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Water as Earths Lifeblood: Droughts (Drought Precursors (Unusual dry…
Water as Earths Lifeblood: Droughts
Drought types
Meteorological drought: related to precipitation deficits which cause decreases in water supplies
Hydrological drought: Results from low stream flows that directly affect established water uses under a given water resources management system
Agricultural Drought: Linked to crop failure as a consequence of decreases in soil moisture and have no reference to stream flow
Annual Droughts occur on a yearly basis, most common around the equator
Other droughts can be deemed irregular, as they occur in regions which usually have lots of rainfall and moisture
Drought Definition
Long period of unusually low precipitation
Lack of precipitation due to weak storms or storms that are less frequent than normal
Most major droughts last for months or years
Definitions are region specific
Where Do Droughts Occur
Occur annually in arid and semi-arid areas
Usually most severe around the 30 degree latitude, where cool dry air is falling
Occur in regions where the sun is almost directly over the tropics for most of the year, and has a high constant temperature, little precipitation, and high evaporation
Humid areas are usually prone to short-term droughts, while arid regions are prone to long-term droughts
Drought Climatology: Mechanisms that lead to drought
The presence of widespread and persistent atmospheric calm areas called subsidence areas which do not cause precipitation
Subsidence area can be induced by mountain barriers or other geological barriers (especially the case in mid-latitude regions affected by westerlies wind currents)
absence of humid airstreams lead to lack of precipitation
Absence of rainmaking disturbances which causes dry weather, in the absence of cyclonic disturbances however, there is no mechanism for bringing upper atmosphere moisture down as rain
General Causes
Drought in almost continuously dry climates, which can lead to desert surface conditions and no season of appreciable rainfall
Drought in semi-arid or sub-humid climates with a short wet season in which humid airstreams or rainmaking disturbances penetrate
Drought in sub-humid areas in which rainfall is infrequent but not confined to a special season
Drought conditions enhanced by human activities such as overgrazing, poor cropping methods and improper soil conservation techniques
Aridity Index
The ratio of mean annual precipitation (P) to mean annual potential evapotranspiration (PET)
AI = P/PET
A region is considered to be arid when the aridity index is between 0.05 and 0.2
A region is considered to be semi-arid when the aridity index is between 0.2 and 0.5
Link between drought affects and aridity
People in arid regions are used to drought conditions and have adapted to them
People in sub-humid or humid areas feel the effects of a drought more strongly because of the unusual aspect of it
Drought Indices
Standard Precipitation Index (SPI)
Probabilistic index based on the comparison between current rainfall amount and normal amounts
SPI = xi-x/õ, negative indicates drought, positive indicates wet conditions
Palmer Drought Severity Index (PDSI)
Expresses the current state of dryness in a given region based on recent precipitation and temperature
Attempts to measure the duration and intensity of the long-term drought inducing circulation patterns
Its values are based on soil moisture models and a water balance equation
Palmer Crop Moisture Index (CMI)
Based on the comparison between currently measured soil moisture and normal values
Shows how monthly moisture conditions departt from normal
Measures short-term drought on a weekly scale and is used to quantify droughts impacts on agriculture during the growing season
Drought recurrence interval
Tend to occur at regular intervals
Scientists use historical trends to predict drought periods
Primary and Secondary Effects
Primary Effects
loss of water for hygienic use and drinking
loss of crops, livestock, and other animals
Increased fire hazard
Secondary Effects
Population migration to unaffected areas
Famine
Vegetation changes and increased wind exposure
Severe flash flooding
Desertification: the transformation of arable or habitable land to desert, due to a change in climate or a non-sustainable land exploitation
Drought Precursors
Unusual dry period
Increased number of wind storms
Increased number of dust storms
diminishing water supplies
increase in death rates of animals
changes in migratory patterns of nomads
changes in vegetation, especially the introduction of desert plants such as scrub bushes
increased hygiene related public health issues and diseases linkable to a lack of water