Drought is a shortage of water supply across a region for an extended period of time. Most regions experience drought-like conditions periodically, and some areas can remain in those drought phases for several years. When an area experiences drought for an extended period of time, it has enormous impacts on life in these areas. This shortage of rainfall also has impacts on the soil hydration and animals living within these drought regions. Figure A shows the most current picture of drought in the United States.
|Figure A: Current Drought Conditions
Drought is defined as an extended period of time where rainfall is deficient. The amount of time could be several weeks to several years. This is a general definition of drought, but there are several types of drought:
1) Meteorological drought is a long-term measurement of lower than normal precipitation. However, researchers cannot agree upon a numerical threshold that would define a drought (inches of rain or duration without rain) because it depends on the time of year, the natural climate (even deserts can have droughts, but they might look quite different than a drought in the Southeast), and the plant development stage at the time of occurrence.
2) Hydrological drought is a lack of water in subsurface and surface terms. This includes the water supply that is held in streams, lakes, groundwater and other types of reservoirs. Hydrologic droughts tend to develop over an extended time period.
3) Agricultural drought is when there is not enough water to sustain crops. Rainfall deficits during the critical growth periods of a crop (different from crop to crop) can result in large yield losses, or even complete crop loss. Agricultural droughts can develop quickly during a dry spell if no rain at all is observed or when a heat wave puts unusual stress on plants at critical growing stages.
Shortage of water supply is not the only cause of drought conditions, though usually drought conditions begin with below average levels of precipitation. Any measures taken that reduce the ability of the land to hold water may result in drought impacts. Overfarming techniques do this as well as deforestation. Overfarming takes all the nutrients out of the soil that the plants/crops need to survive and essentially âkillsâ or desertifies the land. In rainforests, the reduced precipitation triggers the drought because the plants and animals in the rainforest depend on the overabundance of rainfall throughout the year. Deforestation within a rainforest adds to the decrease in the amount of precipitation. This decrease in the average amount of rainfall will reduce plant growth and kill the trees. With fewer trees covering the rainforest ground, there is less water vapor to saturate the surrounding air above the trees to form clouds and produce precipitation. Over an extended period of time, the rainforest could dry out and absorb less carbon dioxide because of the reduced plant growth. Essentially this will add enormous amounts of carbon dioxide into the atmosphere.
Lessened precipitation over an area can occur for many different reasons. One cause in particular is a persistent high pressure area or ridge over a region. Sinking air flows within the area of high pressure, which leads to clear, fair skies during the day. This causes relatively warm temperatures across the regions affected by this upper-level ridge. If this high pressure area stalls over an encompassing region for an extended period of time, warm surface temperatures could lead to maximum evaporation of water from the ground and nearby lakes. This evaporation can quickly dry out surface soils and reduce streamflows and reservoir levels in conjunction with the lack of precipitation input, increasing the strength of the drought both by lack of rainfall and by increased removal of moisture by evaporation.
Another possible cause for drought conditions is the wind pattern blowing inland from the ocean. The wind from the ocean brings warm, humid temperatures to the eastern coastal areas during the summer months. If these winds shift direction or weaken during the summer, less water vapor is brought inland from the ocean or Gulf of Mexico and lessened amounts of rainfall will fall over the coastal regions. This below-average precipitation could also lead to a drought across the Southeast US. Lack of tropical storm activity is also a factor in developing drought in the Southeast. If the tropical storms that develop in the Atlantic and Gulf of Mexico follow a track towards Texas instead of towards the Southeast, Texas gets flooded while the Southeast swelters in heat and dry conditions. On the other hand, if a slow-moving tropical storm moves over dry land in the Southeast, a drought can be reduced very quickly as the soils are moistened by the abundant rainfall.
Figure B shows the drought conditions as of June 2, 2009 and one of the drought conditions from the prior year (June 10, 2008). Comparing the graphs, it is very noticeable that the drought conditions have indeed improved over the course of a year. Compared to the 2008 drought conditions, there is minimal drought throughout the Southeast US during this week in June 2009. The drought conditions have drastically changed within a year and most of the Southeast US has been relieved of the previous drought conditions; however, drought conditions can and will return again in the future.
|NOAA US Drought Monitor