Introduction to When the Rains Stop
There's probably nothing more devastating to a farmer than drought. Entire fields of crops can be lost, and a family's finances can be left in ruin. Industries and homeowners, too, can be seriously affected, as their water supplies dwindle. Drought parched most of the Southern Great Plains, the Southwestern United States, and northern Mexico from the fall of 1995 through the summer of 1996. Though the drought was relatively short-lived, crop losses in the region totaled more than $4 billion, according to the National Climatic Data Center. In addition, forest fires, sparked by a combination of the drought and a hotter-than-average spring and summer, burned millions of acres.
Meteorologists believed the drought was caused by two main factors, which worked together to prevent the moisture-bearing jet stream from reaching the region. The first factor was a cool water current, known as La Nina, that periodically flows along the surface of the Pacific Ocean. The current cooled the air above it, producing a high-pressure system in the upper atmosphere. This system diverted the jet stream from its typical path over the Southwest to a path over the Pacific Northwest. The second factor causing the drought was a low-pressure system over the mid-Atlantic Ocean, which pushed the eastern edge of the jet stream away from the Southern Plains. In most of the affected region, the drought was broken in August 1996 when new weather systems, including tropical storm Dolly, brought rain to the area.
It Could Have Been Worse
Though devastating to agriculture, the drought had only a minor affect on ground-water and reservoir levels, because it followed two of the wettest decades in centuries, according to researchers at the University of Arizona Laboratory of Tree-Ring Research. Based on their studies, which reveal wet years as wide rings in tree trunks and dry years as narrow rings, the most severe drought in the recent history of the Southwest and Southern Plains occurred during the 1950's. During that decade, there were two-to-three-year clusters of dryness, each separated by a wet year or two. The region's worst-known drought occurred during an approximately 20-year period in the late 1500's.
Because of the short duration of the 1995–1996 drought, ecologists believed that it was unlikely to have long-term effects on plant and animal populations. More extensive past droughts, however, resulted in the death of large numbers of plants and animals. Droughts kill plants and animals not only by depriving them of water but also by creating conditions favorable to the spread of certain pests and diseases. For example, drought makes it easier for bark beetles to feed on pine trees and to spread fungal diseases among them, because the drought-stressed trees cannot produce enough of the resin that normally protects them from insect attack.
Where Drought Is Most Evident
Drought-induced ecological changes are most evident where different types of plant communities meet. For example, creosote bushes normally live in dry areas, such as deserts. But because severe droughts may deplete the available water in desert soils, creosote bushes may invade adjacent, relatively moist areas, such as grasslands.
Some plants and animals have evolved adaptations to help them survive droughts. Certain amphibians burrow into the ground and surround themselves with a cocoon made of shed skin to help prevent water loss. The animals then go into a state of dormancy, called estivation, emerging when rain soaks the surrounding soil. A number of drought-adapted plants produce seeds that can remain viable for many dry years, germinating when eventually exposed to water.
The Southwest's thin soils, which retain little moisture, provide a poor buffer against the effects of drought. But even in less-arid areas with richer soils, droughts can produce major ecological changes. Ecologist David Tilman and his colleagues at the University of Minnesota tracked the effects of drought on an oak forest and prairie in the Cedar Creek Natural History Area, north of St. Paul. After three consecutive years (1987–1989) of drought, the researchers found that 38 percent of the plant species in their study area had been wiped out. And while one of the area's two main species of oaks suffered a high mortality rate, the other species took over dominance of the forest community. It took six years for plant diversity in the area to return to predrought levels.
Trying to Predict Drought
Although meteorologists cannot accurately predict when droughts will strike or how long they will last, they have found that droughts tend to alternate with wetter-than-normal periods in an irregular cycle. Scientists and engineers are exploring a variety of ways to help us cope better with droughts. Plant researchers, for example, are trying to develop drought-tolerant crops, which would reduce the need for irrigation during dry spells. And in the Southwest, where cities have grown rapidly since the 1980's, engineers have attempted to minimize the effects of drought on municipal water supplies by building pipelines to bring water from distant rivers. Homeowners in drought-prone areas can help by landscaping with native plants, which generally use less water than large expanses of lawn.
Ecologists warn that plans to cope with droughts should address the potential effects of major, prolonged dry periods such as those that struck the Southwest and Southern Plains during the 1950's. If a similar drought were to strike the Southwest today, with all of the residential and industrial development that the region now has, the economic consequences would be much more serious than in the 1950's.