The Concept of Drought

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The Concept of Drought

• Drought is a normal, recurrent feature of climate
  
• It occurs in virtually all climatic zones
• Its characteristics vary significantly from one
  region to another
• It differs from aridity, which is restricted to
  low rainfall regions and is a permanent feature
  of climate.

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The Concept of Drought-Cont.

• Drought is an insidious hazard of nature.
• Drought should be considered relative to some
  long-term average condition of balance between
  precipitation and evapotranspiration.
• Drought should not be viewed as merely a physical
  phenomenon or natural event.

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Drought and Famine

• The relationship between the two is rarely
  direct.
• Drought is an a geophysical hazard whereas famine
  is a cultural phenomenon or complex emergency.
• Drought results from a lower than expected amount
  of rainfall.
• Famine may be associated with low rainfall but
  the severity of any failure in the food supply is
  tied up with factors such as poverty,
  malnutrition, environmental degradation, poor
  governance, ethnic or religious differences.

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Annual number of droughts and famines
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The Drought Bowl Years

• Causes deficient rainfall, high temperatures,
  high winds, insect infestations, and dust storms.
• Duration In the 1930s, for almost a decade. In
  particular, 193031, 1934, 1936, and 193940 were
  distinct.
• Geographic coverage drought covered virtually
  the entire Plains.
• Impacts the Great Depressions bank closures,
  business losses, increased unemployment, and
  other physical and emotional hardships.

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Want some definitions of drought?

• Conceptual definition Drought is a protracted
  period of deficient precipitation resulting in
  extensive damage to crops, resulting in loss of
  yield.
• In early 1980s, 150 published definitions of
  drought were uncovered (Drs. Donald Wilhite and
  Michael H Glentz).

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Categorizing the definitions

• Meteorological drought precipitations
  departure from normal over some period of time.
• Agricultural drought occurs when there isnt
  enough soil moisture to meet the needs of a
  particular crop at a particular time.
• Hydrological drought deficiencies in surface
  and subsurface water supplies.
• Socioeconomic drought occurs when physical
  water shortage starts to affect people,
  individually and collectively.

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1930s Drought Costs

• 1988-89 drought was the most economically
  devastating natural disaster.
• Close second is undoubtedly the series of
  droughts that affected large portions of the
  United States in the 1930s.
• In all, assistance may have reached 1 billion
  (in 1930s dollars) by the end of the drought.
• The total cost (social, economic, and
  environmental) would be impossible to determine.

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The dust bowl area
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Costs and losses(Comparison)
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Some Indices

• Drought indices assimilate thousands of bits of
  data on rainfall, snowpack, streamflow, and other
  water supply indicators into a comprehensible big
  picture.
• A drought index value is typically a single
  number, far more useful than raw data for
  decision making.
• Several indices have been devised based on
• historically established norms.

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Some Indices-Cont.

• Percent of Normal
• Palmer Drought Severity Index (PDSI)
• Standardized Precipitation Index (SPI)
• Crop Moisture Index (CMI)
• Surface Water Supply Index (SWSI)-Colorado
• Deciles

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-NDVI Not a real time
monitoring tool. Not sensitive to
water. Difficult to compare different
ecosystems.
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PDSI during the dust bowl period
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http//drought.unl.edu/dm/monitor.html
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SPI
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CMI
-Weather station network-based
indicators (PDSI, CMI, SPI, ... ) Not
applicable outside US. Sparse meteorological
network. Coarse spatial resolution.
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Assessing the current situation