‘Drought’ refers to an extended period of months or years when a region experiences a deficiency in its water supply generally because of a consistently below average precipitation (rainfall) rate. Droughts are considered billion dollar weather events and are amongst the top three threats to populations in the world along with famine and flooding.
Drought can be caused by a number of factors which all result in a deficit in water supply. The main causes of drought include:
(1) Atmospheric related
The amount of water vapor in the atmosphere which causes precipitation is a main factor. Rain, sleet, hail and snow occur where there are moist, low pressure air systems. Therefore, if there is an above average presence of dry, high pressure air systems, less moisture becomes available to produce precipitation since these high pressure systems cannot hold as much water vapor. This results in a deficit of water for the areas over which they circulate.
Reduced precipitation also occurs when winds shift air masses and warm, dry, continental air moves over an area as opposed to cooler, moist, oceanic air masses.
The El Nino phenomenon, which affects the ocean’s water temperature, also has an impact on precipitation levels because in years when the temperature cycle is present it can shift the air masses above the ocean, often making wet places dry (drought prone) and dry places wet.
(2) Man-made
Deforestation for agriculture, infrastructure development or whatever other reason with the resultant erosion can cause drought to begin because as soil is moved away from an area it is less able to absorb moisture when it falls.
There are three main types of drought which include: (1) Hydrological drought whereby there are depleted amounts of available water in watersheds which results in a lack of water in river systems and reservoirs; (2) Meteorological drought which is mainly due to a lack of precipitation and is defined based on a regions ‘normal’ climatic conditions; and (3) Agricultural drought which refers to when soil moisture becomes a problem as a result of shortages in precipitation, changes in evapo-transpiration and reduced ground water levels which creates stress for agricultural crops.
Prolonged drought can be ‘measured’ in a region by utilizing many methods some of which include:
(a) the river flow
(b) precipitation values (in mm (under usual values))
(c) rainfall minus evapo-transpiration
(d) the size of the permanent bed of the river (problems to support water dependant ecosystems)
(e) the level of water within the river
(f) the level of water in the aquifer
(g) level of water in reservoirs
(h) the flow (or not) between the river and the aquifer
(i) environmental indicators (e.g. associated plant and fish species of the river ecosystem)
(j) the number of rainfalls (days)
(k) Soil humidity
(l) Snow reserves levels and glaciers (where present)
Environmental consequences of prolonged drought
The consequences of prolonged drought can be categorized mainly as: economic impacts, social impacts and environmental impacts. This article addresses only the environmental impacts of drought which are outlined below:
(1) Impact on water
Water is a basic necessity and without it living organisms, including humans, can die from dehydration. Also, a decrease in water reserves affects water storage systems such as rivers, springs, reservoirs, lakes, ponds and wetlands which ultimately lead to a reduction in aquatic flora and fauna. This obviously has a negative effect on the food chain, thereby affecting terrestrial animals and humans as well. In addition, as groundwater depletion occurs, a decrease in water quality occurs as a result of contamination (by algae for instance), increased salinity (due to salt water intrusion) and turbidity etc.
(2) Impact on soil, agriculture and food production
With a water deficit, vegetation loss occurs along with soil erosion. With the loss of soil which is the capital and resource base of farming, there is diminished crop or growth yields production. A sufficient decrease in agricultural production over a period of time can lead to hunger and famine as a result of lack of water for irrigation etc. This also affects grass and grain used to feed livestock and poultry and thus reduces the carrying capacity of animals. Soil erosion also leads to habit and land degradation.
(3) Impact on species biodiversity
As already mentioned, with decreased water supply both aquatic and terrestrial plants and animals are affected.
(4) Wildfires and dust storms
The low moisture and precipitation that often characterize droughts can quickly create hazardous conditions in forests and across range lands, setting the stage for wildfires. This may cause injuries or deaths as well as extensive damage to property and already shrinking food supplies. Also, with the loss of vegetation and soil erosion dust storms can occur. Both cases contribute to a decrease in air quality which affects health of living organisms.
(5) Disease
Drought often creates a lack of clean water for drinking and sanitation. This can lead to a wide range of life-threatening diseases as well as insect infestations and plant diseases.
A current example of the environmental effects of prolonged drought can be seen with droughts in Argentina and Australia, which is presently wrecking havoc to their agricultural production. As noted by the United Nations Convention to Combat Desertification (UNCCD) Secretariat, the two countries belong to the largest producers in the world yet current climatic patterns are subjecting their farmers to extremely dry conditions unlike any in recent memory. In both countries, the farmers have watched their plants and livestock wither under the effects of a heat wave this summer. Since March 2008 in Argentina, rainfall has been significantly below normal. Argentine farmers report that some 800,000 heads of cattle have been lost as a result. Wheat, maize and soy harvests are expected to drop by as high as 80 percent in the usually bountiful Pampas and Entre Ros regions. Rural associations estimate that total Argentine grain production will fall 39 percent and as many as 1.5 million head of cattle could die.
The world needs to increase its efforts to tackle the unpredictable and extreme occurrences of drought,” says the Executive Secretary of the UNCCD, Mr. Gnacadja. “In its latest assessment the Intergovernmental Panel on Climate Change (IPCC) forecast that the length and severity of droughts would intensify in the future. The panel particularly pointed to Australia as a region that would be severely affected. The IPCC report also revealed that climate-related disasters in Latin America have more than doubled since 1970. The drought in Argentina is just a further example.”
Sources used:
http://www.wmo.ch/pages/prog/hwrp/HN/documents/prolonged_drought.pdf
http://weather.about.com/od/drought/f/droughts.htm
http://geography.about.com/od/globalproblemsandissues/a/drought.htm
http://environment.about.com/od/environmentalevents/a/droughteffects.htm
http://www.bom.gov.au/climate/drought/livedrought.shtml
http://www.unccd.int/publicinfo/pressrel/showpressrel.php?pr=press09_02_09
http://www.un.org/esa/dsd/susdevtopics/sdt_desedrou.shtml
