial results indicated significant decreases in discharge, chlorophyll a, DO and sediment to downstream ecosystems. Headwater stream burial within wetlands representing 33% of the headwaters within Kemptville Creek, had the most significant impact on downstream discharge, chlorophyll a, DO and sediment. Changes for these variables correlated with several benthic macroinvertebrate families, which supported predictions of community structure shifts following headwater stream burial. Results from this work highlight the importance of headwater streams to ecohydrology within watersheds and contributes to the understanding of ecosystem response as hydrologic regimes are altered following stream burial.