How Solar Radiation, GDDs Compare with Normals

June 19, 2015

Nobody can argue that moisture has been lacking since the beginning of May across a broad swath of the central and southern High Plains region.  Well over two feet of moisture has been reported in portions of Texas, Oklahoma, and Kansas.  Closer to home, unofficial precipitation totals of 15 to 22 inches have been reported across a substantial portion of south central, central, east central, and southeast Nebraska.

An endless string of storm systems resulted in precipitation being reported somewhere in the state of Nebraska on 45 of 48 days from May 1 through June 16. Of the 45 days when precipitation was recorded, 37 had at least one location recording more than a quarter inch of moisture. All of this moisture has led to planting delays, flooding, and reports of yellowing crop due to lack of adequate sunshine.

A preliminary analysis of solar radiation values from the High Plains Regional Climate Center's automated station network supports the notion that sunshine has been below normal the past six weeks. Every station in the network shows below normal solar radiation for the periods May 1-June 16 and May 15-June 16. Since June 1, solar radiation values have begun to push closer to normal, but continue to be well below normal across southeast Nebraska.

To give readers a quick overview of solar radiation as a percent of normal, eight sites were selected across the state to represent each of the major cropping districts.  Solar radiation values (percent of normal) were calculated for the periods May 1-June 16, May 15-June 16, and June 1-June 16:

• Scottsbluff (84, 84, 99),
• Ainsworth (88, 86, 98),
• Concord (95, 93, 100),
• North Platte (91, 91, 93),
• McCook (92, 92, 96),
• Clay Center (90, 88, 95),
• Lincoln (77, 80, 85), and
• Falls City (76, 79, 83).

The solar radiation data suggests that the past two weeks have been more favorable to crop photosynthesis and there has been a corresponding drop in reports of yellow corn. Air temperatures have rebounded with the increase in solar radiation.  Growing Degree Day unit accumulations since May 1, May 15, and June 1 show unit accumulations are within plus or minus 2 percent at all sites east of the Panhandle.

GDD unit accumulations are running 3-5% below normal for the Panhandle since May 1, primarily due to very cold temperatures during the first 10 days of May.  The southern half of the Panhandle saw average temperatures 3-5 °F below normal, while the northern half of this region experienced average temperatures 5-9 °F below normal.

GDD unit calculations for defined time intervals are problematic as they fail to show the impacts of planting delays.  A producer could easily have normal or above normal GDD accumulations if his emergence was on June 1 or later.  However, his crop might normally emerge during early May and GDD accumulations since emergence can't compensate for these lost growing days.

Calculating Delayed Planting Impact

There is a quick and easy method to determine the impacts of late planting on GDD accumulations, regardless of whether GDDs are ahead or behind normal from this year's emergence date.  For each day your crop emerged after the average emergence date for your farm, multiply by 13.  For every 10 days, you fall behind 130 GDD units.  Add that to your accumulated GDDs since this year's emergence to get a true picture of the impact of temperatures on early crop development.

How long will it take your corn crop to reach tassel?  GDDs accumulate in July at a rate of twice the average daily rate in May.  If you are 10 days behind at the end of May, tassel during July will likely be delayed approximately five days.  A corn crop that emerges at the end of May instead of a normal emergence of May 10, will reach tassel about 10-11 days later than normal given normal temperatures from emergence to tassel.

Al Dutcher
State Climatologist