3.4 Droughts and their impacts on agriculture and water resources

Droughts are prolonged dry spells that wreak havoc on water resources and agriculture. They're sneaky, creeping up slowly and lasting for ages. From parched crops to empty reservoirs, droughts can seriously mess things up.

There are different types of droughts - meteorological, agricultural, hydrological, and socioeconomic. Each impacts water availability in its own way. Understanding what drives droughts helps us predict and prepare for these water-starved times.

Drought Definition and Types

Defining Drought and Its Characteristics

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• Drought is a prolonged period of abnormally low precipitation, leading to a shortage of water
• Key characteristics of drought include:
  • Extended duration of dry conditions compared to the average precipitation in a region
  • Deficiency in water supply affecting various sectors (agriculture, water resources, ecosystems)
  • Slow onset and gradual development, making it difficult to determine the beginning and end of a drought event

Types of Drought

• Meteorological drought
  • Defined based on the degree of dryness and the duration of the dry period compared to the average precipitation in a region
  • Determined by the departure of actual precipitation from the long-term average or normal amount
  • Examples: Below-average rainfall for several months or seasons, extended periods of high temperatures and low humidity
• Agricultural drought
  • Occurs when the soil moisture is insufficient to support crop growth and development, affecting agricultural productivity
  • Depends on factors such as crop type, stage of growth, and soil characteristics
  • Examples: Reduced crop yields, increased irrigation requirements, pasture and rangeland degradation
• Hydrological drought
  • Refers to the depletion of surface and subsurface water supplies, such as rivers, reservoirs, and groundwater
  • Characterized by reduced streamflow, lake and reservoir levels, and groundwater recharge
  • Examples: Low river flows, dried-up wetlands, decreased water availability for irrigation and hydropower generation
• Socioeconomic drought
  • Associated with the impact of drought conditions on the supply and demand of economic goods and services
  • Occurs when the demand for water-dependent products and services exceeds the available supply due to drought-induced shortages
  • Examples: Reduced industrial output, increased energy costs, economic losses in tourism and recreation industries

Atmospheric Drivers of Drought

High-Pressure Systems and Reduced Precipitation

• High-pressure systems can lead to sinking air and reduced cloud formation, resulting in decreased precipitation and increased evaporation
• Subsiding air under high-pressure systems warms and dries, inhibiting the development of convective clouds and precipitation
• Examples: Persistent high-pressure ridges over drought-affected regions, such as the "Ridiculously Resilient Ridge" during California droughts

Jet Stream Patterns and Moisture Transport

• Jet stream patterns and blocking high-pressure systems can divert moisture-bearing weather systems away from drought-prone regions
• Changes in the position and strength of jet streams can alter the distribution of precipitation and contribute to drought conditions
• Examples: Blocking high-pressure systems over the North Atlantic, diverting storm tracks away from Europe and leading to drought in the Mediterranean region

Teleconnections and Global Circulation Patterns

• Teleconnections, such as El Niño and La Niña, can influence global atmospheric circulation patterns and contribute to drought conditions in certain regions
• El Niño events can lead to reduced rainfall in Southeast Asia, Australia, and parts of Africa, while increasing precipitation in the eastern Pacific and South America
• La Niña events can cause drier conditions in the southern United States, Mexico, and parts of South America
• Examples: Severe droughts in Australia during strong El Niño years, such as 1982-1983 and 1997-1998

Feedback Loops and Drought Amplification

• Positive feedback loops, such as reduced vegetation cover and soil moisture, can amplify and prolong drought conditions
• Drought-stressed vegetation has lower evapotranspiration rates, leading to reduced atmospheric moisture and further suppression of precipitation
• Dry soils have lower heat capacity and higher albedo, leading to increased surface temperatures and evaporative demand
• Examples: Dust bowl drought of the 1930s in the United States, where overgrazing and poor land management practices exacerbated drought conditions

