Conservation Practices for Reducing Snowmelt Runoff Phosphorus
Conservation Practices for Reducing Snowmelt Runoff Phosphorus
Why consider snowmelt?
Snowmelt runoff is runoff associated with melting snow in the landscape. It includes rain on snow or rain on frozen ground. Runoff from melting snow is not the same as rainfall-generated runoff. Some conservation practices that are effective for reducing particulate phosphorus (P) losses in rainfall generated runoff can be ineffective or even detrimental during snowmelt. In most regions that experience multiple months of frozen soil and snow accumulation, most annual runoff and therefore phosphorus loss are associated with snowmelt or winter thaws (Macrae et al., 2021; Baulch et al., 2019). To successfully reduce P delivery to surface water, it is essential that conservation practices function during these periods.
What’s different about snowmelt?
We categorize P forms in runoff as either dissolved or particulate (sediment-bound). In fields with rainfall-induced erosion, particulate P is usually the dominant form. In contrast, dissolved P dominates in snowmelt because its impact on the surface has lower energy than rainfall and does not cause as much sheet and rill erosion. This is especially the case in regions with little slope. Regions with snow accumulation may also have frozen ground or partially frozen ground. Frozen ground can impede particle detachment and reduce particulate P loss, but can also impede infiltration and drainage, increasing runoff volumes and prolonging contact between surface runoff and soil, which can increase dissolved P loss. As a result of these different conditions, some conservation practices may be less effective in these landscapes or may have unintended consequences.
Is there a solution?
Not all winters are the same, and the effects of winter processes on P loss can vary across different landscapes. The challenge is identifying which conservation practices are most appropriate for a given field or region. Snowmelt runoff is experienced in different ways in different regions, landscape types and/or cropping systems. Place-based solutions are necessary to reduce P loss in runoff and avoid unintended consequences.
The weather influences snowmelt runoff in three ways: through snowfall amounts/cover, soil frost, and frost in vegetation. The amount of snow that falls, how long it accumulates, and how quickly melt events proceed influence the volume of runoff, the duration of its contact with soil, and the energy with which it scours the soil surface. Winter temperatures that cause the soil to freeze deeply and remain frozen during snowmelt increase the volume of runoff through reduced infiltration but also reduce erosion of particles and transport of particulate P. However, prolonged surface ponding resulting from reduced infiltration on frozen ground can increase dissolved P loss from soil and vegetation. Weather also determines how effectively snow-covered buffer strips intercept particulate P, and whether vegetation can actively take up dissolved P during snowmelt or release it when extreme temperatures cause cells to rupture (Liu et al., 2019; Cober et al., 2018).
Position in the landscape influences the potential for snowmelt runoff to erode soil and transport particulate P. Steep slopes and gullies where runoff concentrates are erosion-prone. In these conditions, practices to reduce channelized flow and erosion or intercept particulate P are appropriate. Landscape also determines drainage efficiency and the likelihood of snowmelt runoff reaching receiving waters.
Key considerations relevant to snowmelt runoff
Nutrient Management Planning
The 4R principles (Right source, right place, right rate and right time) apply to both snowmelt and rainfall generated runoff in all landscapes and cropping systems. Timing is the most important for snowmelt runoff as winter applications of fertilizers or manure should be avoided. Large proportions of applications made to snow or on frozen ground are transported in snowmelt runoff. Late fall applications (or grazing) should be made with care with cut-off dates set locally depending on average frost date or weather forecasts. Surface applications should be avoided in the fall with nutrient either banded or broadcast and incorporated (right place). Application rates should be managed to meet crop requirements as larger snowmelt P losses are associated with greater soil test P (STP). Drawing down soil P fields with elevated STP is important to manage snowmelt P losses.
Over-winter Cover Crops
Vegetation can slow erosion of particulate P but can release dissolved P following winter kill. This is most important in regions with severe winters and does not readily occur in more moderate conditions or regions with significant snow cover (Liu et al., 2019). Cover crops and residues on fields should be used with caution. They can be effective at reducing erosion in areas with warmer winters and/or where erosion is significant. They should be used with caution in areas with severe winters where dissolved P losses are the greatest source and freeze-thaw processes are significant.
Although perennial forages reduce the erosion of particulates compared to other systems, the plant materials on the surface encourage snow accumulation, which increases snowmelt volumes and the risk of mobilization of dissolved P. Late season harvesting may reduce P inputs to snowmelt runoff.
