Figure 2: Mean yearly corn and soybean yields in Iowa between 2000 and 2022.

Drought Indices

This study utilized six drought indices to capture various dimensions of drought and their potential impacts on crop yields. The indices were selected based on their ability to represent short-term and long-term drought conditions. Each index is described below, along with its limitations and rationale for inclusion.
Standardized Precipitation Index (SPI)
SPI quantifies precipitation deficits or surpluses over multiple timescales, such as 1, 3, 6, and 12 months (McKee et al., 1993). Contrary to earlier statements, SPI values can range widely but typically fall between -3.0 and +3.0, with negative values indicating drought conditions and positive values indicating wet conditions. Precipitation data from the Iowa Mesonet (https://mesonet.agron.iastate.edu/) were used to calculate SPI values.
Standardized Precipitation Evapotranspiration Index (SPEI)
SPEI combines precipitation and potential evapotranspiration (PET) to better account for temperature effects on drought (Vicente-Serrano et al., 2010). SPEI values were derived using the SPEI R package, incorporating monthly precipitation and temperature data. SPEI is particularly useful for capturing the combined impact of temperature and rainfall, a critical factor for agricultural applications.
The Standardized Precipitation-Evapotranspiration Index (SPEI) combines precipitation (Pi) and potential evapotranspiration (PETi) data to compute drought conditions. Pi and PETi were utilized to calculate the monthly water balance, represented as Di.