An Improved Water-Driven Sediment Yield Model for Cold Agricultural Regions

Excessive transport of sediment in the world’s freshwater, typically from water-driven erosion occurring in agricultural regions, is a significant source of pollution and damage to the aquatic environment. Excess sediment erosion promotes the transport of phosphorus to lakes, which can increase cyanobacterial growth, cause sedimentation that limits the lifespan of wetlands, and increase turbidity, reducing sunlight access for aquatic life. ese conditions are influenced by, and potentially mitigated by, agricultural land management. Long-term observation and simulation help to understand and predict the effects of current and future sediment load risk. Due to the challenges in measuring sediment load and the need for evaluating future or potential loading conditions, in-situ observations need to be augmented by model outputs to inform policy development. Cold agricultural regions occupy large areas of North America, Europe, and Asia, and calculating sediment erosion here requires consideration of snow redistribution and melt, frozen soils, and ponding over grainfields and grasslands with well-developed soils. Soil erosion models typically target temperate climate hillslope-scale processes driven by rainfall-runoff during the growing season and fail to capture the critical dynamics of spring snowmelt over frozen or partially frozen soils and the “fill-and-spill” flow regimes characteristic of low-gradient, depressional areas of cold regions. is research addresses this critical knowledge gap by developing, evaluating, and applying a new soil erosion and sediment transport model specifically tailored to cold agricultural regions. e model captures overland flow and channelized flow using separate parameterisations, treats snowmelt with an energy-balance approach, calculates the impact of thawed and frozen soils on runoff, and explicitly accounts for fill-and-spill hydrology. e model was evaluated with observational data from three research basins across the Canadian Prairies, and a sensitivity analysis was performed to determine sensitivity to parameter and forcing variation. Model behaviour for sediment load was shown to have high sensitivity to surface slope, soil texture, and storage capacity of the soil as would be expected. Modelled results were generally in-line with observations. e model was then applied over more than 4000 small ~100 km² virtual basins across the Canadian Prairies to evaluate the spatial and temporal trends in in-stream sediment load from 1950 to 2020. e results suggest that basins in western Alberta and southern Manitoba have the highest trend of excessive sediment load, and that the trend of excess sediment load in southern Manitoba basins has increased significantly since 1950. is is primarily due to the increase in rainfall early in the growing season. Overall trends across the Prairies show a shi toward a larger role of rainstorms as a driver for sediment load in streams, and a reduced role of snowmelt, as the climate has warmed and the onset of spring freshet has advanced. In semi-arid regions, such as the Palliser triangle, where summer rainfall is low, there has been a net decrease in trend of excess in-stream sediment load. Separation of land-use and climate effects suggests that the introduction of continuous cropping has reduced sediment erosion by 14 to 30%. Modelling suggests that the loss of depressional storage across the Prairies has led sediment transport to increase by 15 to 29%, and climate change from the 1950-1980 epoch to the 1990-2020 epoch has caused changes in sediment load between -19% and +32%. e advantages of this model are due to the parameterisation of cold regions processes with a physically based approach, incorporation of a sub-daily timestep for modelling flashy streamflow in small basins, and erosion parameterisations for simulating overland flow and channel flow respectively. e outcome of this research is a more robust and reliable tool for predicting sediment load for changing climate and land-use regimes in cold agricultural regions, and a diagnosis of changing sediment erosion and transport regimes across the Canadian Prairies since the 1950s.

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