In regions prone to particulate losses, fall tillage can increase erosion in snowmelt or early thaw. No-till or conservation till is preferred. If tillage must be done, doing it as early in fall as possible or in spring is better (Macrae et al., 2021). Conversely, where dissolved losses dominate snowmelt runoff, residues and stratification of STP close to the soil surface can increase dissolved loads in runoff. Reducing stratification using a tillage pass every few years or deeper placement of P fertilizer is recommended in these situations. In addition, fall tillage can create surface roughness, allowing snowmelt to collect in small surface depressions and leading to more infiltration rather than runoff and keeping P in the field (Stock et al., 2019).
Riparian Buffer Zones
Vegetative zones between the field and water resource can slow and spread runoff during the growing season and encourage the deposition of particulates and the retention of dissolved P. However, these systems can be ineffective during snowmelt when plants are dormant or dead and covered by ice or snow, and the soil is still frozen (Kieta et al., 2018). As with forages, late season harvesting of riparian zone vegetation is recommended if it is possible.
Control of Runoff Volumes or Intensity
The use of detention structures or erosion control structures can retain water so that it reduces downstream nutrient load. Examples include structures such as retention ponds, drainage water management, and maintenance of floodplains, grassed waterways or water and sediment control basins (WASCoBs) to slow or spread runoff, thus promoting infiltration. Water retention/detention structures restrict connectivity in the landscape and may reduce flooding in downstream areas. Although structural practices are largely for flood control, they have promise for nutrient control simply by reducing water flow. This may be especially important during snowmelt, as they may hold back some of the large volumes of water that result from the melting snow or rain-on-snow events that can exit fields, thereby reducing P transport processes. However, these can be easily overwhelmed during peak flows and thus should be used with caution.
Livestock wintering sites should not be located close to streams, rivers, channelized flow, or drainage ways as dissolved P can be lost during snowmelt. This is easily accomplished with in-field grazing but is more difficult when permanent wintering sites (e.g. corrals) have already been located where runoff feeds streams. In these cases, diversions or holding ponds to contain effluent for later treatment or use are potential solutions.
Baulch, H., J. Elliott, H. Wilson, M. Cordeiro, D. Lobb and D. Flaten. 2019. Soil and water management: opportunities to mitigate nutrient losses to surface waters in the Northern Great Plains. Environmental Reviews 27: 447-477. doi:10.1139/er-2018-0101
Cober, J. R. , Macrae, M. L. , & Van Eerd, L. L. . (2018). Nutrient release from living and terminated cover crops under variable freeze–thaw cycles. Agronomy Journal. The American Society of Agronomy, Inc.
Kieta, K.A., Owens, P.N., Lobb, D.A., Vanrobaeys, J.A. and Flaten, D.N., 2018. Phosphorus dynamics in vegetated buffer strips in cold climates: A review. Environmental Reviews, 26(3), pp.255-272.
Liu, J., M. Macrae, J. Elliott, H. Baulch, H. Wilson and P. Kleinman. 2019. Impacts of cover crops and crop residues on phosphorus losses in cold climates: A review. Journal of Environmental Quality 48:850-868 doi:10.2134/jeq2019.03.0119
Macrae, M., Jarvie, H., Brouwer, R., Gunn, G., Reid, K., Joosse, P., King, K., Kleinman, P., Smith, D., Williams, M. and Zwonitzer, M., 2021. One size does not fit all: Toward regional conservation practice guidance to reduce phosphorus loss risk in the Lake Erie watershed. Journal of Environmental Quality, 50(3), pp.529-546.
Stock, M., F. Arriaga, P. Vadas, L. Good, M. Casler, K. G. Karthikeyan, and Z. Zopp. 2019. Fall tillage reduced nutrient loads from liquid manure application during the freezing season. Journal of Environmental Quality 48:889-898.
|Merrin L. Macrae
University of Waterloo
Environment and Climate Change Canada &
University of Saskatchewan
Editing and Design
NC State University
University of Tennessee
Macrae, M., J. Elliott, and L. Good. 2023. Conservation Practices for Reducing Snowmelt Runoff Phosphorus. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/conservation-practices-for-reducing-snowmelt-runoff-phosphorus/