Soil erosion and nutrient loss through agricultural runoff are major causes of land degradation and water quality deterioration in receiving water bodies in different regions across the world. This has become a particular concern in the Lake Winnipeg Basin (LWB) for which excessive nutrients, phosphorus (P) and nitrogen (N), loads from agricultural non-point sources and wastewater discharges have resulted in a decline in ecological conditions of Lake Winnipeg, the tenth largest freshwater lake in the world and the sixth largest in Canada (Rattan et al., 2017). The LWB covers nearly one million square kilometers, encompassing four Canadian provinces and four American states. The hydrological regime of LWB is dominated by spring snowmelt runoff, often occurring over frozen ground (Shrestha et al., 2012). In the southern Manitoba portion of LWB, a wide range of P export coefficient, from 0.01 to 1.58 kg/ha/yr, had been reported at multiple sampling stations along tributary streams and main rivers depending on conditions of climate, geology, soil, vegetation, and land use (Stewardship, 2011). Therefore, the study of runoff, sediment, and nutrient load variation in relation to climate and land management practices is essential to support the primary goals of reducing nutrient input from sources and improving lake water quality.
Agricultural water quality management in cold climate regions is more challenging and complex because of its unique agronomic, bio-geochemical, and hydrometeorological characteristics. This includes short growing seasons, limited water infiltration on frozen ground, vegetative nutrient release after freeze-thaw, and enhanced transport of late fall and winter nutrient applications, as well as the challenges that also exist in warm regions such as soil legacy nutrients and nutrient stratification (Liu et al., 2019a). Redistribution of snow by the wind results in irregular snow accumulation and snowmelt in spring, leading to an uneven distribution of runoff, sediment, and nutrient losses within a watershed (Costa et al., 2021). In addition to the climate effect, agricultural operations such as crop rotation, fertilizer, and manure application, tillage, irrigation, surface, and subsurface drains, and associated structural and non-structural best management practices (BMPs) also have a significant impact on nutrient export from crop fields (Bishop et al., 2005; Tiessen et al., 2010). These impacts complicate nutrient export processes at both field and watershed scales.
In the field of soil and water conservation, field studies are essential for gathering real-world information, identifying the main drivers of soil erosion and nutrient dynamics, and quantifying ecohydrological responses to climate and land use changes. However, short-term or event-based monitoring studies have a limited replication potential, resulting in less power and, therefore, potentially unreliable estimates when scaling up to a watershed level for a long-term assessment (Yang et al., 2022). Long-term field monitoring on a watershed scale can help to minimize the bias of low-frequency field studies and help to address runoff and pollutant delivery from individual land uses with a high temporal and spatial resolution (Remund et al., 2021). Therefore, long-term field studies in small watersheds are very valuable to better understand the mechanisms of runoff, sediment, and nutrient load variations in relation to climate and land management practices.
The South Tobacco Creek (STC) watershed in southern Manitoba, Canada was one of the pilot study watersheds in the Agriculture and Agri-Food Canada (AAFC) watershed evaluation of BMPs (WEBs) project from 2004 to 2012. Extensive field and modeling studies have been implemented in the study area over the past two decades based on long-term field monitoring data. Based on a paired field study within the STC, Tiessen et al. (2010), Liu et al. (2013), and Liu et al. (2014a) found that conservation tillage reduced the export of sediment and total N (TN) but increased dissolved P (DP) and total P (TP) with the snowmelt runoff the top driving factor affecting field-scale losses of N and P in the study area. Liu et al. (2014bconcluded that annual crop and perennial forage had no effects on loadings of sediment, particulate N (PN), and particulate P (PP) under snowmelt runoff conditions but had significant effects on the dissolved N (DN) and DP based on 2005–2012 filed monitoring data collected at four experimental fields within the STC watershed. Based on data collected at two small dams within the STC watershed during 1999–2007, Tiessen et al. (2011) concluded that small dams were effective in reducing peak flows from agricultural land and could reduce sediment and nutrient loads significantly during both snowmelt and rainfall runoff. Li et al. (2011) studied the effects of multiple BMPs in two small subwatersheds within the STC watershed by comparing monitoring results between pre-BMPs and post-BMPs. Results showed that the collective reduction in runoff was mostly nonsignificant, but the implementation of these BMPs resulted in a significant reduction in nutrient losses from the treatment subwatersheds. Koiter et al. (2013) investigated the role of connectivity and scale in assessing the sources of sediment in the STC watershed using sediment source fingerprinting, while an economic analysis of agricultural BMPs in the STC watershed was implemented by Khakbazan et al. (2013). Chen et al. (2017) analyzed the changes in runoff chemistry and soil fertility after multiple years of cattle winter bale feeding on annual cropland based on 2007–2015 field monitoring data collected at two experimental sites within the STC watershed. The results showed that more runoff was produced from the bale feeding areas but with a lower nutrient concentration in comparison to feedlot sites. These earlier field studies focused on the effects of BMPs on runoff and nutrient losses from agricultural fields but lacked systematic analysis of their spatio-temporal variations in relation to climate and land management practices.
In addition to field studies, modeling attempts were also conducted in the STC watershed to assess the effects of small dams on stream flow and water quality, and sediment yield from upland and channel erosions using the Soil and Water Assessment Tool (SWAT) (Liu et al., 2014c; Liu et al., 2015). In a recent study, Wilson et al. (2019) analyzed surface soils as a source of P in snowmelt runoff from cropland based on 2013–2017 measurement data collected at 16 edge-of-field monitoring sites across the Red River Basin (RRB) and the Assiniboine River Basin (ARB) in Southern Manitoba. Results showed a clear link between near-surface soil P concentration and the potential of P loss with surface runoff for fields located in the LWB over a range of fertilizer management and tillage practices. Soil P was also the focus of a study by Liu et al. (2019a) based on 1997–2014 measurement data in the paired experimental fields within the STC watershed. Results showed that soil P drawdown by lowering fertilizer inputs had a large potential to reduce DP concentrations in both snowmelt runoff and rainfall runoff without affecting crop yields. A review by Liu et al. (2019b) on processes, drivers, management options, and research needs for agricultural water quality improvement in cold climates concluded that the nongrowing season had a critical role in annual nutrient losses, seasonal nutrient transport, and BMPs efficiency among years and across regions. An assessment of cross-region synthesis of edge-of-field results showed that snowmelt dominated runoff volume and P loss across Canada with varying drivers on P runoff pattern with most losses in a dissolved form in the Prairie region and most in particulate form in the Great Lakes region (Liu et al., 2021). These findings demonstrated a need for further understanding the interactions among water quality, climate, hydrology, land use, and land management in cold climate regions, which helps BMP management to reduce nutrient losses under the changing land use and climate variability.
This study aimed to better understand the spatio-temporal variation of runoff, sediment, and nutrient losses and their relations with climate, landscape features, and land management practices in the Steppler subwatershed (SSW) within the STC watershed based on eleven years (2005–2015) of field monitoring data. The specific objectives are (1) to quantify runoff, sediment, and losses of nutrient and suspended particulate carbon (SPC) from nine fields within the SSW, (2) to analyze their spatial distribution under different landscape and land management conditions, (3) to analyze their annual and seasonal variation during the monitoring period, and (4) to assess these losses with environmental variables and land management practices. Results from this study can provide site-specific information on runoff, sediment, and nutrient losses under Canadian prairie cold climate conditions and support policy development for the promotion of suitable agricultural BMPs on the non-point source (NPS) pollution control and their spatial management in the LWB.