GCRI · Water Nexus
Drought and flood
Droughts and floods are the two ends of hydrological variability, and they increasingly occur together: a dry season that empties reservoirs and dries soils, then intense rain on hard ground that overwhelms drainage. This page sets out how the two extremes develop, how they are measured and forecast, and what the forecasts can and cannot tell the people who must act on them.
Drought and flood on the record.
Drought and flood: how the risk develops.
Compound water extremes, forecasts and limits. The dossier below sets out how the risk develops, what is measured, where the data come from, how it is modelled and who decides.
Deficit builds slowly
A rainfall deficit becomes a soil-moisture deficit, then low river flow and falling reservoirs and groundwater. Heat deepens it: higher evaporation and higher demand for irrigation and cooling at the same time.
Floods arrive fast
Flash floods from intense convective rain, river floods from sustained basin-wide rain or snowmelt, and coastal floods from surge and tide. The same storm can produce all three.
The two compound
A warmer atmosphere holds more moisture and releases it in heavier bursts. Dry, compacted or burnt ground sheds water instead of absorbing it, so a drought raises the flood risk of the rain that ends it.
Impacts cascade
Reservoir storage sets hydropower output; turbine curtailment strains the grid; pumping and treatment lose power; hospitals fall back on fuel. Each step carries its own evidence label and its own uncertainty.
Water now in the Nile Basin.
Rain, flood water and root-zone soil moisture over the Nile Basin, from NASA GIBS. Each layer carries its own date.
The interactive map needs JavaScript; the layer list and sources are below.
Layers 3
Water now, for this page
About this layer
- Evidence
- Estimated
- Source
- GPM IMERG (NASA GES DISC)
- Provider
- NASA GES DISC
- Licence
- NASA open data
- Vintage
- daily; Satellite estimate, near real time
- Newest at build
- 2026-10-12
- Resolution
- about 2 km grid
- Uncertainty
- Satellite estimate, near real time. Satellite or model raster as the provider publishes it. Clouds, swath gaps and retrieval error leave gaps and noise; values are indicators, not measurements at a place.
- Reached through
- gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
- Evidence
- Observed (clouds leave gaps)
- Source
- VIIRS Global Flood Product (NASA LANCE)
- Provider
- NASA LANCE
- Licence
- NASA open data
- Vintage
- daily; Satellite detection; clouds leave gaps
- Newest at build
- 2026-10-14
- Resolution
- about 250 m grid
- Uncertainty
- Satellite detection; clouds leave gaps. Satellite or model raster as the provider publishes it. Clouds, swath gaps and retrieval error leave gaps and noise; values are indicators, not measurements at a place.
- Reached through
- gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
- Evidence
- Modelled
- Source
- SMAP Level 4 (NASA NSIDC DAAC)
- Provider
- NASA NSIDC DAAC
- Licence
- NASA open data
- Vintage
- daily; Model with satellite data assimilated
- Newest at build
- 2026-10-11
- Resolution
- about 2 km grid
- Uncertainty
- Model with satellite data assimilated. Satellite or model raster as the provider publishes it. Clouds, swath gaps and retrieval error leave gaps and noise; values are indicators, not measurements at a place.
- Reached through
- gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
- Evidence
- Observed
- Source
- VIIRS NOAA-20 corrected reflectance (NASA LANCE)
- Provider
- NASA LANCE
- Licence
- NASA open data
- Vintage
- daily; Satellite image
- Newest at build
- 2026-10-14
- Resolution
- about 250 m grid
- Uncertainty
- Satellite image. Satellite or model raster as the provider publishes it. Clouds, swath gaps and retrieval error leave gaps and noise; values are indicators, not measurements at a place.
- Reached through
- gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
- Evidence
- Modelled (latest year GIBS publishes)
- Source
- MODIS Terra and Aqua land cover (MCD12Q1) (NASA LP DAAC)
- Provider
- NASA LP DAAC
- Licence
- NASA open data
- Vintage
- yearly; Satellite classification; latest year GIBS publishes
- Newest at build
- 2024-01-01
- Resolution
- about 500 m grid
- Uncertainty
- Satellite land cover classes for the year shown, including croplands; a classification with known errors, not a farm register.
