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Soil moisture, more than inches of rain, is key

September 05, 2006|by JEFF SEMLER

As I sit down to write this week's column, Ernesto is headed our way and will surely break our long dry spell.

I have already received the obligatory call from the media asking the question, "What will the impending rainfall mean for agriculture?"

For most of our crops, not much at this point because they have already passed the point of no return in their lifecycle.

Of course, since all crops grow in soil and our soil moisture is short to say the least, then the rain will be good for perennial crops like the orchards, pastures and hay fields. It will also bode well for the crops that will be planted this fall, such as wheat, barley, rye and grass.

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But the question that is more difficult to answer is the drought question. I often get asked during dry periods about the "D" word. That is very hard to answer.

Droughts, unlike hurricanes, are harder to define.

A hurricane is a low pressure system characterized by cyclonic winds in excess of 74 mph. Of course, there is also the associated rain and, if you are on the coast, the storm surge which can cause beach erosion.

To us here in the Cumberland Valley, it usually means heavy rain. Those my age or older can remember Agnes in 1972 or Isabel in 1985. Both of these ladies brought flooding to our area.

But back to drought - how do we define a drought?

According to NOAA, a drought is as follows: "a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area." - Glossary of Meteorology (1959).

In easier-to-understand terms, a drought is a period of unusually persistent dry weather that persists long enough to cause serious problems, such as crop damage and/or water supply shortages. The severity of the drought depends upon the degree of moisture deficiency, the duration, and the size of the affected area.

There are actually four different ways that drought can be defined:

· Meteorological - a measure of departure of precipitation from normal. Due to climatic differences, what might be considered a drought in one location of the country may not be a drought in another location.

· Agricultural - refers to a situation where the amount of moisture in the soil no longer meets the needs of a particular crop.

· Hydrological - occurs when surface and subsurface water supplies are below normal.

· Socioeconomic - refers to the situation that occurs when physical water shortages begin to affect people.

So as you can see, there are dry spells and then there are droughts. As they say, the devil is in the details.

However, there are a few important things to remember about rainfall.

One, it is not only the amount of rain but how it comes. Short intense rains have more runoff and thus less effective moisture for both soil and ground water.

Conversely, light sporadic showers don't give much in the way of moisture penetration either.

This is what makes looking only at average annual rainfall a poor indicator of drought.

You also have to look at soil moisture.

If you are interested, you can follow the dry conditions both here and around the country by checking out the Drought Monitor web site at: http://www.drought.unl.edu/dm/monitor.html

So what will Ernesto bring?

By the time you read this article, you will know.

Any rain is welcome.

As the old saying goes, "It will rain again and when it does, we will need it."




Jeff Semler is an Extension educator, specializing in agriculture and natural resources, for the University of Maryland Cooperative Extension. He is based in Washington County. He can be reached weekdays by telephone at 301-791-1404, ext. 25, or by e-mail at jsemler@umd.edu.

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