The Drought Monitor Comes of Age

The 20th century featured immense scientific discoveries and advances. Astrophysics gained Einstein’s life-altering theory of relativity, opening the door to nuclear weaponry and the mind-bending Big Bang theory. The medical field achieved stunning success in suppressing or vanquishing a host of deadly diseases, including polio and smallpox. And through advances in computing technology, meteorological forecasting moved from backof- the-envelope calculations to supercomputers. However, drought monitoring fell behind the curve of scientific advancement. Not until 1965, when the U.S. Department of Commerce published Wayne C. Palmer’s “Research Paper No. 45: Meteorological Drought,” was there even a complex mathematical definition of drought. In his foreword, Palmer explained that “meteorological science has not yet come to grips with drought. It has not even described the phenomenon adequately.” The Palmer Drought Severity Index (PDSI) was the earliest attempt to describe an imbalance between water supply and water demand, by integrating water supply (precipitation) and water demand (evapotranspiration, as computed from temperature) in a water-budget calculation that also included water storage in the soil. It also established an intensity scale for drought and identified when drought began and ended. Yet the PDSI was never really designed for national drought monitoring, as Palmer’s focus was on the Great Plains and the western Corn Belt; born in 1915, he grew up in south-central Nebraska, shaped by the 1930s Dust Bowl. Clearly, Palmer did not create the PDSI from thin air. He worked for years perfecting his equations, and many of his studies of U.S. droughts of the 1890s, 1910s, 1930s, and 1950s were published in the federal Weekly Weather and Crop Bulletin and other outlets, including the Monthly Weather Review and the Bulletin of the American Meteorological Society. Though not among six dozen references listed in “Research Paper No. 45,” “A Simple Index of Drought Conditions,” an article by James McQuigg of the U.S. Weather Bureau published in a 1954 issue (Volume 7, Issue 3) of Weatherwise might have influenced Palmer. Palmer’s 1965 work, as remarkable as it was for that time, was not the final word on drought. In 1968, three years after introducing the PDSI, he added the complementary Crop Moisture Index, recognizing that drought affects agriculture and hydrology on differing time scales—and at different soil depths