- Reached through
- gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
- Evidence
- Estimated
- Source
- Gridded Population of the World v4.11 (NASA SEDAC (CIESIN, Columbia University))
- Provider
- NASA SEDAC (CIESIN, Columbia University)
- Licence
- CC BY 4.0
- Vintage
- null; Census-based estimate, static (2020)
- Resolution
- about 1 km grid
- Uncertainty
- Census-based estimate of people per square kilometre in 2020, shown as exposure context; not a count of people affected by any hazard.
- Reached through
- gibs.earthdata.nasa.gov, only after you allow the live map
Keyboard, with the map focused
Hold Ctrl or ⌘ and scroll to zoom
Latest available
NASA GIBS / EOSDIS · ASTER GDEM (METI, NASA) · Blue Marble (NASA) · © OpenStreetMap contributors (ODbL) · Natural Earth
Loading the latest NASA imagery.
Drag to move. Ctrl or ⌘ and scroll, pinch or the buttons to zoom; hover for the values under the pointer; reset returns to the region.
Water now, framed on the Nile Basin.Satellite and model products as NASA publishes them, each with its own date. They are indicators, not assessments of any place.
Data: NASA GIBS · ASTER GDEM · Blue Marble · © OpenStreetMap contributors · Natural Earth · NASA EONET · NASA GPM IMERG Early · NASA JPL OPERA DIST-ALERTSources and licences
- We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS). Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS). Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- GPM IMERG precipitation, NASA Precipitation Processing System; distributed by the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC); imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- VIIRS near-real-time products (flood, land surface temperature, snow cover, corrected reflectance), NASA LANCE; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- SMAP Level 4 soil moisture and snow mass, NASA Global Modeling and Assimilation Office; distributed by the NASA National Snow and Ice Data Center DAAC; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- MODIS land products (vegetation indices, gross primary production, land cover), NASA LP DAAC and LANCE; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- Center for International Earth Science Information Network (CIESIN), Columbia University. 2018. Gridded Population of the World, Version 4 (GPWv4): Population Density, Revision 11. NASA Socioeconomic Data and Applications Center (SEDAC). Licensed under CC BY 4.0. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). doi:10.7927/H49C6VHW. CC BY 4.0
- ASTER GDEM is a product of METI and NASA (ASTER Global Digital Elevation Model V003, NASA LP DAAC); imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). Cite as: NASA/METI/AIST/Japan Spacesystems and U.S./Japan ASTER Science Team (2019). ASTER Global Digital Elevation Model V003. doi:10.5067/ASTER/ASTGTM.003. NASA open data
- Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com. Public domain
- © OpenStreetMap contributors (ODbL), via NASA GIBS (coastlines, boundaries, roads and place names). Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). ODbL
- Blue Marble, NASA Earth Observatory; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
- Context on the globe and flat map: coastlines, boundaries, roads and place names from NASA GIBS reference layers (OpenStreetMap data, ODbL, © OpenStreetMap contributors); Blue Marble imagery as an optional base; 3D terrain and the elevation read-out from the ASTER GDEM elevation colour index (5 m to 200 m classes, exaggerated for legibility). Boundaries are drawn as published and imply no position on any territory.
- Live events: NASA Earth Observatory Natural Event Tracker (EONET); events are indicative and keep their original source links.
- Live: NASA EONET Events are indicative, not authority warnings.
- Live: NASA GPM IMERG Early. We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS).
- Observed (EO): NASA JPL OPERA DIST-ALERT (contains modified Copernicus Sentinel data 2026). We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS).
- NASA GIBS: We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS).
- River discharge forecast, 30 days with range (GloFAS via Open-Meteo): Contains modified Copernicus Emergency Management Service information 2026; not a flood warning.
