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The planting of trees on land that has never had forest, or has been without forest for a long time.
Agricultural drought
A rainfall deficiency from meteorological drought that leads to deficiency of soil moisture and soil water availability, which has a knock on effect on plant growth and reduces biomass.
A permeable or porous rock which stores water.
Base flow
The normal, day-to-day discharge of the river.
Blue water
Water stored in rivers, streams, lakes and groundwater in liquid form (the visible part of the hydrological cycle).
The area of land drained by a river and its tributaries.
Closed system
A sequence of linked processes with a transfer of energy but not matter between the parts of the system (the inputs and outputs happen within the system). An example is the global hydrological cycle.
The change from a gas to a liquid, such as when water vapour changes into water droplets.
Convectional rainfall
Often associated with intense thunderstorms, which occur widely in areas with ground heating such as the tropics and continental interiors.
Areas of the Earth where water is frozen into snow or ice.
Cyclonic rainfall
A period or sustained, moderately intensive rainfall; it is associated with the passage of depressions.
The cutting down and removal of all or most trees in a forested area.
The process of converting saltwater to freshwater suitable for human consumption and industry.
Land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities.
Dew point
The temperature at which dew forms; it is a measure of atmospheric moisture.
Drainage basin
The catchment area from which a river system obtains its supply of water.
The definition varies internationally. According to the UN, drought is an extended period (a season, a year or several years) of deficient rainfall relative to the statistical average for a region measured over a very long period of time.
Economic water scarcity
Occurs when water resources are available but there is insufficient human, institutional and financial capital to access the water in order to meet demand.
ENSO cycle
Involves the movement of a mass of very warm water in the equatorial Pacific due to changes in the surface trade winds, atmospheric circulation and ocean currents. There are two phases- El Niño (warm water to the east) and La Niña (warm water to the extreme west).
Excessive richness of nutrients in a lake or other body of water, frequently due to run-off from farming land, which causes a dense growth of plant life and death of animal life from a lack of oxygen.
The change in state of water from a liquid to a gas.
The combined effect of evaporation and transpiration.
Falling or recessional limb
The part of a storm hydrograph in which the discharge starts to decrease and return to base flow.
Famine drought
A humanitarian crisis in which the widespread failure of agricultural systems leads to food shortages and famines with severe social, economic and environmental impacts.
Flash flooding
A flood with an exceptionally short lag time- often minutes or hours.
Fossil water
Ancient, deep groundwater from former pluvial (wetter) periods.
Frontal rainfall
Occurs frequently in mid-latitudes when a warm tropical air mass meets a cooler polar air mass. The warmer air is less dense and rises over the colder air, which causes the warm air to cool, leading to condensation of water vapour, clouds of different types and precipitation.
Green water
Water stored in the soil and vegetation (the invisible part of the hydrological cycle).
Groundwater flooding
Flooding that occurs after the ground has become saturated from prolonged heavy rainfall.
Groundwater flow
The slow transfer of percolated water underground through pervious or porous rocks.
Hard engineering
The use of man-made, artificial structures to manage flooding or water supply.
Hydrological drought
Associated with reduced stream flow and groundwater levels, which decrease because of reduced inputs of precipitation and continued high rates of evaporation. It results in reduced storage in lakes and reservoirs, often with marked salinization and poorer water quality.
The movement of water from the ground surface into the soil.
Infiltration capacity
The maximum rate at which rain can be absorbed by a soil.
Integrated water resource management (IWRM)
A process which promotes the co-ordinated development and management of water, land and related resources in order to maximise economic and social welfare in an equitable manner without compromising the sustainability of vital ecosystems.
Inter-tropical convergence zone (ITCZ)
A concentration of warm air that produces rainfall as part of a global circulation system (the Hadley cell). It moved north and south across the equator seasonally. Small shifts in location can cause drought.
Meteorological drought
Defined by shortfalls in precipitation as a result of short-term variability, or longer-term trends, which decrease the duration of the dry period.
A seasonal change in the direction of prevailing winds of a region, causing wet and dry seasons in many sub-tropical areas.
Orographic rainfall
Concentrated on the windward slopes and summits of mountains.
Peak discharge
The time when the river reaches its highest flow.
The transfer of water from the surface or from the soil into the bedrock beneath.
Potential evaporation (PEVT)
The water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation.
The movement of water in any form from the atmosphere to the ground.
The physical mechanisms that drive the flux of material between stores.
Residence time
The average time a water molecule will spend in a reservoir or store.
Rising limb
The part of a storm hydrograph in which the discharge starts to rise.
River regime
The annual variation in discharge or flow of a river at a particular point or gauging station, usually measured in cumecs.
Saltwater encroachment
The movement of saltwater into freshwater aquifers due to sea level rise, storm surges and/or human abstraction of groundwater which lowers the water table.
Reservoirs where water is held, such as the oceans.
Storm hydrograph
Shows changes in a river’s discharge at a given point on a river over a short period of time (usually before, during and after a storm).
Surface run-off
The movement of water that is unconfined by a channel across the surface of the ground. Also known as overland flow.
Surface water flooding
Flooding that occurs when intense rainfall has insufficient time to infiltrate the soil, so flows overland.
Systems approach
Systems approaches study hydrological phenomena by looking at the balance of inputs and outputs, and how water is moved between stores and flows.
Technological fix
A human innovation using technology to solve a problem such as water supply issues.
Thermohaline circulation
The global system of surface and deep water ocean currents is driven by temperature (thermos) and salinity (haline) differences between areas of oceans.
This is when the rainfall persists or is relatively intense, and the water drops from the leaves, twigs, needles etc.
Water moving sideways through the soil, downslope under the influence of gravity.
Transboundary water
A water resource, including rivers, lakes and aquifers, that occupies a territory shared by more than one state.
The diffusion of water from vegetation into the atmosphere involving a change from a gas to a liquid.
An agreement signed between states, recognised under international law.
Water budget
The annual balance between inputs (precipitation) and outputs (evapotranspiration and channel flow) at a place.
Water conservation
Strategies to reduce water usage and demand.
Water insecurity
Occurs when the economic, social and environmental requirements for water supplies are not met.
Water recycling
The treatment and purification of waste water so that it is clean and safe to be re-used for industrial or domestic purposes.
Water scarcity
Occurs when renewable water resources are only between 500 and 1,000m3 per capita per year.
Water security
The capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining human wellbeing, and socio-economic development, protection against water-born pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability (UN definition).
Water stress
When renewable water resources are only between 1,000 and 1,700m3 per capita per year.
Water transfer
Hard engineering projects, such as pipelines or aqueducts, that divert water from drainage basins with surplus water to those with shortages.
An area of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing fresh, brackish or salt.
The highland which divides and separates waters flowing into different rivers.