Moisture situation: How much snow-melt enters the soil?

Farmers can count on 20-50% of the moisture from snow-melt to enter the soil. This variability depends a lot on surface soil moisture conditions. A North Dakota study (Willis and Haas) concluded that 50% of snow-melt moisture runs off or evaporates when surface soils (top 30-40 cm) are dry and up to 80% runs off when surface soils are wet.

If, for example, soils were bare and dry in March, then got 25cm of “regular” snow in April, that snow would contribute at best about 1.25cm (1/2”) of water to the soil. (The ratio of snow depth to rainfall equivalent varies, but is about 10:1 on average. So 25cm of snow is 2.5cm of water. If more than half the snow-melt runs off, then 25cm of snow will contribute about 1.25cm of moisture to the soil.)

Snow depth is also highly variable across the landscape. Winter winds can move as much as 75% of annual snowfall away from open, exposed fallow fields in southern Saskatchewan, according to this Fang et al report. And even if snow depth was equal across a field, infiltration of meltwater into the soil tends to be higher in low areas where water pools before moving on.

Stubble helps

The same Fang report also made this statement:

“Pomeroy et al (1990) found that southern Saskatchewan wheat stubble fields had substantially smaller losses to blowing snow than did fallow fields. Vegetation height in these agricultural fields plays an important role. As the stubble height increases from 1cm to 40cm on agricultural fields near to Regina, the loss to blowing snow decreases by 22% of the mean seasonal snowfall.”

Another study by A. E. Coles (University of Saskatchewan) et al* looked at snow cover across the Great Plains from 1962 to 2013 and found that while snow cover on fallow (tilled) land and overall snowfall in general dropped over that time, snow cover on land with stubble stayed steady.

From Coles: “In years where the hillslopes were left fallow, snow cover SWE [snow water equivalent] decreased by 88% over the 1962–2013 period. The decrease in snow cover was over four times greater than the decrease in snowfall (a 21% decrease). No significant trends in snow cover were found over the same period when the fields were covered in stubble.”

While this snow may not contribute a lot to required moisture for crops (canola needs 2.5cm (1”) of water for every 3.5 – 4 bu./ac. of yield), the early season soil-surface moisture it provides is often essential to good crop establishment.

Sublimation and snow moisture loss

Phillip Harder, researcher with the Centre for Hydrology at the University of Saskatchewan, says stubble also “drastically reduces” sublimation from the snowpack in the middle of winter. Sublimation is the conversion of water from the snow (solid) to vapour phase, bypassing the intermediate liquid stage. Harder says sublimation is most intense in windy and open areas where chinooks are common and there are not a lot of obstructions to slow down the wind. It is less extreme in areas with more bush and without chinooks.

Research shows that winter sublimation rates during a chinook can be up to 2.1mm/day – if water is available for sublimation**. Over a winter, models show that up to half of the winter precipitation can leave through sublimation. This is about snow loss to the atmosphere before it even has a chance to soak into the soil.

Considerations for dry soil conditions in spring

Reduce seed-placed fertilizer. Risk of seedling loss from seed-placed fertilizer goes up in dry conditions. In a dry spring, be more conservative with seed-placed fertilizer rates for canola. Limit seed-placed fertilizer to no more than 20 lb./ac. of phosphate (40 lb./ac. of MAP, for example) and nothing else.

*A.E. Coles et al, “Climate change impacts on hillslope runoff on the northern Great Plains, 1962–2013” Journal of Hydrology 550 (2017) p538-548.

**M.K. MacDonald et al, “Water and energy fluxes over northern prairies as affected by chinook winds and winter precipitation.” Agricultural and Forest Meteorology 248 (2018) 372–385 . The abstract says: Daily sublimation plus evaporation rates during chinooks at the sites with heaviest and lightest precipitation were 1.3–2.1mm/day and 0.1–0.3 mm/day, respectively.