W4 Droughts and Wildfires

Why is drought important ?

• An estimated 55 million people globally are directly affected by droughts every year, making it the most serious hazard to livestock and crops in nearly every part of the world (WHO, 2021)

• Droughts have deep, widespread, and underestimated impacts on societies, ecosystems, and economies, with only a portion of the actual losses accounted for (UNDRR, 2021)

Drought Characteristics

• Usually develop slowly as opposed to floods

• Can last for decades

• Effects can linger for years - even after the termination of the event

• The onset and end of drought are difficult to determine

• Large spatial scale: can affect entire countries/ continents

• Difficult to measure societal impacts of drought

Drought definition

a prolonged absence or marked deficiency of precipitation (rainfall) that results in water shortage’

Drought causes

• high temperatures → very low humidity → less rainfall

• poor water management

• loss of soil moisture → influences agriculture

Drought impacts

• impact on agriculture → can lead to famine sometimes

• vanishing streams, lakes, and water tables → drinking water shortage

• risk of wildfires

This can lead to major loss of life of people and animals, loss of livelihoods and loss of habitats.

What re the different types of droughts?

• Agricultural

• Meteorological

• Hydrological

• Socioeconomic

Meteorological drought definition

definition based on:

• Degree of dryness or lack of rainfall with respect to “normal” or “average” for that region

• Duration of the dry period: Number of days with precipitation less than some specified threshold (<1 mm/day).

What do meteorological droughts depend on?

• for example, no rain in summer in the Mediterraean in normal, not a drought.

• applicable only for regions which have year-round precipitation

Definition and characteristics of Agricultural droughts

Links meteorological drought to agricultural impacts (e.g. less rain, soil water deficit, reduced groundwater)

Characteristics:

• Water amounts are lower than what is needed by crops

• Conditions that hinder crop growth and agricultural production

• Not enough soil moisture to meet the needs of a particular crop at a particular time - e.g. less topsoil moisture at planting → hinder germination.

• AGRICULTURAL DROUGHT HAPPENS AFTER A METEOROLOGICAL DROUGHT BUT BEFORE A HYRDROLOGICAL DROUGHT.

Hydrological Drought characteristics

• Reduced rainfall has impacts on the water supply

• Deficiencies in surface and subsurface water supplies (e.g. lake, reservoir, and groundwater) levels

• Not the earliest indicator of drought – time lag between lack of rain and low water

• Takes longer for precipitation deficiencies to show up

• The frequency and severity of hydrological drought are often defined on a small scale (watershed or river basin scale) (not large continental scale but sometimes that happens too!). When would that happen?

• This happens due to a lack of precipitation however hydrologists are more concerned with how this deficiency plays out through the hydrologic system.

• Indicators: flow, soil moisture, groundwater, etc.

Socio-economic drought definition

Occurs when physical water shortage starts to affect people: individually and collectively

Demand > Supply

Characteristics of socio-economic droughts

• Differs from the other types of drought - occurrence depends on supply and demand to identify or classify droughts. The supply of many economic goods, such as water, forage, food grains, fish, and hydroelectric power, depends on the weather.

• Because of the natural variability of climate, water supply is ample in some years but unable to meet human and environmental needs in other years.

Drought Indices

•Percent of Normal

•Percentiles

•Standardized Precipitation Index (SPI)

•Palmer Drought Severity Index (PDSI)

Planners may consult one or more indices before making decisions

How do we measure drought intensity?

1. Measuring Rainfall

2. Understanding averages

3. Comparing rainfall

% of normal equation

dividing actual precipitation by normal precipitation (typically 30-year mean)

what do percentiles help with ?

• Help us understand how a particular value compares to others in a dataset

• Gives a sense of ranking in a group

Standardised Precipitation Index (SPI) Overview

• Measures how likely the observed level of precipitation is compared to long-term data

• Very low probability à Rare precipitation deficit

• The SPI index is based on the likelihood of precipitation (only) for any time scale

SPI advantages

• The SPI can be computed for different time scales, from days to months

• Can provide early warning of drought, and assess drought severity

• Less complex than the Palmer Drought index

• Normalized index so it is possible to compare drought in different regions

Gamma distribution to the data to show skewness

Values in Gamma distribution meaning

α (shape parameter) controls the skewness.

β (rate parameter) controls the scale.

Γ(α) Γ(α) is the Gamma function, which generalizes the factorial function.

The Gamma distribution parameters (α and β) are estimated using statistical methods such as the maximum likelihood estimation (MLE).

SPI Parameter Meanings

• An index of zero indicates the median precipitation amount

• Negative values mean dry

  • Exceptionally dry: -2 and below

• Positive values mean wet

  • Exceptionally wet = +2 and above

Palmer Drought Severity Index (PDSI) Definition

• A soil moisture index that differs with region

• Based on the concept of water balance: water supply (precipitation), demand (evapotranspiration), and loss (runoff)

Many US government agencies and states rely on the Palmer to trigger drought relief programmes.

Pros and cons of PDSI

Pros

• The first comprehensive drought index was developed in the US (1965)

Cons

• Palmer values may lag emerging droughts (meteorological), by several months; complex

Calculation of P (avg moisture)

P = ET + RO + SS

P → precipitation

ET → Evapotranspiration

RO → Runoff

SS → Soil-water storage

Calculation of PDSI

Other drought factors

• Overpopulation → many people living in an area and using more water than can be provided reliably

• Over-cultivation → more agriculture than can be sustained during the dry years

• Over-extraction → use of stores at a faster rate than they can be replenished e.g. for irrigation

• Inefficient use → pollution or wastage of water

• Degradation of soil and landscape → leading to reduced capacity of soil/ ecosystems to provide water to agriculture or urban populations.

Definition of wildfire

An unplanned, uncontrolled, and unpredictable fire that burns in combustible vegetation such as forests, grasslands, or bushlands.

Causes of wildfires

• Very dry and warm conditions can also lead to wildfires

• Natural phenomena such as lightning can start forest fires, however, wildfires are often caused by human activity (e.g. campfires, smoking, arson etc.)

• Out of 50% of wildfires recorded, it is not known how they started

Direct impacts of wildfires

• loss of life - animals and people

• disruption in transportation

• communications

• power and gas services

• water supply

• deterioration of the air quality

• water pollution

• loss of property, crops, resources

• biodiversity + natural habitats

Indirect impacts of wildfires [from smoke and ash]

• Burns and injuries

• Eye, nose, throat, and lung irritation, decreased lung function, including coughing and wheezing

• Pulmonary inflammation, bronchitis, exacerbations of asthma, and other lung diseases

• Release significant amounts of mercury into the air, which can lead to impairment of speech, hearing, walking, muscle weaknesses, and vision problems for people of all ages

What’s the problem in progressively longer summers in Australia ?

• Increases the length and intensity of the bushfire season

• Long periods of high temperature → dries out soil and moisture in the environment → increasing fire danger

Forest Fire Danger Index

• estimates the fire danger on a given day

• based on observations of temperature, rainfall, humidity, and wind speed