- Past year, daily temperature (ERA5 via Open-Meteo): Contains modified Copernicus Climate Change Service information 2026.
- Past year, daily temperature and sunshine (NASA POWER): These data were obtained from the NASA Langley Research Center (LaRC) POWER Project funded through the NASA Earth Science/Applied Science Program..
- Weather data by Open-Meteo.com (https://open-meteo.com/), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
- Optional layers (14 sources in the layer list: live, forecast, observed and context) print their provider credit on the map and here when switched on; forecasts are model output and derived indices are estimates.
Layers and dates
- Precipitation rate, daily mean: GPM IMERG, NASA GES DISC · 12 Oct 2026
- Flood water, 3-day composite: VIIRS Global Flood Product, NASA LANCE · 14 Oct 2026
- Root-zone soil moisture, 0 to 100 cm: SMAP Level 4, NASA NSIDC DAAC · 11 Oct 2026
- True colour: VIIRS NOAA-20 corrected reflectance, NASA LANCE · 13 Oct 2026
- Land cover type (IGBP), yearly: MODIS Terra and Aqua land cover (MCD12Q1), NASA LP DAAC · 2024
- Population density, 2020: Population: CIESIN, Columbia University, Gridded Population of the World v4.11 (2018), NASA SEDAC, CC BY 4.0 · static
How droughts and floods reach energy, food, health and nature.
| Domain | How it couples here | What to watch |
|---|---|---|
| Energy | Low flows cut hydropower and force thermal plants to curtail when cooling water is scarce or too warm; floods inundate substations. See hydropower and water. | Reservoir storage against rule curves; river temperature at plant intakes |
| Food | Rainfall deficits at sensitive crop stages reduce yields; floods destroy standing crops and stored grain. See food system shocks. | Soil moisture anomaly during flowering; flooded cropland in hectares |
| Health | Drought concentrates contaminants and drives unsafe water use; floods spread waterborne and vector-borne disease. See environmental health. | Diarrhoeal disease and cholera alerts after floods |
| Biodiversity | Wetlands and floodplains buffer both extremes, and lose that capacity when drained or disconnected. See wetlands and freshwater. | Floodplain connectivity; wetland extent |
Basin stress monitor.
The screen a team would use to follow this subject: its measures, states and decisions. Names are examples and values are illustrative.
Where drought and flood evidence is designed to go, and how to add to it.
Drought and flood analyses are designed for the Country Systems Risk and Resilience series, and verification of drought indicators and flood forecasts for the Methods and Technical series. Under the proposed Nexus Standards, each indicator series (a Standardized Precipitation Index for a basin, a flood extent for an event) would carry an NXS-GRX (canonical data, provenance and risk) record identifier under NS-3 so that a figure in a report can be traced to its record. Warnings remain with national hydrometeorological services; the Global Centre for Risk and Innovation (GCRI) issues none.
- Digitised historical gauge or rainfall records that lengthen a short series.
- Post-event flood marks and dates surveyed on the ground, for validating satellite extents.
- Drought impact records from farms, utilities or clinics, so indices can be tested against what happened.
Route a storm through a river network.
Rain becomes runoff by the curve number method, drains each sub-catchment through a linear reservoir and travels down the main stem by Muskingum routing; play it to watch the flood wave move and flatten. Teaching model with stated illustrative values, not a prediction. The map on this page shows The Nile Basin; the model itself is not tied to a place.
