Page 271 - Nutrient Management Planning Guide

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Chapter 8.1

Ice

Snow Ice

Frozen topsoil

First major snowfall occurs late in the season, after surface soil layers have frozen.

A.

Snow melt

Snow

No ice

Unfrozen topsoil

First major snowfall occurs in the fall, before soil surface has a chance to freeze.

B.

Snow melt

Infiltration

No ice

Runoff

Likewise, a slow, steady springmelt event is associated with less erosion than a fast springmelt event, which may release a large quantity of water over a short period of time.

The condition of the soil at the time of rainfall or runoff is also important. Wet soils generally have lower infiltration rates than dry soils since pore spaces are already filled with water. In addition, certain types of clay swell upon wetting, which reduces the size and number of pores or small channels in the soil making it more difficult for subsequent precipitation to infiltrate.

The state of the soil at the time of the first major snowfall (i.e., snow remains until the following spring) also has an important influence on the amount and extent of runoff from a site (Figure 8.1.1).

Figure 8.1.1 Effect of Timing of First Major Snowfall on Runoff

In the first scenario (Figure 8.1.1A), the first major snowfall occurs late in the year (e.g., mid to late November) after surface soil layers have frozen. The snow cover insulates the frozen ground resulting in increased runoff during spring thaw since the ground remains largely impermeable. In the second scenario (Figure 8.1.1B), the first major snowfall occurs earlier in the year (e.g., early to mid October). The insulation properties of the snow cover influence the extent to which ice crystals form in the surface layers of the soil. The result can be less runoff during spring thaw since surface soil has greater permeability to infiltration of melt water.

Forms of Precipitation

The majority of Alberta’s surface water runoff is generated by snowmelt. Snowmelt runoff usually occurs in the early spring (March to April) as daytime temperatures warm to above zero. The water released from the melting snow is unable to infiltrate into partially or completely frozen soil resulting in surface water flow or runoff. This increases the risk of snowmelt water flowing from fields into surface water bodies compared to runoff from rainfall.

Water infiltration dynamics during snowmelt into thawing soils are complex. An important factor is the moisture content of the soil at the time of freezing. Soils that have drained prior to freezing allow greater water infiltration. Soils that were saturated at the time of freezing will have formed ice crystals, which effectively plug soil pores. Soil structure can be degraded in frozen saturated soils as aggregates break down from the force exerted by expanding water as it freezes. The resulting degradation of soil structure results in slower drainage and less water infiltration.