509
Soil and Vegetation Moisture Changes for Forest Fire Events
21.2 LITERATURE REVIEW
21.2.1 dRought definition
Drought is a recurring phenomenon that has plagued civilization throughout history. It affects natural habitats, ecosystems, and many economic and social sectors, from the foundation of civilization—agriculture—to transportation, urban
water supply, and the modern complex industries (Richard and Heim 2002).
Drought is the most complex and least understood of all natural hazards, affecting
more people than any other hazard (Wilhite 2000). Bryant (1991) ranked natural
hazards on the basis of various criteria, such as severity, duration, spatial extent,
loss of life, economic loss, social effect, and long-term impact, and found that
drought ranks first among all natural hazards (Narasimhan 2004). Compared with
other natural hazards such as flood and hurricanes that develop quickly and last
for a short time, drought is a creeping phenomenon that accumulates over a period
across a vast area, and the effect lingers for years even after the end of drought
(Tannehill 1947). Hence, the loss of life, economic impact, and effects on society
are spread over a long time, which makes drought the worst among all natural
hazards.
In spite of the economic and social impact of drought, it is the least understood
of all natural hazards because of its complex nature and varying effects on different
economic and social sectors (Wilhite 2000). The wide variety of sectors affected by
drought, its diverse geographical and temporal distribution, and the demand placed
on water supply by human-use systems make it difficult to develop a single definition
of drought (Richard and Heim 2002). The difficulty of recognizing the onset or end
of a drought is compounded by the lack of any clear definition of drought. Drought
can be defined by various factors, such as rainfall amounts, vegetation conditions,
agricultural productivity, soil moisture, levels in reservoirs and stream flow, or economic impacts. In the most basic terms, a drought is simply a significant deficit in
moisture availability due to lower than normal rainfall (http://www.ncdc.noaa.gov/
paleo/drought/drght_what.html).
After analyzing more than 150 definitions of drought, Wilhite and Glantz (1985)
broadly grouped those definitions into four categories: meteorological, agricultural,
hydrological, and socioeconomic drought.
• Meteorological drought: A period of prolonged dry weather condition due
to precipitation departure
• Agricultural drought: Agricultural impacts caused by short-term precipitation shortages, temperature anomaly that causes increased evapotranspiration, and soil water deficits that could adversely affect crop production
• Hydrological drought: The effect of precipitation shortfall on surface or
subsurface water sources such as rivers, reservoirs, and groundwater
• Socioeconomic drought: The socioeconomic effect of meteorological, agricultural, and hydrologic drought associated with the supply and demand of
the society
Soil and Vegetation Moisture Changes for Forest Fire Events
21.2 LITERATURE REVIEW
21.2.1 dRought definition
Drought is a recurring phenomenon that has plagued civilization throughout history. It affects natural habitats, ecosystems, and many economic and social sectors, from the foundation of civilization—agriculture—to transportation, urban
water supply, and the modern complex industries (Richard and Heim 2002).
Drought is the most complex and least understood of all natural hazards, affecting
more people than any other hazard (Wilhite 2000). Bryant (1991) ranked natural
hazards on the basis of various criteria, such as severity, duration, spatial extent,
loss of life, economic loss, social effect, and long-term impact, and found that
drought ranks first among all natural hazards (Narasimhan 2004). Compared with
other natural hazards such as flood and hurricanes that develop quickly and last
for a short time, drought is a creeping phenomenon that accumulates over a period
across a vast area, and the effect lingers for years even after the end of drought
(Tannehill 1947). Hence, the loss of life, economic impact, and effects on society
are spread over a long time, which makes drought the worst among all natural
hazards.
In spite of the economic and social impact of drought, it is the least understood
of all natural hazards because of its complex nature and varying effects on different
economic and social sectors (Wilhite 2000). The wide variety of sectors affected by
drought, its diverse geographical and temporal distribution, and the demand placed
on water supply by human-use systems make it difficult to develop a single definition
of drought (Richard and Heim 2002). The difficulty of recognizing the onset or end
of a drought is compounded by the lack of any clear definition of drought. Drought
can be defined by various factors, such as rainfall amounts, vegetation conditions,
agricultural productivity, soil moisture, levels in reservoirs and stream flow, or economic impacts. In the most basic terms, a drought is simply a significant deficit in
moisture availability due to lower than normal rainfall (http://www.ncdc.noaa.gov/
paleo/drought/drght_what.html).
After analyzing more than 150 definitions of drought, Wilhite and Glantz (1985)
broadly grouped those definitions into four categories: meteorological, agricultural,
hydrological, and socioeconomic drought.
• Meteorological drought: A period of prolonged dry weather condition due
to precipitation departure
• Agricultural drought: Agricultural impacts caused by short-term precipitation shortages, temperature anomaly that causes increased evapotranspiration, and soil water deficits that could adversely affect crop production
• Hydrological drought: The effect of precipitation shortfall on surface or
subsurface water sources such as rivers, reservoirs, and groundwater
• Socioeconomic drought: The socioeconomic effect of meteorological, agricultural, and hydrologic drought associated with the supply and demand of
the society