Analyzing the long-term effects of drought after recent rain

Drought is probably not top of mind for many Mid-Missourians after a wetter-than-normal spring, but dry weather is still a recent memory and continues to have impacts long after rain starts falling again.

There is currently no drought across central Missouri, but the latest U.S. Drought Monitor shows 25% of the state is drier than normal, and nearly 10% is in severe drought near the Bootheel. Southern Missouri has continued to battle drought through the spring as storm tracks remained a few miles too far north.

The U.S. Drought Monitor updates weekly and calculates drought categories based on precipitation totals and streamflows. This approach is useful for illustrating drought impacts right now, but there are other ways to see how drought has affected the environment over months or even years.

Analyzing drought can get tricky when long stretches of dry conditions are broken up by intermittent, heavy bursts of rain. Climate change is causing this to happen more frequently as well, so scientists have developed several ways to look at drought across different timescales.

The Standardized Precipitation Index (SPI) was developed as a statistical method for analyzing drought over weeks to months. The SPI compares rainfall to long-term climate normals, rather than only to the average, because rain totals are distributed unevenly and heavily skewed toward lower amounts.

Overall, the SPI categorizes how far rainfall deviates from normal, with categories for above- and below-normal conditions. Right now, the SPI in Missouri over the last 60 days indicates moderately wet to severely wet conditions across the state, or 1 to 2 standard deviations from normal.

But there are still limitations to the SPI; mainly, it doesn't account for extra water loss due to heat or vegetation. This is especially problematic for agriculture, since plants absorb and release over 10% of rainwater before it can reach the surface.

Using satellites, scientists can directly measure the amount of moisture in plants and compare it to historical averages to monitor crop health. This data can then be compared with the SPI, the Drought Monitor, and several other indices to provide a more comprehensive analysis of drought conditions.

An experimental product run by the National Drought Mitigation Center does just that. Known as the Composite Drought Indicator (CDI) Percentiles, it combines the Drought Monitor, SPI, soil and vegetation moisture, and more across several timescales.

And sure enough, while the short-term SPI shows the entire state is much wetter than normal, when including the effects of evaporation and transpiration, the long-term CDI percentiles indicate Missouri is still mostly drier than normal, with much of the Bootheel in moderate drought.