February 13, 2012

Southeast and South Central Nebraska Fares Better

With spring planting around the corner, it’s time to analyze the impact of recent precipitation events on soil moisture recharge and whether drought concerns are warranted for the northern Corn Belt. The early February snow brought valuable moisture to the southern two-thirds of the state, but little benefit to the driest region, northeastern Nebraska.

Most of south central and southeastern Nebraska received 1.50-2.00 inches of liquid equivalent moisture from the February 3-4 event, with southwestern Nebraska receiving 0.74-1.25 inches. Most of this moisture fell on soils with little, if any, frost. The lack of frost and high temperatures in the upper 20s to upper 30s promoted a slow melt, allowing thirsty soils to capture the moisture and reverse the dry pattern established since early December.

Unfortunately, northeastern Nebraska was on the extreme northern periphery of this system and saw liquid equivalent moisture totals in the trace to 0.50 inch range. This area has been on the southern flank of an area that has consistently experienced 25%-50% of normal moisture since October (Figure 1). The dry pattern actually began to develop in late August and was intensified by the lack of snowfall this winter.

Since October 1 much of southeast and south central Nebraska has received 4-5 inches of moisture, with areas in extreme south central and southeast Nebraska receiving 6-8 inches (Figure 2). Assuming that these precipitation events had a 70% effective infiltration rate, then soil moisture recharge should be about 3.5 inches, with about 5 inches in the wettest locations. Effective recharge across northeast Nebraska would be 1-2 inches using a 70% effective rate.

These recharge rates do not take into consideration the quantity of soil moisture at the end of the 2011 growing season. This is probably more relevant for irrigated acreage where late season water applications could have enhanced soil moisture if maturing crops didn’t fully extract the added moisture. More relevant is the accumulated precipitation departures since October 1 and whether this region will be susceptible to drought development or strengthening as the growing season progresses.

Accumulated moisture deficits since October 1 are running 1.5-3.0 inches over all of northeast Nebraska, with an area between Sioux City and West Point having deficits of 3.0-4.5 inches (Figure 3). Under normal conditions this region would receive 5 inches of moisture from mid-February to the end of April. These deficits can be made up, but it will require an increase in the number and intensity of storm systems.

Using simple statistical analysis, the average likelihood of erasing 1-inch of accumulated deficit since October 1 is 38%. For a 2-inch deficit, the likelihood is 28%, and for a 3-inch deficit, it’s 20%. Conversely, there is a 36% chance that the deficits could increase another inch, a 19% chance they would increase 2 inches, and a 5% chance they would increase by 3 inches.

Moisture in the Great Plains

One of the major differences between this winter and last winter has been the absence of snowfall across the northern Great Plains. Storm systems have remained well to the south and north of the region. Last year most locations in North Dakota were approaching 50 inches of snow for the winter, while this year you would be hard pressed to find many locations that have received 12 inches so far this winter.

The disappointing snow totals across the northern Plains have definitely reduced the flood risk for the Missouri River basin. Unfortunately, the miserable snow season has extended westward to include most of the central Rocky and Sierra mountains. With 40% of the snow season yet remaining, there is time to benefit from late snow storm activity, but current streamflow estimates don’t paint a rosy outlook.

If normal moisture is received through the remainder of the winter, streamflow estimates for the Platte watershed indicate flows of less than 80% of normal (Figure 4). More concerning is that the average snowpack in the central Rockies is under 85%. When it drops below 85%, it may likely disappear before mid-June as it did in 2000, 2002, and 2006. This would increase the odds that heat and dryness would build eastward until the Monsoon season begins across the desert Southwest during the second half of July.

The next couple of months are critical for cutting into precipitation deficits that have accumulated since the end of the last growing season across the northern Plains. If there is below normal mountain snowpack and March storms don’t substantially cut into these moisture deficits, a drought alert will likely be issued for northeastern Nebraska by early April.

Al Dutcher
Extension State Climatologist