Climate Change and Drought Frequency

• Climate change can exacerbate drought frequency and severity by altering precipitation patterns and increasing evaporation rates
• Rising global temperatures can lead to increased atmospheric moisture demand, accelerating the drying of soils and vegetation
• Changes in the timing and intensity of precipitation events can affect water availability and increase the risk of drought in some regions
• Examples: Projected increases in drought frequency and severity in the Mediterranean region, southern Africa, and parts of South America under future climate scenarios

Drought Impacts on Resources

Agricultural Productivity and Food Security

• Droughts can lead to reduced crop yields, crop failure, and decreased livestock productivity due to insufficient water availability
• Agricultural droughts can result in economic losses for farmers, reduced food supply, and increased food prices
• Examples: Severe drought in the United States Midwest in 2012, leading to significant crop losses and increased global food prices
• Droughts can threaten food security by affecting the availability, accessibility, and affordability of food, particularly in regions reliant on rainfed agriculture
• Subsistence farmers and rural communities in developing countries are particularly vulnerable to drought-induced food insecurity
• Examples: Recurring droughts in the Horn of Africa, contributing to widespread food shortages and humanitarian crises

Water Resource Management and Competition

• Reduced water availability during droughts can lead to increased competition for water resources among various sectors, such as agriculture, industry, and domestic use
• Drought-induced water scarcity can exacerbate existing water management challenges, such as overexploitation of groundwater resources and degradation of water quality
• Examples: Depletion of the Ogallala Aquifer in the United States due to excessive groundwater pumping for irrigation during drought periods
• Droughts can strain water infrastructure, such as reservoirs and distribution systems, leading to supply disruptions and conflicts among water users
• Transboundary water disputes can arise when shared water resources become scarce during drought events
• Examples: Tensions between states in the Colorado River Basin during prolonged droughts, leading to negotiations over water allocations and conservation measures

Drought Monitoring and Mitigation

Drought Monitoring Techniques and Tools

• Drought monitoring involves the continuous observation and assessment of meteorological, hydrological, and agricultural variables to detect the onset and severity of drought conditions
• Key variables monitored include precipitation, temperature, soil moisture, streamflow, and vegetation health indices
• Examples: Standardized Precipitation Index (SPI), Palmer Drought Severity Index (PDSI), and Normalized Difference Vegetation Index (NDVI)
• Remote sensing techniques, such as satellite imagery and soil moisture monitoring, can provide valuable data for drought monitoring and assessment
• Satellite-based sensors can measure land surface temperature, vegetation health, and soil moisture at regional to global scales
• Examples: NASA's Gravity Recovery and Climate Experiment (GRACE) mission for monitoring changes in groundwater storage during droughts

Drought Forecasting and Early Warning Systems

• Drought forecasting uses statistical and dynamical models to predict the likelihood and severity of future drought events based on current conditions and historical data
• Statistical models rely on historical relationships between drought indicators and climate variables, while dynamical models simulate the physical processes that influence drought development
• Examples: National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center's Seasonal Drought Outlook for the United States
• Early warning systems integrate drought monitoring and forecasting information to provide timely alerts and enable proactive decision-making for drought preparedness and response
• Effective communication and dissemination of drought information to stakeholders, including farmers, water managers, and policymakers, are crucial for implementing appropriate mitigation measures
• Examples: Famine Early Warning Systems Network (FEWS NET) for monitoring drought and food security risks in developing countries

Drought Risk Assessment and Preparedness

• Drought risk assessment and vulnerability mapping can help identify regions and communities most at risk and guide the allocation of resources for drought preparedness and resilience-building
• Risk assessment considers factors such as drought hazard, exposure, and vulnerability of different sectors and populations
• Examples: Drought Risk Atlas for the United States, developed by the National Drought Mitigation Center
• Drought preparedness measures include developing drought contingency plans, implementing water conservation strategies, and promoting sustainable land management practices
• Diversifying water supply sources, such as through rainwater harvesting and wastewater reuse, can enhance resilience to drought
• Examples: California's Sustainable Groundwater Management Act, which requires local agencies to develop plans for sustainable groundwater management in drought-prone regions