Controls
- Storm rainfall depth
- 120 mm
- Storm duration
- 12 h
- Curve number (soil and land cover)
- 78
- Catchment area
- 800 km²
- Reaches on the main stem
- 5
- Travel time per reach (Muskingum K)
- 6.0 h
- Muskingum X (storage weighting)
- 0.20
- Channel capacity per unit area
- 300 L/s per km²
Readouts
- Peak discharge at the outlet
- 452 m³/s
- Time to peak
- 21.6 h
- Attenuation by the channel
- 61%
- Hours over capacity
- 26.4 h
- Runoff coefficient
- 52%
Numbers, methods and sources
| Hour | Outlet (m³/s) | Unrouted (m³/s) | Upper reach (m³/s) |
|---|---|---|---|
| 2.4 | 0.0 | 678.9 | 0.0 |
| 12.0 | 308.3 | 1,151.4 | 181.8 |
| 21.6 | 452.4 | 35.1 | 73.7 |
| 31.2 | 363.3 | 1.1 | 11.1 |
| 40.8 | 198.8 | 0.0 | 1.5 |
| 50.4 | 79.3 | 0.0 | 0.2 |
| 60.0 | 25.4 | 0.0 | 0.0 |
| 69.6 | 6.9 | 0.0 | 0.0 |
| 79.2 | 1.7 | 0.0 | 0.0 |
| 88.8 | 0.4 | 0.0 | 0.0 |
| 98.4 | 0.1 | 0.0 | 0.0 |
| 108.0 | 0.0 | 0.0 | 0.0 |
| 117.6 | 0.0 | 0.0 | 0.0 |
| 127.2 | 0.0 | 0.0 | 0.0 |
| 136.8 | 0.0 | 0.0 | 0.0 |
Methods
- Curve number runoff USDA Soil Conservation Service (1972), National Engineering Handbook, Section 4: Hydrology
- Linear reservoir Nash (1957), The form of the instantaneous unit hydrograph, IAHS 45
- Muskingum channel routing McCarthy (1938); Cunge (1969), Journal of Hydraulic Research 7
- Uncertainty fan (parameter Monte Carlo) Spiegelhalter, Pearson and Short (2011), Visualising uncertainty about the future, Science 333
- One-at-a-time sensitivity (tornado) Saltelli and others (2008), Global Sensitivity Analysis: The Primer, Wiley
Data
The model runs on the teaching values set by the controls. To calibrate it for a place, read it against these admitted layers and series, which this page’s map can draw:
- Rivers Natural Earth
- Lakes and reservoirs Natural Earth
- Cities Natural Earth
- Rain rate, last half hour (IMERG) NASA GPM IMERG Early · register nasa.gibs.context-live-layers
- River discharge forecast, 30 days with range (GloFAS via Open-Meteo) Open-Meteo.com (CC BY 4.0), Copernicus GloFAS · register open-meteo.public-pages
- Surface soil moisture (SMAP) NASA SMAP · register nasa.gibs.context-live-layers
- Surface water, 30 m (OPERA DSWx-HLS) NASA JPL OPERA DSWx-HLS (contains modified Copernicus Sentinel data 2026) · register nasa.gibs.context-live-layers
- Surface water through cloud, 30 m (OPERA DSWx-S1 radar) NASA JPL OPERA DSWx-S1 (contains modified Copernicus Sentinel-1 data 2026) · register nasa.gibs.context-live-layers
Across the six pillars, led by Nexus Consortium
- Experiments Labs
- Built in Water Nexus in Foundry
- Tested at Water Nexus in Universe
- Codified in Water and Food Systems in Standards
- Published in Water Nexus in Reports
- Taught in Faculty of Earth and Environmental Sciences in Academy
How the shock moves through the field.
Select a scenario. Real geography, example values: the cells show a design of how stress spreads, not a measurement.
Fig. 01Drought and flood on a reference basin with real elevation, coastlines, lakes and rivers. Cell levels are examples, not results; a national programme replaces them with its own labelled evidence.
Before a shock
The reference basin in a normal season: storage within rule curves, flows near the median, no indicator in deficit.
Stress builds inland
Deficit indicators agree inland first; hydropower and treatment feel it weeks later. Each step keeps its own label and confidence.
The coast and the lake plain take the second shock
Intense rain on dry ground overwhelms drainage and inundates treatment works and pumping stations. The same model shows where the two shocks compound.
Levels in the figure are examples in a design, not findings. A programme replaces them with labelled evidence it owns.
Model drought and flood as a system. Run the cascade.
