Tuesday, May 02, 2006

Drought Defined

Author: Chris Orr

Drought is usually talked about as single event. In reality, there are two types of drought: agricultural and hydrologic. Drought is further defined by its duration.

The earth's water or hydrologic cycle is a closed system, meaning the water is never lost. The rainfall we lack is a surplus somewhere else on the planet, either above ground, below ground or in the atmosphere. Rather than a spontaneous change in the weather, drought is the cumulative affect of moisture deficit. The cumulative affect also works to relieve drought.

Last year's rainfall was close to normal across the West River plains - certainly better than the year before - and that is starting a positive accumulation of moisture. There is at least hope that the drought will be overcome by additional moisture this year.

The Black Hills, however, remained very dry last year and suffered a net loss of moisture. This winter's snow-pack was minimal and they are starting the wet season with a cumulative loss of moisture. That is not a good position to be in.

Agricultural drought is the most familiar type of drought and it can vary in duration from a few weeks to a few years. Plants do not have enough moisture when this type of drought occurs. Agricultural drought can be remedied when there is enough rainfall to bring moisture several feet into the soil.

Hydrologic drought has a more significant impact on society because it affects reservoir levels as well as the amount of water in underground storage or aquifers. Hydrologic drought occurs during long dry spells and it takes a much longer time to recover. Rainfall and snowmelt must drain into reservoirs and seep deep down into the earth to reach aquifers.

It is very important to understand the context when you talk about drought.

Agricultural drought is more common and its visual affects are clearer than hydrologic drought. It is easier to see parched soil, dry stock dams and withered plants than peer down into limestone aquifers. We identify with agricultural drought because it dries up our lawns and forces livestock producers to buy hay because pasture grass will not grow. We become aware of hydrologic drought when wells cannot keep up with demand and springs dry up.

I still think that the West River plains will see significant improvement this year. The higher elevations of the Black Hills, eastern Wyoming and eastern Montana are a different story because they still very dry.

The dry condition of the Black Hills is even more troublesome because a lot of the rain and snow received at the high elevations is used to recharges aquifers. Therefore, the drought in the Black Hills has a far-reaching impact.

I am devoting the month of April to exploring the impact of drought on you and me. I will dispel myths about how long it takes to recover from drought and suggest ways you can help minimize the impact of drought. I will also take the bull by the horns and answer that burning question: Is there ever enough rain?

About the author: Chris Orr is a Certified Consulting Meteorologist with more than 25 years of experience. His private practice includes work as an expert witness, weather forecasting and forecaster training. His column appears in the Rapid City Journal every Sunday. He can be contacted at weather@rapidwx.com or through his Web site www.rapidwx.com .


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