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.