Lecture 6: extremes Flashcards
What is an extreme event?
• Multiple definitions - language used in climate science and
hydrological science is not very precise
• High impact (but not really extreme)
• Exceedance over a relatively low threshold e.g. 10th, 90th
percentile of daily temperature or precipitation
• Rare events (long return period)
• Unprecedented events (in the available record)
• Very wide range of space and time scales
• From very small scale (tornadoes, hail storms) to large scale
(drought, heat waves)
• Extremes in one location may be normal in another
What are the extreme events relevant to water resources?
extreme precipitation
droughts
floods
extreme heat and heat waves
(all have connections)
`
scale of extreme events
bb
What are extreme values and extreme value analyis?
• Extreme values are the values of a process (e.g. precipitation or streamflow)
that are exceptionally high or low
• Extreme value analysis is the statistical analysis of extreme values. These
values are analysed separately, and at a range of time (duration) and space
(extent) scales. For example, extreme precipitation at the daily time scale at a
point, or extreme drought at the annual scale for a country.
• Why is it important for water resources?
• In hydrology, it is important to understand the occurrence of extreme high
values (floods) and extreme low values (droughts) for water resources
management and disaster risk reduction
• For example,
• knowing how often an extreme flood will happen on average every 25
years allows us to design how big to build flood defenses or dams.
• knowing how severe a drought could be, helps planning in agriculture.
- Some concepts from engineering hydrology
- Design rainfall, design flood, Intensity-Duration-Frequency (IDF) curves
What is the extreme value analysis concept: exceedance probability
Exceedance probability – how often a value is exceeded over a time period
• Cumulative distribution function (CDF) is a plot that shows the proportion of values
less than or equal to a given value
Fx(x) = P(X <= x)
• Exceedance probability is the complement of the CDF
Ex(x) = P(X > x) = 1 - Fx(x)
diagram of bb
Extreme value analysis concepts- return period
• Exceedance probabilities are often expressed as return periods or recurrence
intervals:
• The return period or recurrence interval of an event of magnitude x, is the
average number of years between exceedances of x:
• Tr(x) = 1 / E(x)
• One can also think about the reverse of this: the 10-year return value, for
example, is the value of X that occurs on average every 10 years.
• In the design of engineering structures, the return period is used to design the size and strength of structure. A return period is generally chosen to be higher for projects that have higher costs of failure • For example, the flood with a return period of 25 years is used for designing culverts; the 100 year flood is used to define floodplains for land use planning
what are examples of annual maximum flows
diagrams on bb
what are the global impacts of extreme events?
“Every year, disasters related to weather, climate and water hazards cause
significant loss of life and set back economic and social development by years, if
not decades.”
“From 1970 to 2012, 8,835 disasters, 1.94 million deaths and US$ 2.4 trillion of
economic losses were reported globally as a results of droughts, floods,
windstorms, tropical cyclones, storm surges, extreme temperatures, landslides
and wildfires, or by health epidemics and insect infestations directly linked to
meteorological and hydrological conditions.”
GDP of the UK is $2.7 trillion
GDP of Ethiopia is $48 billion
what are the global database of distatsers and impacts?
- what are the costs of disasters?
bb
What are the changing impacts over time?
- Impacts have mainly increased in the developed world because of assets
- Impacts have both increased and decreased in the developing world
- Decreases due to better warning and management
- Increase due to more exposed population and assets
What are the cost of drought? bb
…
explain:
risk, vulnerability, exposure, hazards
Risk (R) = Hazard (H) x Exposure (E) x Vulnerability (V) { x Insurance (I) }
Hazard The degree of hazard, i.e. the full range of intensities of threatening natural phenomena, including their probabilities of occurrence.
Exposure
The total exposed values or values at
risk – real/personal property – present at
the location affected/threatened.
Vulnerability
The degree of vulnerability, i.e. the lack
of resistance to damaging forces, or the
ratio of exposure that can potentially be
damaged.
Insurance penetration
The proportion of insured values at risk.
Risk
The integral over the hazard density
function multiplied by the corresponding
consequences.
What is a drought related distster?
2011 horn of africa drought and famine
Weather conditions over the Pacific, including an unusually strong La Niña, interrupted seasonal rains for two consecutive seasons. The rains failed in 2011 in Kenya and Ethiopia, and for the previous two years in Somalia. In many areas, the precipitation rate during the main rainy season from April to June, the primary season, was less than 30% of the average of 1995–2010.
The lack of rain led to crop failure and widespread loss of livestock, as high as 40%–60% in
some areas, which decreased milk production as well as exacerbating a poor harvest. As a
result, cereal prices rose to record levels while livestock prices and wages fell, reducing
purchasing power across the region. The crisis was compounded by rebel activity around
southern Somalia from the Al-Shabaab group.
Droughts arguably cause the most impacts of all natural hazards
in terms of the number of people affected and the long-term
economic costs and ecosystem stresses.
