Figure 3: Annual correlation of corn yield with different drought indices.
Higher correlations observed for indices such as PDSI and EDDI during 2004 and 2010 suggest that these indices may more effectively capture specific drought dynamics during favorable or extreme weather conditions. The variability in correlation coefficients reflects the complex interplay between climatic variables and corn productivity. This underscores the importance of using multiple indices to understand drought impacts comprehensively. Higher correlations between drought indices and productivity suggest that these indices describe the conditions better than other low correlations. The variability in the data is sometimes different, which shows how complicated the relationship is between climate variables and agricultural productivity. It also demonstrates the importance of using more than one drought index in planning farms and managing risks to help mitigate production loss due to short-term and long-term droughts. Drought indices provide a comprehensive understanding of how drought impacts agricultural production by analyzing various aspects of drought, such as soil moisture, plant health, and precipitation deficits. Previous studies have typically considered only a few drought indices, and research is scarce in Iowa that correlates crop yields with all the common drought indices in this region. This research addresses this gap by identifying which drought indices correlate most with corn and soybean yields in Iowa. The effectiveness of each index depends on the type of drought being tracked, the agricultural context, and the local climate. Some indices are more adept at detecting short-term droughts, while others are better suited for assessing long-term droughts. By evaluating the full range of standard drought indices, this study aims to provide a more complete picture of drought impacts on crop yields in Iowa.

Yearly Correlation of Soybean Yields with Drought Indices

Figure 4 highlights the annual correlation coefficients between soybean yields and drought indices. Unlike corn, soybeans exhibit greater resilience to short-term indices such as SPI-1 and SPEI-1, as indicated by smaller fluctuations in correlation values. However, longer-term indices like SPI-6, SPI-12, SPEI-6, and SPEI-12 show stronger and more consistent correlations with soybean yields. This suggests soybeans benefit more from sustained moisture availability over extended periods, particularly during key growth stages like pod setting and filling.
The oscillations observed around 2005, 2012, and 2018 reflect the complex interaction between climatic conditions and soybean yields. For example, 2012, severe drought conditions led to negative correlations across most indices, although soybeans demonstrated slightly higher resilience than corn. This can be attributed to the crop’s ability to recover from moderate drought stress during non-critical growth phases.