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.