Seasonal Variation in Sediment and Phosphorus Yields in Four Wisconsin Agricultural Watersheds

Abstract

Agricultural water quality projects in two distinct topographic regions in Wisconsin collected 5 to 10 yr of continuous stream discharge, suspended sediment (SS), total P (TP), and total dissolved P (TDP) in four watersheds (2100-5000 ha) from 2006 to 2016. Previous agricultural nonpoint SS and TP reduction efforts in two of these watersheds documented cold versus warm season differences in water quality response. The goal of this study was to identify seasonal partitioning of SS, TP, and TDP in storm event loads to inform stream water quality protection efforts. We used National Weather Service Coop Observer frost depth reports to identify dates when watershed soils were frozen. By comparing daily mean event discharge for dates relative to frost, we identified a 32-d post-frost high-discharge "thaw" period. Combined, the frozen and thaw periods contributed about half of the annual SS and TP runoff event loads, ranging from 47 to 63% for SS and from 45 to 51% for TP. The proportion of runoff event TDP during this time was even higher, 62 to 79%, with the majority during thaw. Watershed average volumetric runoff coefficients (event flow/precipitation and snowmelt) were two to four times higher during the freeze and the thaw compared with the rest of the year. To reduce total stream loads in regions with similar climates to Wisconsin, this study indicates that using management practices that curb sediment and P delivery to streams in the winter and early spring may be as important as those designed for nonfrozen conditions.

Agricultural Water Quality in Cold Climates: Processes, Drivers, Management Options, and Research Needs

“…Across 26 fields in Wisconsin and Minnesota, USA, Zopp et al (2019) observe that the flow‐weighted mean total P concentrations in runoff from mixed precipitation events (rainfall and snow) are dominated by dissolved P forms when the soil is frozen, and for nonfrozen soil conditions, total P is frequently dominated by particulate P forms. The results are supported by Good et al (2019), who emphasize that runoff over frozen soils or partially frozen soils is particularly important for the loss of total dissolved P. However, Wilson et al (2019b) report that although the P in snowmelt runoff is frequently dominated by dissolved forms in Manitoba, Canada, high rates of particulate P loss can result from erosion of frozen soils in some circumstances, necessitating further research to elucidate the drivers of this erosional P loss from frozen soils during snowmelt (Wilson et al, 2019b).…”

“…A diagram showing key agronomic, biogeochemical, and hydrological characteristics influencing nutrient losses in cold agricultural regions. The papers in this special section investigate different aspects of these characteristics, including the importance of the nongrowing season (Good et al, 2019; Plach et al, 2019; Ulén et al, 2019) and snowmelt (Hoffman et al, 2019; Kokulan et al, 2019; Wilson et al, 2019a) in annual flow and nutrient transport, the impacts of vegetative nutrient release (Cober et al, 2019; Costa et al, 2019; Liu et al, 2019b; Schneider et al, 2019; Vanrobaeys et al, 2019), the impacts of winter and fall nutrient applications (He et al, 2019; Sadhukhan et al, 2019; Smith et al, 2019; Stock et al, 2019; Vadas et al, 2019; Vetsch et al, 2019; Zopp et al, 2019), the influence of soil on nutrient loss (Dharmakeerthi et al, 2019; Liu et al, 2019a; Satchithanantham et al, 2019; Wilson et al, 2019b), and the patterns of nutrient concentrations in streams (Casson et al, 2019). …”

“…In the Laurentian Great Lakes region, for example, Plach et al (2019) report that the nongrowing season contributes an average of 81% of annual edge-of-field runoff (surface + subsurface), 76% of soluble reactive P, and 78% of total P losses from three cropped fields over 5 yr in Ontario, Canada. In Wisconsin, USA, Good et al (2019) observe that 62 to 72% of total dissolved P export and 45 to 51% of total P export occur when soils are frozen or during the thaw period.…”

Influence of Soil and Manure Management Practices on Surface Runoff Phosphorus and Nitrogen Loss in a Corn Silage Production System: A Paired Watershed Approach

“…Much of the annual runoff in cold climate regions is from snowmelt [11,13,34,35,39,68,69]. An average of 53% of annual runoff (averaged across watersheds and control/treatment periods) was derived from snowmelt.…”

“…Much of this is from field plots, however, which largely ignores the dynamic nature and complex hydrology of larger fields/watersheds [31][32][33]. Evaluating the effectiveness of practices at the landscape and watershed scales is also challenging due to the heterogeneity of runoff processes [34][35][36]. Paired watershed designs account for inherent physiographic differences between watersheds and can help to isolate management effects on runoff water quality [9,32,37,38].…”