Soil Moisture Recharge Starts Early, but Will it Be Enough? - UNL CropWatch, Sept. 18, 2012
September 18, 2012
As the end of the 2012 production season rapidly approaches, growers are asking: What is the likelihood that this year’s devastating drought will be repeated in 2013?
A repeat is certainly possible, but droughts of this year’s magnitude and intensity are infrequent. In fact, based on land area coverage alone, you would have to go back to 1988 to find a drought with similar national coverage.
Thankfully, national droughts of this year’s intensity materialize an average of once every 15-20 years. When multiyear droughts do occur, western areas of the U.S. Corn Belt are particularly susceptible. In this area “off season” moisture is the primary driver for drought risk while in the central and eastern Corn Belt, normal off-season moisture is more than sufficient to replenish soil profiles.
UNL research has found that 70% of the moisture that falls from October through April will make its way into the soil profile. As bad as the drought was this year across the nation’s midsection, the prospects for normal soil moisture recharge are higher than normal. Although this sounds counter intuitive, remember that in a normal year, corn and soybeans are still actively growing in September. With corn and soybeans shutting down early, soil moisture recharge can start early, as long as the atmosphere cooperates.
With the remnants of hurricane Isaac dropping 2-5 inches of precipitation across portions of Missouri, Illinois, Indiana, and Ohio, soil moisture is building there. However, Isaac offered little reprieve for the western Corn Belt. We’ll be watching precipitation patterns through November before predicting the potential drought risk for 2013.
Understanding the Two Types of Drought — Agricultural and Hydrological
It is important to remember that when we quantify drought, we are looking at two types of drought. Agricultural droughts respond quickly to short-term precipitation events, while hydrological droughts extend longer than six months and can still be relevant event when agricultural conditions are optimum.
As long as your specific region has received normal moisture during the fall and spring, your risk of an agricultural drought during the growing season is entirely dependent on the mean jet stream pattern. Just because the fall is dry, doesn’t mean that you are going to experience drought conditions the next growing season. A wet spring can make up for fall precipitation deficits. What we have found here in Nebraska is that our more significant drought risks occur when moisture during the fall/spring recharge period falls below 80% of normal.
Hydrological droughts are much more difficult to quantify, especially when they span several consecutive years. It may take several years of above normal moisture to undo the damage of a single, intense drought year. Conversely, because the annual precipitation of the central and eastern Corn Belt may be two or more times greater than that in the western Corn Belt, a hydrological drought of equal magnitude can be eradicated in half the time as one in the western Corn Belt.
Our long range forecasting ability is weak at best. However, the long range models that we rely on work best when La Nina or El Nino conditions develop across the eastern and central equatorial Pacific. Weak El Nino conditions are now prevalent in the eastern Pacific and expected to strengthen slightly this fall and winter. These events usually dissipate as we move through spring, unless they are moderate to strong.
Several areas of the country respond to La Nina or El Nino events with enough regularity that they can be described with an accuracy of 70% or greater. During an El Nino event:
- In the southern third of the U.S. precipitation is usually above normal during the fall through spring and temperatures are below normal. (During La Nina years this same region will typically see below normal moisture and above normal temperatures.)
- The Pacific Northwest and the eastern Corn Belt will be drier and warmer than normal. (During a La Nina they will have above normal moisture and below normal temperatures.)
For the remainder of the country, fall and spring weather is entirely a function of where the mean jet stream pattern develops, even in the absence of El Nino or La Nina conditions. In the western Corn Belt, if precipitation patterns this fall are similar to last fall, the drought risk will remain elevated into next spring. If precipitation is close to normal, the risk of drought really won’t be definable until next March, at the earliest.
For the central and eastern Corn Belt, I believe that the drought risk for 2013 will be no different than any other year. Most of the eastern and central Corn Belt droughts develop during the growing season and not during the fall/spring period as happens in the western Corn Belt.
Extension State Climatologist