Figure 2. Discrete soil moisture levels for southeast Lincoln at depths of 10 cm (4 inches), 25 cm (12 inches), 50 cm (24 inches), and 100 cm (4 feet).
December 19, 2012
Rains Moisten Topsoil, Do Little at Greater Depths
Two snow storms in the next week may help more
Figure 1. Seven-day average soil moisture levels, based on data from the High Plains Regional Climate Center soil moisture monitoring network.
A welcome precipitation event moved through the southeastern third of Nebraska last weekend, bringing 0.40-0.80 inch of rainfall. A few isolated pockets of 1.00-1.30 inches fell in Seward, Saline, Jefferson, Gage, and Lancaster counties. On the backside of the precipitation shield, light snowfall accumulations were reported just north of the I-80 corridor from Grand Island to Omaha.
The significance of this moisture event was two-fold. First, it was the first widespread significant moisture event for eastern Nebraska since October. Second, the ground was not frozen and most of the moisture was captured with little appreciable runoff. Unfortunately, the rainfall only replaced surface soil moisture lost through evaporation.
Figure 1 indicates the status of soil moisture levels across the state and reflects the most recent rain. For most of the state, soil moisture levels are in the lowest 10 percentile when compared to historical patterns dating back to the late 1990s.
Lincoln Airport officially received 0.86 inches of moisture and Figure 2 indicates that soil water content increased slightly in the 4-inch depth, but there was no improvement at the 12-inch depth.
What these two figures are indicating is that it will take numerous events to substantially increase stored soil water. The best case scenario would to see several more rains before the ground freezes. A much better solution would be to see a significant foundation of wet snow (more than 6 inches) fall on unfrozen soil surfaces. This would allow for melted water to infiltrate into the soil profile and minimize surface runoff.
I am optimistic that the winter storm predicted to hit the state Wednesday and Thursday (12/19-12/20) will provide 4-8 inches or another 0.25–0.50 inch of liquid equivalent. Yet another major storm is being indicated by weather models to impact the central Plains as early as Christmas day, with the 26th currently favored as the most likely date for significant accumulations.
Also see Nebraska precipitation data by city and region
Although no one wants to deal with a long winter, our accumulated soil moisture deficits since October 2011 are still more than 12 inches at most locations, with portions of northeastern Nebraska approaching 18 inches.
The Climate Prediction Center will issue their new 30- and 90-day outlooks on Thursday. It will be interesting to see if they depict any precipitation trend for the western Corn Belt. I expect that above normal moisture will be indicated for the eastern Corn Belt, with equal chances of normal, above normal, or below normal moisture for the western Corn Belt.
Temperatures will depend on the amount of snow cover on the ground across the northern and central Plains. With a more extensive snow pack across southern Canada and the northern Plains as compared to this time last year, I expect temperatures will be more typical of a normal winter. Hopefully, these cooler winter conditions will materialize and delay spring dormancy until late March, reducing the impact of soil moisture extraction through early evapotranspiration.