Drought causes more economic damage than any other natural hazard, affecting agriculture, water supply, energy production, and ecosystems worldwide. Yet drought risk assessment remains one of the harder climate hazards to quantify because drought develops slowly, varies by type, and depends heavily on regional precipitation patterns.
A drought risk assessment evaluates how likely a location is to experience extended dry periods and how severe those periods may become under future climate conditions. Unlike acute hazards such as floods or storms, drought is a chronic risk that builds over weeks or months, making early identification through systematic assessment essential for planning.
Below, you will learn the types of drought, how drought risk is measured and scored, what data sources drive the assessment, and how climate change is shifting drought patterns globally.
What Is Drought Risk Assessment?
Drought risk assessment is the process of evaluating a location’s exposure to prolonged precipitation deficits and the resulting impacts on water availability, agriculture, and operations. The assessment combines historical climate data with future projections to estimate how many months per year a location may experience drought conditions under different climate scenarios.
For organizations, drought risk assessment answers practical questions: Will our manufacturing sites face water supply disruptions? Are our agricultural supply chains exposed to crop failure risk? How does water stress at our key locations change under future climate scenarios?
The core metric in drought risk assessment is the number of severe drought months per year. A location experiencing less than 1 drought month per year faces low risk, while locations projected to experience 4 to 6 drought months face severe risk. These thresholds are calibrated against the Standardized Precipitation Index (SPI), the most widely used drought measurement standard.
Types of Drought
Drought is not a single phenomenon. It occurs in several forms, each affecting different systems and measured by different indicators. A comprehensive drought risk assessment considers all relevant types.
Meteorological drought. Defined by a sustained deficit in precipitation relative to historical norms. Meteorological drought is the starting point for all other drought types. It is measured using the Standardized Precipitation Index (SPI), which compares observed precipitation against the long-term statistical distribution. An SPI value below -1.5 indicates severe drought conditions.
Hydrological drought. Occurs when reduced precipitation leads to depleted rivers, reservoirs, and groundwater levels. Hydrological drought typically lags behind meteorological drought by weeks or months as surface and subsurface water storage slowly depletes. It directly affects municipal water supply, hydropower generation, and industrial water users.
Agricultural drought. Results from soil moisture deficits that stress crops and reduce yields. Agricultural drought can occur even during periods of normal rainfall if temperatures are unusually high (increasing evapotranspiration) or if rainfall timing does not align with crop growth stages. It is the most economically damaging drought type in many regions.
Socioeconomic drought. Occurs when water supply can no longer meet demand for economic goods. This fourth type reflects the human dimension of drought and depends on water infrastructure, population density, and water management practices as much as on climate conditions.
How to Measure Drought Risk
Drought risk assessment relies on precipitation data from climate models, processed through standardized indices to produce comparable risk ratings across locations.
Standardized Precipitation Index (SPI). The SPI is the most widely accepted drought measurement tool, recommended by the World Meteorological Organization. It calculates how far observed precipitation deviates from the long-term mean for a given time period. An SPI below -1.0 indicates moderate drought; below -1.5 indicates severe drought; below -2.0 indicates extreme drought.
CMIP6 precipitation projections. Future drought risk is assessed using precipitation projections from CMIP6 climate models. By comparing projected future precipitation against the historical baseline, the assessment estimates how drought frequency and severity may change. Each 10% decrease in annual precipitation corresponds to approximately one additional drought month per year.
Risk thresholds. Drought risk assessment translates projected drought months into standardized risk ratings:
| Risk Rating | Drought Months per Year |
|---|---|
| Low | Less than 1 |
| Moderate | 1 to 2 |
| High | 2 to 4 |
| Severe | 4 to 6 |
| Extreme | More than 6 |
How Climate Change Affects Drought Risk
Climate change is altering drought patterns in complex ways. While global average precipitation is expected to increase slightly as warmer air holds more moisture, this increase is unevenly distributed. Many subtropical and mid-latitude regions are projected to become drier, while high-latitude and some tropical regions may see more precipitation.
Higher temperatures also intensify drought through increased evapotranspiration. Even in regions where precipitation stays constant, warmer temperatures dry out soils and vegetation faster, effectively creating drought conditions from the demand side rather than the supply side.
The NOAA Drought Monitor and IPCC AR6 both document expanding drought risk in the Mediterranean, southern Africa, southwestern North America, and parts of South America. Under high-emissions scenarios (SSP5-8.5), some of these regions face a doubling of drought months by 2050 compared to the historical baseline.
For organizations with agricultural supply chains or water-dependent operations in these regions, drought risk assessment provides the data needed to prioritize adaptation investments and diversify sourcing strategies.
Drought Risk Assessment Data Sources
Effective drought risk assessment draws on multiple data sources. The primary sources include:
NASA NEX-GDDP-CMIP6. Provides downscaled daily precipitation projections at 25 km resolution from over 30 climate models. The precipitation variable (pr) is the primary input for calculating future drought months.
WRI Aqueduct 4.0. Provides basin-level water stress scores and drought risk indicators. Aqueduct data complements CMIP6 by adding demand-side water risk (how much water is being withdrawn relative to supply) alongside the supply-side climate projections.
NOAA U.S. Drought Monitor. Provides real-time drought classification for the United States, updated weekly. Useful for validating model projections against observed conditions and for understanding current exposure.
Together, these sources enable drought risk assessment that accounts for both the climate signal (will precipitation decrease?) and the water management context (is demand already high relative to supply?).
Frequently Asked Questions
What is the risk assessment of drought?
Drought risk assessment evaluates how likely a location is to experience extended dry periods based on historical precipitation data and future climate projections. It uses the Standardized Precipitation Index (SPI) to measure drought severity and translates projected drought months per year into risk ratings from Low (less than 1 month) to Extreme (more than 6 months).
What are the 4 types of drought?
The four types of drought are meteorological (precipitation deficit), hydrological (depleted rivers, reservoirs, and groundwater), agricultural (soil moisture deficit affecting crops), and socioeconomic (water supply cannot meet demand). Each type has different indicators and affects different sectors, but meteorological drought typically triggers the other three.
How to measure drought?
Drought is most commonly measured using the Standardized Precipitation Index (SPI), which compares observed precipitation against long-term averages. An SPI below -1.0 indicates moderate drought, below -1.5 is severe, and below -2.0 is extreme. Other measurement tools include the Palmer Drought Severity Index (PDSI) and soil moisture monitoring systems.
How does climate change affect drought?
Climate change affects drought through two mechanisms: shifting precipitation patterns (making some regions drier) and increasing evapotranspiration (warmer air dries out soil faster). Subtropical and mid-latitude regions are projected to face more frequent and severe drought under future climate scenarios, with some areas seeing drought months double by 2050 under high-emissions pathways.
Conclusion
Drought risk assessment combines precipitation projections from CMIP6 climate models with water stress data to evaluate how drought exposure may change at any location. Understanding the four drought types, the SPI measurement framework, and the risk thresholds helps organizations interpret their drought risk assessment results and prioritize locations where water supply disruptions are most likely to intensify under future climate conditions.
