UNL CropWatch April 1, 2011 Drought and Floods Possible for Spring
|Figure 1. Precipitation in inches from Oct. 1, 2010 to March 27, 2011. Source: High Plains Regional Climate Center.||Figure 2. Departure in inches from normal precipitation from Oct. 1, 2010 to March 27, 2011.|
April 1, 2011
Within the next few weeks, planting of warm season crops will begin in earnest. Conditions this spring are considerably different than last year, with drought conditions steadily advancing from the southern Plains to the Nebraska border while Platte River flows and snow melt from the Rockies remains high..
The vast majority of the state has failed to receive five inches of moisture since October 1, the starting date most commonly used to assess fall and spring soil moisture recharge (Figure 1). The eastern half of Nebraska has soil moisture deficits of more than two inches, with areas of 3-5 inch deficits in east central, south central, and southeastern Nebraska (Figure 2).
The western half of the Sandhills and pockets in the Panhandle are the only areas showing surplus moisture since Oct. 1. Much of this moisture fell during winter in the form of snow. Average snow to water ratios were 15:1 and from systems that often included high winds. I am somewhat concerned that many of these events may have overestimated liquid equivalent moisture due to the inherent problem of accurately measuring snow moisture from high wind events.
Previous studies by the High Plains Regional Climate Center have found that the average recharge efficiency of precipitation equates to 70% for non-growing season events. Using this ratio, average soil moisture recharge (in inches) for each agricultural crop district is: Panhandle (2.50), North Central (3.50), Northeast (3.75), Central (2.50), East Central (3.25), Southwest (1.75), South Central (2.25), and Southeast (3.50).
With normal moisture in April, the eastern third of the state can expect 2 more inches of recharge, the central third of the state, 1.50 inches, and the western third, 1.25 inches. However, if the March precipitation trend continues through April, cumulative deficits from October 1, 2010 could increase by 1-2 inches across southeastern Nebraska.
The U.S. Drought Monitor currently shows abnormally dry conditions for the southern half of Nebraska with moderate drought conditions perilously close to the Nebraska/Kansas border from Red Cloud west to the Colorado border (Figure 3). Expect moderate drought conditions to move into southwest and south central Nebraska in the next two weeks unless these areas receive at least 1 inch of rain.
The wheat crop has broken dormancy across the state and water use will increase as the crop moves toward the jointing stage. Below average soil moisture levels, poor emergence, and periodic high wind events should all contribute to increasing plant stress. Add into the mix the likelihood of below normal temperatures and at least a couple hard freezes in April.
Timely rains will be needed over the next 12 weeks to minimize further crop degradation.
Simply stated, the general consensus forecast is for established weather trends to continue into May, then weaken into the early summer. Wet and cool conditions are likely for the upper Midwest and eastern Corn Belt, while warmer and drier than normal conditions are likely for the southern and western High Plains region.
The Climate Prediction Center shows this trend for April (Figures 4 and 5), but doesn’t continue it into summer due to variability in its forecasting method
It appears Nebraska will be sandwiched between these two polar opposite areas and I am concerned that the southern Plains dryness will over spread much of state during the next few months. Storm systems are crossing the state on a frequent basis, but moisture associated with these storms are being shunted to our north and east and continue to contribute to our drying trend.
As the mean position of the jet shifts northward toward the Canadian border by mid summer, the number of frontal passages crossing the nation,s mid-section typically decrease to an average of one per week. Combine this with the fact that soil moisture recharge has been below normal and there is an above average risk that dryland crop could face an elevated drought risk.
Snowpack in the upper Platte River basin is well ahead of normal for the second consecutive year. Cool, wet springs the past few seasons coupled with stringent water restrictions have led to a steady increase in reservoir storage.
Platte River levels are at or above minor flood stage from the Nebraska/Wyoming border eastward through central Nebraska. Snow runoff could exceed available reservoir storage by over 250,000 acre feet. Current flows into Lake McConaughy are close to 4,000 cubic feet per second and four times greater than last year.
A widespread heavy rain within the basin or a rapid warm-up over the central Rockies could cause widespread low land flooding in many areas. Reservoir releases and stream flows may need to be increased above current levels to keep reservoirs below the 90% capacity set by federal guidelines to reduce wave action on dam facings. This mandate will expire April 30.
Gravity fed irrigators can expect a full 18 inches of water from the Central Nebraska Public Power and Irrigation District. In addition, the water will be available earlier than normal it fills canals to make way for inflows into Lake McConaughy. In addition, the Elwood reservoir will be filled to capacity for a second straight year.
Another challenging production season is nearly upon us. On one hand, agricultural drought conditions are knocking at our back door at the same time that the flood risk along the Platte remains high as snow pack in the Rockies begins to melt. This is Mother Nature at her best in the Central Plains.
Extension State Climatologist, Lincoln