Aridity and Drought

Drought is a prolonged period of abnormally low precipitation that can lead to water shortages, impacting ecosystems, agriculture, water supply, and various human activities. It is a complex natural hazard with significant environmental, social, and economic consequences. Here are key aspects of drought:

Types of Drought

Depending on the variables used to characterize it and the systems or sectors being impacted, drought may be classified in different types, such as

• Meteorological drought: Lack of rainfall or below-average precipitation for an extended period characterizes meteorological drought. This is the most common form of drought.

• Hydrological drought: Hydrological drought refers to water shortage in streams or storages such as reservoirs, lakes, lagoons, and groundwater which usually occurs due to prolonged dry conditions.

• Agricultural and ecological drought: Agricultural drought relates to soil moisture deficits that affect crop growth. It occurs when there's not enough water in the soil to meet the needs of crops.

Impacts of Drought

Drought has extensive impacts across multiple sectors, affecting ecosystems, agriculture, water resources, energy production, commerce, public health, and infrastructure stability.

• Ecosystems and Biodiversity: Changes in soil moisture and surface water can alter the rate of carbon uptake by ecosystems, affecting the carbon cycle and overall ecosystem health. They also alter suitable climate zones for wild species, potentially leading to habitat loss and changes in species distributions. The altered conditions can increase can increase the prevalence of pests and pathogen-carrying vectors. Prolonged dry conditions increase the risk of wildfires, as vegetation becomes more susceptible to ignition.
• Agriculture: Reduction in water availability challenges the water supplies needed for irrigation, affecting crop yields. Insufficient water can stress crops and forests, affecting crop quality. Severe water stress can lead to crop failure, especially during key developmental stages or extended drought periods. Drought can also contribute to insect outbreaks, increases in wildfire, and altered rates of carbon, nutrient, and water cycling—impacting agricultural production.
• Power and Energy (copied from drought.gov) Production of all types of energy, including electricity, requires water. Because the energy sector is dependent on water availability, drought can severely impact energy systems.
  • Thermoelectric electricity generation. Thermoelectric power plants use steam turbines to generate electricity using a variety of fuel sources. Large amounts of water are needed to generate steam and for cooling. Drought conditions can result in reduced plant efficiency and generation capacity and can also impact the supply chain for coal, natural gas, biofuel, and nuclear fuel.  
  • Hydroelectric power generation. Hydroelectric power is generated by funneling water through power plants contained in dam structures. When water levels in reservoirs become low, the force of water pressure required to turn hydro turbine blades is reduced, which affects productivity.
  • Hydraulic fracturing and refining. Reduced water availability affects the production and refining of petroleum and natural gas. During droughts, hydraulic fracturing (or fracking) and fuel refining operations can require alternative water supplies or may be forced to temporarily shut down. Shutdowns can increase costs, which in turn can raise consumer prices.
  • Biofuels. Reduced water availability and decreased soil moisture during drought can reduce the cultivation of biofuel feedstocks.
• Manufacturing: Manufacturers use water for many processes, including fabrication, washing, cooling, and transporting goods. Water is also often incorporated into products themselves. Reductions in the amount of available water can reduce manufacturing productivity or even lead to temporary closures of key manufacturing facilities.
• Navigation and Transportation Drought impacts port and waterway transportation and supply chains, resulting in increased transportation costs. Higher temperatures that often coexist with drought can impact roads, airport runways, and rail lines.
• Recreation and Tourism Drought impacts various water-dependent activities like boating and skiing, leading to reduced revenues in tourism and recreation sectors. Increased wildfire risk during droughts further affects tourism by limiting access and deterring visitors due to safety concerns. Additionally, degraded water resources from drought can lower water quality and reduce streamflow, impacting hunting and fishing opportunities.

• Public health Drought increases disease infection rates. Low flow volume and higher stream temperature during drought can lead to reduced dissolved oxygen, and more concentrated pollutants, posing health risks for human and aquatic life.
• Water Utilities Drought can result in impacts to water utility operations, including:
  • Loss of water pressure and water supply
  • Poor water quality from the source that may require additional treatment to meet drinking water standards
  • Inability to access alternative and supplementary water sources because of high demand by and competition from other users
  • Increased customer demand
  • Increased costs and reduced revenues related to responding to drought impacts.
• Infrastructure The alternation of dry and wet spells can lead to swelling and shrinkage of clay soils, causing structural damage to buildings.

