5 Causes Of Drought

Drought is easy to recognize and hard to define. The federal drought portal describes it as a shortage of precipitation over an extended period, usually a season or more, that results in a water shortage, and notes that researchers once counted more than 150 published definitions. Meteorologists track missing rain, farmers track dry soil and failing crops, and water managers track falling reservoirs and streams. Each of those can begin and end at different times.

This autumn gave a clear example of how quickly it can happen. In September 2019 a ridge of high pressure sat over the eastern half of the country. NOAA’s National Centers for Environmental Information reported that the contiguous United States tied for its second warmest September on record, and that Mississippi, Alabama, Georgia, Florida, Kentucky and West Virginia each had their driest September on record. Across the region pastures stopped growing, and soil became too dry for fall planting.

1. High pressure that will not move

Most droughts begin with weather patterns that stall. Under a strong ridge of high pressure, air sinks and warms, clouds struggle to form, and the storms that would normally cross a region are steered around it. A ridge that lingers for weeks, as it did over the Southeast this September, can wipe out a season’s rainfall surplus.

2. Ocean temperatures far away

Where those ridges set up is influenced by sea surface temperatures, above all in the tropical Pacific. The warm and cool phases of the El Niño Southern Oscillation shift the jet stream and with it the paths of winter storms. La Niña winters, for instance, tend to be drier than normal across the southern tier of the United States. These patterns load the dice rather than decide the outcome.

3. Heat and wind

Rain is only half of the water balance. Heat, sunshine, low humidity and wind all increase evapotranspiration, the water drawn out of soil and plants into the air. When these arrive together with a dry spell, soil moisture can collapse within weeks, which is what scientists call a flash drought. A garden needs noticeably more water in a hot, windy week than in a cool, still one, even with the same rainfall.

4. Dry soil feeding itself

Moist soil cools the ground as its water evaporates, and some of that moisture returns as rain. Once soil dries out, more of the sun’s energy goes into heating the surface and the air above it. Hotter, drier air then pulls harder on whatever moisture is left. This feedback is one reason droughts tend to persist and spread once they take hold.

5. Demand and timing

The last cause is human. A growing region can use more water than its rivers and aquifers supply even in a normal year, and a dry spell then turns a tight balance into a shortage. Timing matters as well: much of the year’s precipitation may arrive in winter, while gardens and crops need it in summer. Reservoirs, ponds, rain barrels and healthy soil that holds water are all ways of moving water from the season that has it to the season that needs it.

None of these causes can be switched off by a gardener, but the last two can be eased. Mulched beds, deep and infrequent watering, and soil rich in organic matter all slow the loss of water from the root zone when the rain stops.