The Global Centre for Risk and Innovation (GCRI)
A catchment is the area that drains to one point: every drop of rain that falls inside it either evaporates, soaks in or runs to that outlet. Catchment hydrology is the accounting of that water through time, and it is the base on which every water decision in the basin rests.
Water NexusWireframe of the working environment, not a live screen. Names and values are placeholders.
Catchment hydrology and the complete water balance. The dossier below sets out how the risk develops, what is measured, where the data come from, how it is modelled and who decides.
Precipitation equals evapotranspiration plus runoff plus the change in storage in soils, aquifers, snow and reservoirs. The balance closes only when every term is measured or estimated with a stated error.
Water reaches rivers by overland flow when rain exceeds infiltration, by shallow subsurface flow, and by groundwater discharge that sustains dry-season flow. Land cover and soil decide the split.
A flood wave attenuates and lags as it moves downstream; channels, floodplains and reservoirs each change its shape. Travel time is what gives a warning its lead.
Dams, abstraction, drainage, urban surfaces and irrigation returns rewrite the natural balance. A model calibrated on the natural regime misreads a regulated one.
Where this page’s subject sits in the world: boundaries and context now, the field layers when the interactive map loads.
MapWorld map in the Equal Earth projection, land and boundaries from Natural Earth (public domain). The interactive map loads through the Nexus Foundry plugin (globe or flat view).
The systems this subject touches, the method at its core and the decisions it informs.
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 |
|---|---|---|---|---|
| Catchment precipitation | millimetres per month | monthly | Gauge and satellite rainfall | ESTIMATED |
| Actual evapotranspiration | millimetres per month | monthly | Satellite energy balance, land-surface model | MODELLED |
| Runoff coefficient | dimensionless | seasonal | Gauged flow against rainfall | ESTIMATED |
| Baseflow index | dimensionless | annual | Hydrograph separation | ESTIMATED |
| Flow duration curve percentiles | cubic metres per second | period of record | Stream gauges | OBSERVED |
| Water yield | millimetres per year | annual | Water balance | MODELLED |
The topic as a computational graph: named sources feed the methods, an evidence review gates the outputs, and the decisions stay with the authorities that hold them.
Fig. 02Nodes carry their type, two metric rows and a state; connectors show what feeds what; dashed frames name the owners; the timeline is the cadence. Design of the pipeline, not a record of a run.
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 |
|---|---|---|
| HydroSHEDS and HydroBASINS | Catchment boundaries and river networks derived from elevation | Free, attribution and terms |
| Copernicus DEM and SRTM | Elevation for delineation and slope | Free and open, attribution |
| CHIRPS and ERA5 | Rainfall and atmospheric forcing | Public domain; free and open |
| MODIS evapotranspiration | Satellite evapotranspiration at five hundred metres | Public domain |
| Global Runoff Data Centre | Gauged discharge for calibration | Data policy |
| Copernicus land cover and ESA WorldCover | Land cover for runoff parameters | Free and open, attribution |
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 |
|---|---|---|
| Basin authorities | How much water the basin yields in a dry year, and how that yield is shared | Water balance with stated error; flow duration curves at the points of allocation |
| Dam and irrigation operators | How inflows respond to a wet or dry season and to land-use change upstream | Calibrated rainfall-runoff model with its test-period skill |
| Urban drainage authorities | How development changes peak runoff and travel time | Runoff coefficients before and after, routing with the new surfaces |
Over any period, precipitation equals evapotranspiration plus runoff plus the change in storage, with groundwater flows across the catchment boundary added where they matter. Each term is hard to measure directly. Evapotranspiration depends on energy, vegetation and soil moisture; runoff depends on soil infiltration capacity, slope and antecedent wetness; storage changes slowly in aquifers and quickly in soils.
Climate warming shifts the balance by increasing atmospheric demand for water, changing the timing of snowmelt and altering rainfall intensity. Land change, such as urban paving or deforestation, alters how much water infiltrates and how fast it runs off.
Rain gauges, streamflow gauges, soil moisture probes and groundwater wells, complemented by Earth observation for precipitation, snow, evaporation and storage anomalies. Gauge networks are sparse in many regions and have declined in some.
Conceptual and physically based hydrologic models simulate the balance, analysed with hydrology and water science and geospatial information systems. Models calibrated to past conditions may not hold under changed land or climate.
Different models and parameter sets can fit the same record equally well, so results are reported as ranges using uncertainty quantification. Ungauged catchments rely on regionalisation, which adds uncertainty.
| Method | Applied here | Canonical page |
|---|---|---|
| Hydrology and water science | Hydrology and water science in this field | STEM Commons |
| Geospatial information systems | Geospatial information systems in this field | STEM Commons |
| Earth observation and remote sensing | Earth observation and remote sensing in this field | STEM Commons |
| Spatial statistics and geostatistics | Spatial statistics and geostatistics in this field | STEM Commons |
| Uncertainty quantification and model validation | Uncertainty quantification and model validation in this field | STEM Commons |
Most of the world’s catchments are ungauged, so parameters are transferred from gauged neighbours and carry that uncertainty. Evapotranspiration is the largest and least measured term of the balance. Regulation, abstraction and land-use change break the stationarity that calibration assumes.
The current service level is R0: information and enquiries. An enquiry receives a reference and names the next step; applications, payments, compute allocations and registrations open only when their team, terms and review capacity exist.