How drought works
Drought is not one thing. It is four, they arrive in sequence, and confusing them is why "the rain has ended the drought" is so often wrong.
A drought is a water deficit relative to normal conditions for that place. The word "normal" is doing a lot of work: a summer that would be a severe drought in Scotland is an ordinary year in Spain. Drought is defined against local expectation, not against an absolute threshold.
The four droughts
- Meteorological drought
- A rainfall deficit. Develops within weeks and is the first stage. Measured with indices such as the Standardised Precipitation Index.
- Agricultural drought
- A soil moisture deficit sufficient to stress crops. Develops within weeks to months, and depends on evaporative demand as well as rainfall.
- Hydrological drought
- Reduced river flow and reservoir storage. Develops over months, and is what triggers supply restrictions.
- Groundwater drought
- Depleted aquifer levels. Develops over months to years and recovers slowest of all — sometimes taking several wet winters.
The stages arrive in that order and recover in that order. A wet month can end a meteorological drought while reservoirs remain low and groundwater is still falling. This is why declarations that rainfall has "ended the drought" are frequently premature — they usually refer to the first stage only.
How drought is measured
| Indicator | What it measures | Drought stage |
|---|---|---|
| Standardised Precipitation Index (SPI) | How unusual accumulated rainfall is, in standard deviations | Meteorological |
| SPEI | As SPI, but subtracting potential evapotranspiration | Meteorological and agricultural |
| Soil moisture deficit | Depth of water needed to return soil to field capacity | Agricultural |
| Standardised Streamflow Index | How unusual river flow is against its own record | Hydrological |
| Reservoir storage percentage | Storage against usable capacity | Hydrological |
| Groundwater level percentile | Borehole level against its own historic range | Groundwater |
Each index is defined against a specific place’s own history. An SPI of −2 means the same rarity anywhere in the world, which is what makes it comparable between climates.
Why evaporative demand matters as much as rainfall
A drought is a balance, not just a shortfall. Rainfall is the input; evapotranspiration is the loss. A hot summer increases atmospheric demand for water, so the same rainfall deficit produces a much deeper soil moisture deficit at 30 °C than at 18 °C.
This is why the 2022 European drought was so severe: the precipitation deficit was substantial but not unprecedented, and repeated heatwaves converted it into an exceptional soil moisture and river flow deficit. Indices that account for both — SPEI rather than SPI — capture this and simple rainfall measures do not.
Restrictions and what they mean
- Voluntary appealsRequests to reduce discretionary use. Usually the first response, and generally the least effective in isolation.
- Temporary Use Ban (hosepipe ban)Prohibits specified household uses such as watering gardens with a hosepipe or filling paddling pools. Applied by the water company.Household discretionary use is a small share of total demand, so a hosepipe ban is partly a demand measure and partly a signal.
- Non-essential use banExtends restrictions to commercial uses such as vehicle washing businesses and some industrial processes.
- Drought order or permitRegulatory permission to abstract more than the licence allows, or to reduce compensation flow releases. Has direct environmental consequences and is granted reluctantly.
- Emergency measuresStandpipes and rota cuts. In the UK these have not been used since 1976, and avoiding them is the central objective of drought planning.
The Cape Town case
Cape Town’s 2015–2018 drought is the clearest modern demonstration that demand management works at scale. Facing a named "Day Zero" on which municipal supply would be shut off, the city cut consumption by more than half — restricting personal use to 50 litres per person per day, changing tariffs sharply, managing pressure, and publishing a map showing household-level consumption.
Day Zero was never reached. The episode also demonstrated the harder problem: sustaining low consumption after the rains return, when the political will that made restrictions acceptable evaporates faster than the reservoirs refill.
Sources
- Copernicus Emergency Management Service — European and Global Drought Observatory. Open — attribution required · Copernicus licence
- UK Centre for Ecology and Hydrology — UK Hydrological Outlook and Monthly Hydrological Summary. Open — attribution required · Open Government Licence v3.0
- National Drought Mitigation Center, University of Nebraska–Lincoln — United States Drought Monitor. Open — attribution required · CC BY 4.0