Analysis of an Iowa Aridity Index in Relationship to Climate and Crop Yield

An aridity index and its components, temperature and precipitation, for the period from 1900 through April 1993 were defined and characterized. This index describes the anomalous behavior of both temperature and precipitation over time. Our intent was to examine climate variability in Iowa. Moving mean and standard deviations over various lengths of time were calculated from three time series. We found that these indices fluctuate considerably from year to year and from month to month. The lowest aridity index values occurred in the recent summer of 1992, and the highest occurred during the very drought-prone 1930s. The 12-month moving mean and standard deviations for the aridity index were high during the 1930s and the 1980s. However, the period from the late 1950s to the mid- 1970s exhibited low mean values and low variation when compared with other times of the series. This was a benign weather period for agriculture in the midwest, and production levels were high. By increasing the length of the moving mean and standard deviations, upward and downward trends were shown. Recent variation of the aridity index was as high as it was earlier this century. The probability of having a number of consecutive months with positive, stressful aridity-index values in Iowa was quite small. Power spectrum analyses of the aridity index and its components (temperature and precipitation) suggested there was, at the 95% level, a significant 11-year periodicity. Significant shorter cycles with lengths varying between 3 and 8 years were also found. These may be associated with El Nino events. After adjusting yields to a time trend, correlations between corn and soybean yields and July and August aridity indices, respectively, were negative and significant