Figure 4: Annual correlation of soybean yield with different drought indices.
From 2010 to 2015, notable shifts in climatic trends influenced yield responses, as evidenced by increasing positive correlations with shorter-term indices. After 2015, the influence of rapid precipitation changes (as captured by SPI-1 and SPEI-1) became more apparent, emphasizing the need for adaptive management strategies that consider these dynamics.

Mean Correlation Analysis

Figure 5 presents the mean correlation coefficients for corn and soybean yields with all drought indices over the study period. Soybeans demonstrate stronger correlations with SPI-6, SPI-12, SPEI-6, and SPEI-12, highlighting their dependence on long-term moisture availability. These indices may serve as reliable predictors of soybean productivity in Iowa under similar climatic conditions.
In contrast, corn yields exhibit less consistent correlations with these indices, likely due to the crop’s greater sensitivity to short-term drought conditions. Notably, EDDI shows a strong negative correlation with corn yields, reflecting the crop’s susceptibility to high evaporative demand during critical growth phases. This aligns with corn’s higher water requirements and sensitivity to drought-induced stress.