Observed and Projected changes in Drought (TBA)

or called "How does CO2 increase impact drought"

Drought-related Data

Drought Indices and Indicators

Droughts are often analyzed using indices that are measures of drought severity, duration and frequency. There are many drought indices published in the scientific literature. The Integrated Drought Management Programme provides a comprehensive review of existing drought indices and indicators, evaluating them based on their ease of use and type. A download link for each indicator is also included. Climdex, led by the University of New South Wales in Sydney and involving partners across the globe, provides a list of useful indices and their relevance to each sector.

Below is a list of some of the most commonly used drought indices based on IPCC AR6 report, categorized according to the three drought types introduced earlier.

Indices	Brief Introduction	Data Access	Recommended Use
Standardized Precipitation Index (SPI)	The Standardized Precipitation Index (SPI) is a widely used index to characterize meteorological drought on a range of timescales. It can characterize drought at different time scales which correspond with the time availability of different water resources (e.g. soil moisture, snowpack, groundwater, river discharge and reservoir storage). The SPI can be compared across regions with markedly different climates.	Gridded SPI for Global Land Surface (1949-2012) generated from CRU 3.21 Current SPI maps and tabular data for North America (NCDC) Plots and archive of SPI for contiguous US back to 2003, High Plains Regional Climate Center Python-package to calculate SPI	Meteorological/hydrological/ecological drought
Standardized precipitation evapotranspiration index (SPEI)	SPEI serves as a comprehensive drought index due to its multi-scalar nature, accommodating diverse scientific disciplines for detecting, monitoring, and analyzing droughts. SPEI assesses drought severity based on intensity and duration, identifying onset and cessation of drought episodes. Its versatility allows for comparative analysis across different climates and over time.	SPEIbase (Monthly global SPEI data on a 0.5 degree grid from 1901/01 to 2022/02, based on CRU TS4.07 data set; potential evapotranspiration is estimated using Penman-Monteith method) SPEI data from Global Drought Monitor (Global data on 1 degree grid from 1955/01 to present, updated monthly; potential evaportranspiration is estimated using Thornthwaite method) Annual SPEI data by sovereign regions, presented by the World Bank U.S. Gridded SPEI data derived from the nClimGrid-Monthly dataset (from 1895 to present) R-package to calculate SPEI Python-package to calculate SPEI	Ecological drought
Palmer drought severity index (PDSI)	PDSI is a regional drought index commonly used for measuring the severity of drought conditions. It is a standardized index that ranges from -10 (dry) to +10 (wet), with >4 indicating extreme wet conditions and <-4 indicating extreme dry conditions. Self-calculating PDSI is not easy, as a multitude of computation is involved. Some open-source code is available online. Here is one from Jacobi et al. (2013)(see "Supporting Information" for the code).	Monthly PDSI over global land area on a 2.5 degree grid from Aiguo Dai (also includes ensemble mean monthly PDSI for 1900-2100 calculated using CMIP 6 data) Historical monthly (from 1850/01 to 2014/12) PDSI gridded data Current weekly PDSI maps and info at NOAA CPC NCDC Climate Division Data (text format, see 'pdsi' key)	Meteorological and ecological drought
FAO Agricultural Stress Index System (ASIS)	ASIS monitors agricultural areas with a high likelihood of water stress/drought at global, regional and country level, using satellite technology.	ASIS Raster dataset ASIS interactive system	Agricultural droughyt
U.S. Drought Monitor (USDM)	The USDM is a map released every Thursday, showing where drought is and how bad it is across the U.S. and its territories. The map uses six classifications: normal conditions, abnormally dry (D0), showing areas that may be going into or are coming out of drought, and four levels of drought: moderate (D1), severe (D2), extreme (D3) and exceptional (D4).	USDM Data Agricultural commodities in drought	Meteorological/hydrological/ecological drought
U.S. Climate Extremes Index (CEI)	The CEI evaluates the percent area of extremes in the distribution of much above/below average (top/bottom 10% of occurrence) temperatures, precipitation, drought, and tropical cyclone wind speed across the CONUS and is measured as the percent area of the U.S./region experiencing extremes for a given season.	CEI by NCEI of NOAA (from 1910 to present)
Consecutive Dry Days (CDD)	Maximum number of consecutive dry days with less than 1 mm of precipitation per day	A code for calculating CDD is provided by Earth System Model Evaluation Tool.
Standardized soil moisture index (SSMI)	SSMI is calculated as the soil moisture content normalized by the recent past climatological values at each grid point. The soil moisture content can be the content integrated over the entire soil depth or just the surface layer. Note that soil moisture data in general has greater uncertainties compared to atmospheric data. Cautions should be used when using soil moisture index to assess the drought risk in future climate scenarios.	A list of soil moisture index data is provided here and can be made available upon request.
Evaporative Demand Drought Index (EDDI)

