The influence of climate change and wetland managment on prairie hydrology - insights from Smith Creek, Saskatchewan

Internally drained depressional wetlands are critical landscape features in the Prairie Pothole Region (PPR) of North America. They provide important ecosystems services such as flood attenuation, improved downstream water quality, and diverse species habitat, however they are frequently drained by agriculture producers to manage excess surface water, access more farmland, or improve operational efficiencies. After recent flooding in the Canadian Prairies, there is increased interest in understanding the relative influence of climate change and wetland drainage on prairie hydrology to ensure sustainable economic and social development in the region. Future climate projections show increasing air temperatures and rainfall in the Canadian Prairies, while wetland drainage is expected to persist due to rising land prices. As such, the purpose of this thesis is to determine the influence of wetland drainage and climate change on prairie basin hydrology and develop future wetland management strategies that preserve agricultural land and mitigate downstream impacts during wet periods in the PPR. The objectives are therefore to 1) improve surface water storage capacity estimation methods from high-resolution digital elevation models (DEMs) of agriculturally dominated prairie basins, 2) advance prairie hydrological modelling through improved representation of wetland characteristics, and 3) evaluate the influence of wetland management and projected climate change on prairie basin hydrological responses. A case study of the instrumented and partially drained Smith Creek Research Basin (SCRB) is presented in this thesis. First, surface water storage capacity estimates of depressional wetlands were improved through manual breaching of roads to simulate the function of culverts in surface water drainage and storage modelling, using a 2-m resolution digital elevation model (DEM). Road-breaching at presumed culvert locations was found to decrease estimates of depressional wetland area by 29% and surface water storage capacity by 48% compared to estimates with roads-intact from automated depressional wetland delineation using the 2-m resolution DEM. Importantly, the roads-breached simulation provided wetland area and surface water storage capacity estimates that were 150% higher than estimates from aerial-photos. This result suggests that current prairie hydrological models are subject to uncertainty in estimates of wetland areas and storage capacities depending on wetland delineation methods, which may impact wetland drainage or restoration scenarios modelling results. Next, a new prairie hydrological model was developed for SCRB using the Cold Regions Hydrological Modelling Platform. This model uses primarily physically-based algorithms to simulate cold-regions prairie-specific hydrological processes including precipitation phase, wind redistribution of snow, snow sublimation, snowmelt, infiltration into frozen and unfrozen soils, crop growth, evapotranspiration, soil moisture balance, surface water storage in depressions or wetlands, and runoff routing. The new model, builds upon previous work conducted in the SCRB, but offers improved representation of wetland characteristics using depressional wetlands delineated from the 2-m roads-breached DEM, updated parameters to support multi-year simulations, a new macro to prevent soils from re-freezing after large snowfall events in the late spring, and a novel link to a hydraulic model to simulate culvert-restricted streamflow that occurs in roadside ditches and along stream channels during high runoff events in the SCRB. Finally, the new model was used to evaluate the influence of climate change and wetland drainage on the hydrology of the SCRB. Current and projected future weather variables from the Weather Research and Forecasting model were used to simulate the influence of climate change in the SCRB towards the end of the 21st century. Results suggest that a significantly warmer (5.5 ⁰C) and wetter (44 mm) projected future climate, with less snowfall and more extreme rainfall, will increase mean annual streamflow volume by 26%, with spring peak discharge decreasing by 34% and summer peak discharge increasing by 161%. If wetland drainage continues in the SCRB and wetland area drops below 9% of the basin area, streamflow volume could increase above the climate projected increase. This suggests that continued wetland drainage in prairie basins may have more influence on future streamflow volumes than projected climate change. Wetland restoration to near-historical extents was found to increase storage volumes sufficiently to offset climate projected increases in streamflow volumes, but even complete wetland restoration to historically maximum levels did not offset projected increases in summer peak daily discharge. This means that additional infrastructure upgrades or emergency response plans beyond wetland management strategies will likely be needed to manage future flood risk in the Canadian Prairies. The new methods, analysis, and results presented in this thesis are expected to be relevant to those interested in wetland management in cold-region prairie basins, including policy makers, basin stewardship groups, conservation organisations, water resources engineers, agriculture producers and the public.

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