SIOUX FALLS, S.D. (KELO) — When the temperature gets freezing cold, the soil underground can freeze.
The frost level in soil can make a difference in spring flooding.
The U.S. Army Corps of Engineers for the Missouri River Basin released a runoff forecast last week on chances of river flooding in South Dakota. The runoff from snow is expected to be below average. The National Weather Service flood forecast released on March 7 said there was a below normal risk for flooding on the Missouri, James, Big Sioux and Vermillion Rivers in South Dakota over the next 90 days.
“Frost depth is one of the factors they consider in addition to what are the current river levels, how much snow, and how much water is in the snow across the landscape,” said state climatologist Laura Edwards.
Frost depth is the depth or level at which the soil under the surface is frozen. The soil can absorb the moisture from melting snow if the frost has thawed. The lower the frost depth, the faster the soil can thaw and absorb snow melt.
“Certainly having the soils thawed out or nearly thawed out is ideal for the spring snow melt season,” Edwards said.
South Dakota State University has a mesonet system which tracks temperature, frost level and other climate conditions in the state. Generally, the snow that came quickly after a cold snap in December has stuck around and kept frost levels low across the much of the state, Edwards said.
Edwards said area of the state with low levels of snow cover generally have much deeper frost.
“In the southeastern part of the state, we have areas thawed out or where the frost is a few inches deep,” she said.
The mesonet system also shows areas in the northeastern part part of the state with eight inches of frost but a 1 1/2 feet of snow cover, Edwards said.
The snow cover helped keep the frost levels down but that snow will eventually melt. A fast runoff can cause flooding. The dry weather of the past two years means the dry soil can act like a sponge to absorb snow melt, if the frost is gone.
“There’s a lot of capacity for the soils to take in some of the at snow melt as it occurs,” Edwards said.
Ideally, warm temperatures during day and freezing temperatures at night “give a nice balance to allow the soils to get that moisture back,” she said.
Two scenarios are worrisome when it comes to flooding.
If the state or regions have an extended period of multiple day with highs in the 50s, pushing 60, the snow can melt too fast. “That would generate run off moving over the landscape instead of into the soils,” Edwards said.
The second scenario happened in 2019 and caused flooding in southeastern South Dakota.
“If we get a really heavy rain on top of snow,” Edwards said. A three to four inch rain will melt the snow and the rain, combined with the moisture level in the snow, could mean six to eight inches of moisture running off the surface.
“The longer we hold onto snow on the landscape the chances of having something like that happen,” Edwards said, because the weather is moving toward warmer temperatures and spring.
The upcoming forecast is calling for below average temperatures, Edwards said.
In addition to looking at frost levels, the SDSU mesonet tracks thaw depth.
“Sometimes we see the frost come out from the bottom but we also see it thaw from the top down,” Edwards said. Thaw depth is the level from top down. It can indicate if the upper layer of soil is thawing quicker than the lower layer.
Even if the frost thaws at a comfortable pace this spring, Edwards said there will likely be some chance of flooding, Edwards said. There will be some runoff even after the soil absorbs moisture from the melt.
The James River, for example, is a slow moving river. It’s route drops very little in elevation as it moves from the North Dakota border to the Missouri River.
That river can be at a flood stage more easily because of the terrain and elevation, she said.