Assembled from the Nexus Standards area profile Water risk standards: the hazards it names, the systems and sectors that carry the loss, the controls that dampen it and the data that observe it. Choose the components, pick an initiating hazard and step through the cascade. The Map view places the run on an example geography with open data. Every coupling weight is a stated assumption you can change. A teaching model, not a forecast, an assessment or a certification. To model a place of your own choosing, open the full System Builder.
Hazards
- Flash flood
- Meteorological drought
- Groundwater depletion
- Dam failure
- Debris flow (set off downstream)
- Agricultural drought (set off downstream)
- Hydrological drought (set off downstream)
- Wildfire (set off downstream)
- Peat and underground fire (set off downstream)
- Forest dieback (set off downstream)
- Water supply shortfall (set off downstream)
- Acute food crisis (set off downstream)
- Dust storm (set off downstream)
- Land subsidence (set off downstream)
- Saltwater intrusion (set off downstream)
- River flood (set off downstream)
- Mass displacement (set off downstream)
- Amazon rainforest dieback (set off downstream)
Systems and couplings
- Water risk standards
- Energy risk standards Common driver and cascade
- Food and agriculture risk standards Cascade
- Health risk standards Cascade
- Hydrometeorological hazard standards Shared hazard records
Controls
NS-0Constitution and principlesNS-1The UNI and the artefact catalogue (grammar v0.2: domain, sector, region and environment segments)NS-3Risk record UNIs (grammar, legacy crosswalk, allocation authority)NS-3bDataset and product metadata profile (STAC, DCAT, PROV-O)NS-9Contextual UNIs (deployments, instances, programme instances)NS-10Policies and clauses (machine-readable obligations: definition, binding, evaluation and enforcement)NS-11Verification and monitoring (evidence, attestations, zero-trust conformance observation)NRS-U28Universe platform profile: Water NexusNRS-H61Hazard family: Cascading failure (NHS-1 sys.cascade)NRS-K42Domain pack profile: Cross border basins (DOMAIN-042)NRS-N1Water risk and resilienceNRS-N7Water-energy-food-health-biodiversity-infrastructure nexus couplingNRS-H62Hazard family: Compound and multi-hazard events (NHS-1 sys.compound)
Observation
- USGS Water Data APIs (listed for reference)
- GloFAS global flood forecasts and reanalysis (listed for reference)
- GRACE/GRACE-FO terrestrial water storage mascons (listed for reference)
- Aqueduct 4.0 Water Risk Atlas (listed for reference)
- CHIRPS quasi-global gridded precipitation (listed for reference)
- Level of water stress: freshwater withdrawal as a proportion of available freshwater resources
- Proportion of transboundary basin area with an operational arrangement for water cooperation
- Proportion of population using safely managed drinking water services
- Number of disruptions to basic services attributed to disasters
Example geography
- The Ganges and Brahmaputra Basin: 178 hexagonal cells
- Surface water through cloud, 30 m (OPERA DSWx-S1 radar), NASA GIBS
- Rain rate, last half hour (IMERG), NASA GIBS
- Surface soil moisture (SMAP), NASA GIBS
- Night lights tonight, outages (VIIRS Day/Night Band), NASA GIBS
What is measured, and how well.
Each indicator carries its unit, its cadence, the kind of source behind it and the evidence label a value would carry. No values are shown: a programme supplies them under its own authority.
| Indicator | Unit | Cadence | Source type | Label |
|---|---|---|---|---|
| Standardised precipitation index | dimensionless (3, 6, 12 month) | monthly | Gauge and satellite rainfall | ESTIMATED |
| Soil moisture anomaly | percent of climatology | weekly | Satellite radiometry, land-surface model | MODELLED |
| Streamflow percentile | percent of period of record | daily | Stream gauges | OBSERVED |
| Reservoir storage | percent of live capacity | daily | Operator records | Published statistics |
| Groundwater level anomaly | metres below mean | monthly | Observation wells, satellite gravimetry | OBSERVED |
| Flood return period of peak flow | years | per event | Flood frequency analysis | ESTIMATED |
| Inundated area | square kilometres | per event | Synthetic-aperture radar | OBSERVED |
| Population in the flood extent | persons | per event | Gridded population on the flood layer | ESTIMATED |
Coupled risk, applied response.
