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

(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) From IPCC Ch12.3 (TB audited): Changes in soil moisture and surface water can shift the rate of carbon uptake by ecosystems and alter suitable climate zones for wild species and agricultural cultivation (Feng and Fu, 2013; Garcia et al., 2014; Huang et al., 2016a; Schlaepfer et al., 2017; Fatemi et al., 2018; IPCC, 2019c) as well as the prevalence of related pests and pathogen-carrying vectors (Paritsis and Veblen, 2011; Smith et al., 2020). Water table depth, in relation to rooting depth, is also important for farms and forests under dry conditions (Feng et al., 2006). A reduction in water availability (via aridity or hydrological drought) challenges water supplies needed for for municipal, industrial, agriculture and hydropower use (Schaeffer et al., 2012; Arnell and Lloyd-Hughes, 2014; Schewe et al., 2014; Gosling and Arnell, 2016; van Vliet et al., 2016).

Low flow volume and intermittency thresholds can indicate reductions in dissolved oxygen, more concentrated pollutants, and higher stream temperatures relevant for ecosystems, water resource quality and thermal power plant cooling (Feeley et al., 2008; Döll and Schmied, 2012; Schaeffer et al., 2012; Prudhomme et al., 2014; van Vliet et al., 2016). Low water levels may also restrict waterway navigation for commerce and recreation (Forzieri et al., 2018).

12.3.2.7 Agricultural and Ecological Drought Agricultural and ecological drought indices relate to the ability of plants to meet growth and transpiration needs (Table 11.3; Zargar et al., 2011; Lobell et al., 2015; Pedro-Monzonís et al., 2015; Bachmair et al., 2016; Wehner et al., 2017; Naumann et al., 2018) and the timing and duration of droughts can lead to substantially different impacts (Peña-Gallardo et al., 2019). Drought stress for agriculture and ecosystems is difficult to directly observe, and therefore scientists use a variety of drought indices (Table 11.3), proxy information about changes in precipitation supply and reference evapotranspiration demand, the ratio of actual/potential evapotranspiration or a deficit in available soil water content, particularly at rooting level (Park Williams et al., 2013; Trnka et al., 2014; C.D. Allen et al., 2015; Svoboda and Fuchs, 2017; Mäkinen et al., 2018; Otkin et al., 2018). Severe water stress can lead to crop failure, in particular when droughts persist for an extended period or occur during key plant developmental stages (Hatfield et al., 2014; Jolly et al., 2015; Leng and Hall, 2019). Projections of high wind speed and low humidity (even for just a portion of the day) can also inform studies examining fruit desiccation and rice cracking (Grotjahn, 2021). Drought also raises disease infection rates for West Nile virus (Paull et al., 2017), and the alternation of dry and wet spells induces swelling and shrinkage of clay soils that can lead to sinkholes and destabilize buildings (Hadji et al., 2014).

• Water Scarcity: Reduced precipitation and water availability lead to water scarcity, affecting both surface water and groundwater resources.
• Crop Failures: Insufficient water for crops can result in reduced yields, economic losses for farmers, and potential food shortages.
• Ecosystem Stress: Drought can stress ecosystems, leading to decreased vegetation, habitat loss, and disruptions in wildlife populations.
• Water Supply Challenges: Municipal water supplies may face shortages, impacting communities, industries, and essential services.
• Wildfires: Prolonged dry conditions increase the risk of wildfires, as vegetation becomes more susceptible to ignition.
• Livestock and Wildlife: Drought affects the availability of water and forage for livestock, leading to challenges in animal husbandry. Wildlife may face similar challenges.
• Economic Impact: Drought can have significant economic consequences, including reduced agricultural productivity, increased food prices, and impacts on various industries.
• Human Health: Drought can indirectly affect human health through its impact on water supply, food availability, and the potential for conflicts over scarce resources.

Observed and Projected changes in Drought (TBA)

or called "How does CO2 increase impact drought"

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Drought related risk analysis

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. Below are some metrics and indices used in regional assessments on droughts. They are subdivided into the three drought categories listed above.

Indices for Agricultural and 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.

Data Access

• Monthly PDSI over global land area on a 2.5 degree grid from Aiguo Dai (ensemble mean monthly data from 1900-2100 using CMIP 6 data is also provided)
• Historical monthly (from 1850/01 to 2014/12) PDSI gridded data
• Current weekly PDSI Indices, maps and info at NOAA CPC
• NCDC Climate Division Data (text); see 'pdsi' key

Methodology

Self-calculating PDSI is not easy as a multitude of computation is involved. Some open-source code is available though. Here is one from Jacobi et al. (2013)(see "Supporting Information" for code).

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.

Link for the US Drought Monitor Map (November 2023): https://droughtmonitor.unl.edu/Maps/MapArchive.aspx

Dataset can be accessed here: https://docs.google.com/spreadsheets/d/1k1YhIULXMt4URSIpeJfgfRbiee_YA7oXfl2_s6xfnpk/edit?usp=drive_link

Deaths and Affected numbers on the basis of different Climatological disasters in USA from 2000-2023

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The US Drought Monitor Map

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Reference

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

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