As the climate warms, the risk of extreme weather will continue to increase, causing more frequent droughts and floods and affecting agriculture deeply. Drought projections look particularly dire for the US southwest.

In this article, I use our new County Weather Data App to look back at two recent US droughts: 2012 in the Midwest and 2013-17 in California.

The 2012 drought reduced corn yields by about 25% in Iowa and more than a third in Illinois and Indiana. Other notable drought years include 1983 and 1988. Iowa corn yields were low in 1993 due to flooding. 

Drought is low soil moisture due to low rainfall and high heat. A map of summer rainfall is not very useful for detecting droughts that may affect agricultural yields. It never rains in the summer in the west, but there is little dryland agriculture in the west.

However, if we normalize, i.e., express rainfall in each county relative to its norm, then we can see how unusual a year was.  The corn belt was very dry in the summer of 2012.

Extreme heat predicts non-irrigated crop yields better than precipitation. To measure extreme heat, I use degree days above a 30ºC.  Specifically, each day record the number of degrees above 30C the temperature was that day, e.g., if it was 32ºC today, record 2 degrees for today; if it was 28ºC today, record 0 degrees for today.  Then sum all the degrees above 30ºC over all days in the summer.

There was an abnormally high number of extreme heat degree days in the corn belt in 2012, especially in Iowa, Illinois and Indiana. This explains the low corn yields that year.

Looking over time, we see that the low corn yields in 1983 and 1988 also coincided with lots of extreme heat. Illinois was also particularly hot in the summer of 1980. The early 1950s were also extremely hot in these states.

The west followed the 2012 midwestern drought with a prolonged drought of its own beginning in 2013. Precipitation in California was historically low that year and again in 2015. 

Seasonal weather patterns, and hence agriculture, are very different in California than in the corn belt. California gets almost no rain in the summer, so its agriculture relies on irrigation. The availability of irrigation water depends on winter precipitation. The winter months in 2013 were very dry in California.

The effect of this drought is much less apparent in crop yields than was the midwestern drought because irrigation mitigates the drought's effects in California.  These plots show yields in the 3 largest-producing counties for almonds and processing tomatoes. Yields are somewhat lower in 2013 and later, but not noticeably so.

Crowded House is a rock band, fronted by New Zealander Neil Finn, that was famous in the late 80s and early 90s with songs like "Weather With You". Like the character in that song, California farmers try to make their own weather by irrigating their crops with water transported from the melting snowpack and pumped from under the ground.

That strategy can work for a while, but a prolonged drought will deplete those water sources. During the California drought, parts of the Central Valley sank as much as 60cm per year as farmers pumped groundwater to substitute for the missing rain and snow.

The increasing frequency of droughts will affect agriculture by reducing yields, increasing costs, and changing planting decisions. A recent paper by Jesus Arellano-Gonzales and Fran Moore shows that farmers without access to a large groundwater bank were less likely to convert from low-value annual crops such as wheat and alfalfa into high‐value perennial nut crops such as almonds and pistachio.

We cannot take the weather with us, at least not indefinitely.

The weather plots come from our new US County Weather data app, which displays data assembled by Wolfram Schlenker. The yield plots are from our US Crops and California Crops data apps.. Huge kudos to UC Davis ARE PhD student Seunghyun Lee for putting these apps together.