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Initial Publication Date: March 31, 2017

Droughts

How do you know when you're in a drought?

Identifying an area as 'in drought' is different from identifying it as 'arid'. While the two may seem related, the subtle difference is important. Aridity is defined as the "degree to which a climate lacks effective, life-promoting moisture" (Glossary of Meteorology, American Meteorological Society). Drought, on the other hand, is 'a prolonged period of abnormally dry conditions.' Thus, aridity is a quasi-permanent condition (persistent over human timescales), while drought is a temporary condition (which may persist for weeks, years, or in some cases, decades). The Sahara Desert is an arid environment. The Hoh rainforest in western Washington State is a very humid place that occasionally experiences drought.

Droughts tend to be somewhat elusive phenomena, with severity gradually increasing over many days, weeks, months, or even years. The spatial extent of a drought is also quite difficult to delineate, due to the spatial variability in precipitation. Therefore, they are much harder to define, monitor, and identify (relative to floods) within the 'noisy' background of natural wet and dry cycles. Yet the impacts of drought can be significant on many facets of the economy and environment. All types of drought originate from a deficiency of precipitation from an unusual weather pattern. If the weather pattern persists for a few to several weeks, it is said to be a short-term drought. However, if precipitation remains well below average for several months to years, the drought is considered to be a long-term drought.

Related to the difficulty in defining drought, economic damages related to drought are also difficult to define. But only considering economic damages that can be directly related to drought, it is clear that they too can be costly natural disasters. In 2013, EM-DAT claims only 9 deaths worldwide that were directly attributed to drought (15 is global annual average from 2003-2012), but a total of nearly 8 million people were significantly affected by drought in 2013 (average is over 36 million per year from 2003-2012). Damages related directly to drought in 2013 were estimated in excess of $1 billion (average of nearly $5 billion per year from 2003-2012). However, these numbers do not include related effects of wildfire and indirect effects of decreased food production, water quality, etc.

Table SPM.2 from IPCC, 2007.
Recent trends, assessment of human influence on the trend and projections for
extreme weather events for which there is an observed late-20th century trend.

Phenomenon^a and direction of trend	Likelihood that trend occurred in late 20^th century (typically post 1960)	Likelihood of a human contribution to observed trend^b	Likelihood of future trends based on projections for 21^st century using SRES scenarios
Warmer and fewer cold days and nights over most land areas	Very likely^c	Likely^d	Virtually certain^d
Warmer and more frequent hot days and nights over most land areas	Very likely^e	Likely (nights)^d	Virtually certain^d
Warm spells/heat waves. Frequency increases over most land areas	Likely	More likely than not^f	Very likely
Heavy precipitation events. Frequency (or portion of total rainfall from heavy falls) increases over most areas	Likely	More likely then not^f	Very likely
Area affected by drought increases	Likely in many regions since 1970s	More likely than not	Likely
Intense tropical cyclone activity increases	Likely in some regions since 1970	More likely than not^f	Likely
Increased incidence of extreme high sea level (excludes tsunamis^g	Likely	More likely than not^f,h	Likelyⁱ

Source: IPCC, 2007

Table notes:
^a See table 3.7 for further details regarding definitions.
^b See table TS.4, Box TS.5 and table 9.4.
^c Decreased frequency of cold days and nights (coldest 10%).
^d Warming of the most extreme days and nights of each year.
^e Increased frequency of hot days and nights (hottest 10%).
^f Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies.
^g Extreme high sea level depends on average sea level and on regional weather systems. It is defined here as the highest 1% of hourly values of observed sea level at a station for a given reference period.
^h Changes in observed extreme high sea level closely follow the changes in average sea level. {5.5} It is very likely that anthropogenic activity contributed to a rise in average sea level. {9.5}
ⁱ In all scenarios, the projected global average sea level at 2100 is higher than in the reference period. {10.6} The effect of changes in regional weather systems on sea level extremes has not been assessed.

There are four different kinds of drought.

Meteorological drought refers to a deficit in precipitation that is unusually extreme and prolonged. By definition, meteorological drought must be identified relative to the typical precipitation regime of an area and could be defined as some point out on the long-tail of the distribution of, for example, a plot (histogram or probability density function) similar to Figure 4, except with the x axis indicating the number of consecutive dry days.
Agricultural drought refers to a deficit in soil moisture that affects plant growth and productivity. While this term is most often used to refer to effects on agricultural crops, all plants can be affected by soil moisture drought. Paleoclimatologists (scientists who study past climate) examine the thickness of tree rings in woody plants to estimate the timing, duration and severity of past droughts because water-stressed trees form relatively narrow rings in drought conditions.
Hydrological drought refers to conditions in which stream discharge and/or lake, wetland and water-table elevations decline to unusually low levels.
Socio-economic or operational drought refers to conditions when water supply is significantly below demand such that water/reservoir management must be altered. Typically, when meteorological drought occurs, the effects cascade sequentially to the other three types of drought. Likewise, when the meteorological drought ends, the effects cascade in the same sequence, first restoring soil moisture, then restoring other hydrological 'stocks' within the system, and hopefully restoring the balance between water supply and demand at key water management infrastructure (i.e., reservoirs).