Wednesday, 15 May 2002: 10:45 AM
Drought Risk in Agriculture
Decision risk analysis is a proven method for the optimizing of profits (or for the reduction of loss). Weather is the greatest uncontrollable factor influencing the production and marketing crops. Long-range weather forecasts are produced to be compatible with decision risk analysis techniques. Initial conditions also influence crop yields and must be included in decision risk assessments. During the past century the central North American continent experienced 17 seasons of corn (maize) yield reduction to 90% of the mean (trend) or less. During the same period, there were 23 seasons where yield exceeded 110% of the trend. Initial risk analysis considers a one-in-six risk of serious drought (17 low-yield seasons in a 100-year period) and a one-in-five risk of high production (21 high yields in the 100 year period). Three preseason drought risk assessments have proven effective in decision analysis: First- excessively dry soils at planting time may be found to change the risk of reduced crop yield. Second- the 19-year “Benner” cycle significantly influences drought risk. Third- during La Nina years the "risk" of drought is double the long-term average. When combined with the initial subsoil moisture situation and the phase of the Benner cycle, crop yield of above or below the trend can be anticipated at a confidence level of 80% in the U.S. Corn Belt.
The "Benner Cycle" of some 18-19 years was established by the year 1885. Initially derived from 80 years of variability for grain prices, the cycle was immediately recognized as well correlated with the tree-ring record of climate. The cycle improves on the random 17% annual chance of drought by identifying high and low risk time periods. Commonly, two droughts occur during the first 6 years of the cycle. One mild drought is normal during the subsequent 12 or 13 years.
When subsoil moisture is low during the high-risk six year time period, the risk of severe drought is much greater than during the 12 low-risk years. Using the Benner cycle to manage risk is of greater utility than assuming random occurrence of 17 droughts and 21 high-yield years over a century. Using the Benner cycle and the subsoil moisture situation is an improvement over the Benner cycle alone in that the likelihood of an individual year being of high or low yield can be estimated rather than just assuming the normal 1-in-3 chance of drought during the high-risk years.
El Nino/La Nina status at planting decision time and anticipated El Nino status through the growing season is the final indicator of weather-related crop production risk. As the growing season progresses, a number of additional factors are considered in risk assessment.