The sectors drought touches and how strongly they depend on each other: what a study has to observe and model together.
Choose a context
Sectors · DroughtStress
Pillars · DroughtKPI
In this contextDependence or handoff, heavier is strongerOutside this context
Programme yearAll phases
Context
Drought
Low river flows cut irrigation, cooling water and hydropower at once; food prices, nutrition and ecosystems follow.
W · E · I · F · H · B
Risk
- Sectors
- 5 of 6
- Relations
- 10 of 15
- Coupling
- 0.68 share of total
Hover a corner, a line or a row to trace it across both sides.
Route
Applied to drought
L · F · U · S · R · A
Response
- Pillars
- 6 of 6
- Handoffs
- 18 of 18
- Loops
- 17 closed
Expected value
- What is at stake
- –no published benchmark
- Return on resilience
- $7.1Tnet benefitsGlobal Commission on Adaptation, Adapt Now, 2019
- Avoided loss
- –no published benchmark
Hover a pillar, a line or a row to read how it is applied.
A: a teaching cascade (shock 0.6, gain 0.35, four rounds) over teaching values for a typical city-region, not measurements. B: illustrative applications and the KPI each pillar answers for; values are published benchmarks, cited, not forecasts. Open events: NASA EONET, live.
Where the evidence comes from.
Named public datasets and institutional records, each with its licence class. Sources by agreement are used only under their terms and never republished.
| Source | What it gives | Licence class |
|---|---|---|
| CHIRPS rainfall | Daily and monthly gridded rainfall since 1981, gauge-blended satellite estimates | Public domain |
| ERA5 reanalysis (Copernicus) | Hourly atmospheric and land variables, global, 1940 to present | Free and open, attribution |
| Global Runoff Data Centre | Daily river discharge from national gauge networks | Free for research and public tasks under the data policy |
| GRACE and GRACE-FO | Monthly terrestrial water storage anomalies from satellite gravimetry | Public domain |
| JRC Global Surface Water | Surface-water occurrence, change and seasonality from Landsat, 1984 onwards | Free and open, attribution |
| Sentinel-1 radar | All-weather flood extent mapping at ten-metre resolution | Free and open, attribution |
| National hydrological services | Gauge records, reservoir rule curves, warnings | By agreement with each service |
How drought and flood is modelled, measured and tested.
The model, its assumptions, its data and its uncertainty, in the form a scientist or engineer would check. Structures follow published methods. The values drawn are illustrative, not findings.
System model
The couplings between the systems in drought and flood: signs of influence, a reinforcing and a balancing loop, and cross-links where stress jumps.
Who decides, and what the evidence serves.
Water Nexus prepares evidence. The decisions below are taken by the authorities named, under their own rules, and never by the platform.
| Who decides | The question | Evidence this page serves |
|---|---|---|
| Water allocation authority | When to move from voluntary to mandatory restrictions, and in which sectors | Indicator agreement across SPI, storage and groundwater; outlook skill for the next season |
| Grid and hydropower operators | How much hydropower to schedule and when to secure replacement capacity | Reservoir inflow forecasts with confidence bands; the cascade to generation |
| Disaster management agencies | When to warn, pre-position and evacuate, and for which areas | Flood forecasts by reach, the mapped extent of comparable past events, exposure in the extent |
| Finance ministries and insurers | What a one-in-fifty and a one-in-two-hundred year loss looks like, and who carries it | Return-period losses with retained, transferred and uninsured layers kept apart |
Outputs this topic produces
- Drought monitor with indicator agreement
- Seasonal and sub-seasonal outlooks with skill stated
- Flood hazard maps by return period
- Compound-event scenarios for the coupled model
What experts ask about extremes.
