Trends in 20th century drought over the U.S.

In my previous post I illustrated how decadal climate variability over the last millennium affected populations of pinyon pine in the US south-west. I will now narrow the focus to the 20th century and ask whether droughts are better of worse. (See this earlier post on the intricacies of defining a drought and how to measure one.) For the answer I’ll turn to a study published in Geophysical Research Letters earlier this year by two researchers at the University of Washington.

The researchers used a hydrological model to simulate soil moisture conditions and runoff over the continental U.S. for the period 1925–2003. Good enough records of soil moisture just don't exist over U.S., not even today, so a model is used to reconstruct the record (recall that the best estimate of an agricultural drought is obtained through this type of numerical model, rather than an index based on the climate). They used the Variable Infiltration Capacity model, which calculates the energy and water balance at each point based on climatic inputs and a description of the land surface (soil, vegetation, topography). They examined agricultural and hydrological droughts, using estimates of soil moisture and runoff, spat out by the model, as drought indicators. They then used a statistical test (the Mann-Kendall test, if you really want to know) to see whether there were any trends in the model output. Indeed there were.

Over the 20th century, the soil became wetter over the majority of the country, while drying predominantly occurred in the Southwest. Trends in annual runoff were similar. Decreases in drought severity occurred in the Northeast, Great Lakes, Lower Mississippi, and the Pacific Northwest; increases in severity were located mostly in Texas, the Southwest and intermountain West. The eastern U.S. and Midwest saw a reduction in drought frequency, while the Southwest saw a little increase. Drought duration decreased in the Great Lakes, Mississippi basin, and Pacific Northwest, while duration increased in parts of the west including California and the interior.

While there has been a general increase in precipitation over the U.S., some areas still exhibited worse droughts. In those parts of the country that showed a decrease in soil moisture over the century (locations in the Southwest and the West), the increase in precipitation was offset by higher temperatures, causing more water to be evaporated.

The study made no mention of climate change, but it does illustrate the importance of making regional assessments of any climate change – the hydrology of different regions is different, and so the response to any changes in climate may differ too. The study also demonstrates that we can’t take a simple approach to predictions of drought, like saying more rain means less drought. The whole water cycle must be taken into account.