Indices for hydrological drought

Indices	Brief Introduction	Data Access	Recommendation Rating
extreme low runoff
low flow days
Standardized runoff index (SRI)
Standardized streamflow index (SSI)

Indices for agricultural and ecological drought

Indices	Brief Introduction	Data Access	Recommendation Rating
Consecutive Dry Days (CDD)
Soil Moisture (total and surface)
Evaporative Demand Drought Index (EDDI)
Soil Moisture Anomalies (SMA)
Standardized soil moisture index (SSMI)

Drought Forecast

(from NOAA Global Drought Information System)There currently is not a drought forecast that covers the entire globe. However, there are several national weather and climate centers which produce global seasonal weather and climate forecasts. The World Meteorological Organization (WMO) designates these as WMO Global Producing Centres for Long-Range Forecasts (GPCLRFs). They form an integral part of the WMO Global Data-Processing and Forecasting System (GDPFS). Some of the national centers produce national drought outlooks. Australia - Australian Bureau of Meteorology Brazil - Centro de Previsão de Tempo e Estudos Climáticos (CPTEC) Canada - Meteorological Service of Canada European Centre for Medium Range Weather Forecasts (Intergovernmental, HQ: UK) France - Meteo France Germany - Deutscher Wetterdienst Japan - Japan Meteorological Agency: Tokyo Climate Center North Eurasia Climate Center (Intergovernmental, HQ: Russia) South Africa - South African Weather Service South Korea - Korean Meteorological Administration / Global Producing Center for Long Range Forecast United Kingdom - United Kingdom Met Office United States of America - US National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) World Meteorological Organization (WMO) - Global Producing Centers for Long Range Forecasts (Intergovernmental, HQ: Switzerland)

Other resources

The US Drought monitor

The US Drought Monitor (USDM) is a comprehensive and collaborative effort by several federal agencies, including the National Drought Mitigation Center (NDMC), the United States Department of Agriculture (USDA), and the National Oceanic and Atmospheric Administration (NOAA), among others. It provides up-to-date information and assessment of drought conditions across the United States. Additionally, agricultural commodities in drought is also provided.

National Integrated Drought Information System

The National Oceanic and Atmospheric Administration’s (NOAA's) National Integrated Drought Information System (NIDIS) program is an integrated information system that coordinates drought monitoring, forecasting, planning, and information at federal, tribal, state, and local levels across the country. The Handbook of Drought Indicators and Indices reviews the most commonly used drought indices/indicators.

National Drought Mitigation Center

U.S. drought monitor and related products are listed here

Integrated Drought Management Programme (IDMP)

The IDMP supports governments and other stakeholders at all levels on implementing Integrated Drought Management by providing policy, technical and management guidance and by sharing scientific knowledge and best practices. Specifically, the Drought Monitors and Productssection lists existing tools and resources for monitoring drought conditions.

Global Drought Information System

https://gdis-noaa.hub.arcgis.com/

Notes

(Notes from Xiaojuan: List the sectors impacted by drought and create separate page for each sector; the webpage should be information on what and how climate hazards impact the sector

Reference

1. https://climatedata.imf.org/

2. https://yaleclimateconnections.org/2021/06/california-americas-garden-is-drying-out/

3. Table 11.3 of IPCC WG1