- How likely is an event of a given size, and is that likelihood changing?
- When do several hazards combine, such as heavy rain on saturated ground, a storm surge with river flooding, or drought with heat?
- How far ahead can an event be forecast, and with what confidence?
- Which people, services and assets are exposed and how vulnerable are they?
How extremes develop and combine.
Drought develops slowly: a lack of rainfall becomes a soil moisture deficit, then low river flows and falling reservoirs and groundwater. Higher temperatures intensify it by increasing evaporation. Floods can be fast, as in flash floods from intense convective rain, or slow, as in large river floods driven by prolonged rain or snowmelt.
A warmer atmosphere holds more moisture, which tends to increase heavy rainfall intensity. Compound events matter because their joint probability is higher than independent hazards would suggest, and because they defeat defences designed for one hazard at a time. A drought can harden soils and kill vegetation, increasing runoff when rain returns.
Forecasts, statistics and their limits.
- 01
Frequency analysis
extreme value analysis estimates return periods from records. Short or non-stationary records make rare-event estimates uncertain.
- 02
Forecasting
Ensemble weather and hydrological forecasts, with forecasting and time-series analysis, give probabilities rather than certainties. Skill falls with lead time and is lower for convective rainfall.
- 03
Exposure mapping
Flood and drought maps built with geospatial information systems and Earth observation. Maps reflect their input data and defences as assumed, not necessarily as built.
How it is studied, and what the methods cannot say.
- 01Hydrology and water scienceRainfall-runoff modelling of a basin · Water balance accountingApplied here
- 02Forecasting and time-series analysisFlood forecasting by reach · Seasonal drought outlooks · Inflow forecasts for reservoirsApplied here
- 03Extreme value theory and tail riskReturn-period flows and rainfall · Regional frequency analysisApplied here
- 04Earth observation and remote sensingFlood extent from radar · Surface water and reservoir monitoringApplied here
- 05Uncertainty quantification and model validationEnsemble and parameter uncertainty · Propagation through the cascade · Evidence labellingApplied here
Where this page’s references lead.
The bodies and documents this page is read against. Status and editions change; each link goes to the issuing body.
- Standardized Precipitation Index user guide, WMO-No. 1090 (WMO, 2012)
- National Drought Management Policy Guidelines: a template for action (WMO and GWP Integrated Drought Management Programme, 2014)
- Global Flood Awareness System (Copernicus Emergency Management Service)
- Global Drought Observatory (Copernicus Emergency Management Service)
- IPCC Sixth Assessment Report, Working Group I, chapters 8 and 11 on the water cycle and extremes (IPCC, 2021)
- NS-3, NXS-GRX record UNIs, a proposed standard
Where this page leads across the Consortium.
Questions and answers.
What does a one-in-a-hundred year flood mean?
Why do forecasts give probabilities?
Why are compound events hard to estimate?
Does Water Nexus issue warnings?
Which values on this page are real?
How is drought measured?
Is climate change making droughts and floods worse?
What does a one-in-a-hundred-year flood mean?
Contribute evidence from this page.
Reports is the Consortium’s quarterly publication. What you know about this subject can become evidence, a case or a correction in the next edition, credited to you and published under CC BY 4.0 where it is your own work.
- EmailGateway GW-GCRISend it from this pageEvidence, a correction, a case, a method, a review, a translation or profile input. It arrives tagged with this page, so the editor who owns it receives it.
- ZenodoOpen depositDeposit it and submit to the communityPublish your own record on Zenodo with your ORCID iD, then submit it to the Reports community there. Curators review it; the DOI stays yours.
- ReportsGCRISee every way to contributeThe contribution types, how each is reviewed, what credit you receive, the licence and what is never accepted.
Never send personal data, sensitive infrastructure detail or anything you have no right to share. A contribution is reviewed before use; sending it confers no authorship or editorial rights, and sponsorship never buys a finding.
Bring a method, a dataset or a question.
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