- reduced water levels/ supply: public, industry and power generation
- reduced agricultural, forestry and fisheries productivity
- increased livestock mortality rates
- increased fire hazard/ tree die off
- Damage to wildlife habitat
What drought events of the 20th and early 21st centuries
Europe
Major droughts in 1921, late
1940s to early 1950s, 1976,
and the 2003 fatal heat wave
Western and Central U.S.
Major droughts: 1930s
Dustbowl, 1950s, 2000s.
1988 drought cost $80 billion
Sub-Sahara Severe drought in early mid 1970s and 1980s led to widespread famine and 100,000s deaths
Southeast Asia Strongly affected by El Niño. Severe drought in 1997-98 caused extensive forest fires
define drought
• The simplest definition is “a deficit of water relative to normal conditions”
• Drought may then be defined more specifically as a low amount of water
in one or a combination of these stores (river, lake, reservoir, snowpack,
soil water and groundwater) or fluxes (precipitation, evapotranspiration
and run-off).
• This definition may be further qualified by adding that a drought occurs
when the lack of water is sustained and spatially extensive, and is a
deficit below a threshold that has adverse impacts.
• Many other definitions….
• Note:
• Drought is a normal, recurrent feature of climate that occurs in
virtually all climate zones, from very wet to very dry.
• Drought is different than aridity, which is a permanent feature of
climate in regions where low precipitation is the norm, as in a desert
What are the drought mechanisms?
• The occurrence of drought is governed by the hydrological cycle
• Human activities also play a role in modifying the hydrological cycle, by
removing, adding or storing water at a location.
• When there is no precipitation for a while, perhaps for weeks, months or
even years, the stores of water on the land surface (in snowpack, glaciers,
ice sheets, lakes, wetlands and rivers) and in the ground (in soil water and
aquifers) reduce and there is less water available for use.
• These stores diminish either through evaporation back into the
atmosphere, drainage to lower soil layers, recharge to aquifers, export
through rivers or abstraction by humans.
• There are many other factors that influence the development of drought,
including land cover changes (e.g. deforestation) and disturbances (e.g.
fire).
What are drought feedback mechanisms?
• There is also a set of feedback mechanisms that may have to be overcome for
drought to occur.
• For example, the evaporation of water back into the atmosphere is a process of
recycling water that may fall back again as precipitation.
• To break this cycle, either the available water on the land for evaporation would
have to be depleted, or the evaporated water in the atmosphere would have to
be transported from the region by the wind.
Drought Feedback Mechanisms
• These feedbacks may also work to
prolong a drought. As a drought
intensifies, less water is available to be
recycled, thus intensifying the drought
further.
• There is a link to heatwaves as well,
which may be intensified because less
evaporation to cool the surface and all
energy goes into heating the
atmosphere
what are the different drought types?
In the scientific literature, droughts are typically classified into four major
types:
1. meteorological drought, a significant negative deviation from mean
precipitation;
2. hydrological drought, a deficit in the supply of surface and subsurface
water;
3. soil moisture or agricultural drought, a deficit in soil moisture, driven by
meteorological and hydrological drought, reducing the supply of
moisture for vegetation;
4. socio-economic drought, a combination of the above three types
leading to undesirable social and economic impacts.
These classifications of drought are not rigid, since the definitions
incorporate many different physical, biological and socio-economic
variables. Further definitions may apply, based on environmental impacts
What is drought propagation?
bb
What are drought indicies
• A quantitative expression for the state of drought is generally required to
understand current conditions, and how it compares with past droughts and with
other regions
• This is usually called a drought index and allows a scientist, farmer, manager or
policy-maker to objectively analyse a system and make quantitative management
and policy decisions.
• Drought indices are used in both operational drought monitoring and when
forecasting drought within a warning system.
• There are many different types of index, but they are generally a rescaled version
of a meteorological or hydrological variable at a time step of weekly/monthly or
longer.
Drought indicies continued
• The rescaling is used to allow different levels of drought to be identified
relative to a drought threshold and allow comparisons with other times
and locations (which have different climates and therefore different mean
and variability)
• Z-scores or percentiles are often used
• Percentiles - equivalent to exceedance probabilities:
• Rank your time series of values Vi from 1 to N
• Calculate the empirical percentile value as p(Vi) = 100 * rank / (1+N)
• The lowest value on record will have a percentile value of 1/(1+N)
and the highest, a percentile value of N/(1+N)
• Choose a percentile value for a drought threshold – e.g. 20th
percentile
What are hydrological drought indicies
• Hydrological drought indices are
generally based on low flow
quantities, which characterize the
drier part of the streamflow regime.
• Low flows are usually defined in
relation to the total streamflow
regime, as characterized by the flow
duration curve (percentage of time
that a particular flow is exceeded).
Hydrological Drought Indices
• A metric of low flow can be selected such as the flow that is exceeded 95 per cent
(Q95) of the time. An index of drought can be defined in relation to a low flow
metric such as Q95.
• Examples of indices include the duration of flows continuously below the threshold,
the minimum 7-day average flow, the number of days per year below the
threshold, and the flow deficit volume
• A 7-day low flow index is important for environmental flows; a 90-day low flow
index could be important for a reservoir to understand long periods of low inflows
