A glossary of hydrological and related terms

There are quite a few good glossaries of hydrological terms already ‘out there’ and ready for you to use. The NIWA one is nicely written but doesn’t cover many terms. Two USGS ones (a, b) look very comprehensive, but don’t cover all the terms in use in the UK. There must be some hydrological vocabulary which is particular to Scotland too.

So here’s my own. Prepared initially in some haste soon after a SEPA training event for flood forecasting staff, I naturally hope this will be of value to students in Dundee and even beyond. It will be even better with your suggestions – so you are very welcome to send in.

annual maximum floodThe largest instantaneous flow in any water year.  The standard water year in the UK is 1 October – 30 September.
antecedent conditionsThe state of catchment wetness prior to a rainfall event and its associated rise in a river.
aquiferA geological unit which holds and releases water – this may be important for sustaining watercourses in dry weather and might also be valuable for a private water supply.
baseflowThe flow of a stream or river which continues well after inputs from rainfall or snowmelt cease.  See also BFI.
BFIBase Flow Index.  Calculated as a fraction of unity: baseflow as a fraction of total flow over an extended period, normally a number of years.
convectionOne of the three main causes of atmospheric uplift which in turn leads to condensation and precipitation.  Convective uplift arises due to localised energy transfer and is associated with localised precipitation (< 5km across at any one time) which tends to be intense (often >20 mm/h) and short-lived – normally the most damaging falls are finished within 1 hour.  Where a convective cell keeps on producing precipitation, it may leave a trail of precipitation accumulations which may extend for 100 km or more.  An example would be the rainfall of 11-12 August 2020 in Scotland.  Convection also may be an important feature of a band of frontal precipitation.
cumecCubic metre per second (m3/s) – abbreviation.  Note 1 cumec is the same as 1000 litres per second.
cusecCubic foot per second. Rarely used in UK hydrology, but still commonly in use in the US.
dischargeThe rate of flow of a watercourse.  Think of the volume of water passing every second under a tape measure across a river.  Means the same as flow.  Normally measured in cumecs.
evaporationLoss of water from catchment surfaces (leaves, ground) to the atmosphere.  Requires energy, water at the surface and unsaturated air to occur.  Strictly, transpiration is a special case of evaporation, but hydrologists try to be clear about exactly which losses of water are being referred to.
evapotranspirationThe sum of evaporation + transpiration.
field capacityThe state of water stored in a catchment after rapid drainage has ceased.
flowSee discharge.
frontal precipitationPrecipitation caused by uplift at a weather front.  Fronts occur within a depression of cyclonic weather system.  Cold, warm and occluded types are recognised, each giving rise to precipitation in its own situation.  Frontal precipitation is normally more widespread than convective, but in Scotland rarely produces precipitation intensities of more than 5 mm/h.  A frontal system will normally take several days to form, intensify and then dissolve, during which time it may travel 1000s of km, most often tracking ENE from the Atlantic across NW Europe.  Frontal precipitation is required to produce extreme floods in large Scottish catchments (say >1000 km2), whereas extreme floods in small catchments (say < 10 km2) will always require localised intense rainfall as arises from convection.
gauging stationA site at which equipment has been installed to monitor water levels.  At many sites, a rating has been established by empirical means to convert water level to flow.  At a minimum, a gauging station may be formed of only a a post or other marker against which the water level may be read and recorded by an observer.  Some of Scotland’s longst-running gauging stations started life as daily-read post gauges.
groundwaterWater held in storage in solid or drift geology.  This water drains slowly into watercourses to provide baseflow.
hydrological modelA representation of a catchment system.
hydrologyThe study of water, particularly the storage and movement  of water across the earth’s surface.
hydrometryThe measurement of water in all its forms through the hydrological cycle.
hypsometric curveLet’s do that one another day 🙂
interceptionThe means by which some water is caught above the ground surface by falling onto leaves or other surfaces.  Trees provide the greatest interception capacities.  It’s the first precipitation falling onto a dry canopy which will provide the most noticetable interception loss.  Catchment-wide, such losses will rarely exceed 5 mm, and will be small compared with the total amount of precipitation causing a major flood.
lagThe delay between precipitation and a river flow peak.  A variety of methods have been proposed, either requiring hydrograph separation (with a hydrological model) or not.
mixed phase precipitationPrecipitation incorporating solid and liquid precipitation, e.g. snow which has begun to melt as it descends towards ground level, or precipitation which has formed as hail or snow at high altitude and is then supplemented by rain which has formed at lower altitudes.
orographic enhancementRefers to enhancement of the precipitation rate arising from uplift over a topographic barrier – i.e. if there is frontal rainfall of typically 2 mm/hr as a weather system makes landfall over the west coast, the additional cooling caused by uplift over a range of hills may cause the rainfall to reach 4 mm/hr on the highest ground.
overland flowSee surface runoff
precipitationWater falling in any form from the sky groundwards – including hail, snow, drizzle, sleet, rain.
QMEDMedian flood – the median value in a series of annual maxima.
ratingA site-specific calibration used to relate water levels to flow at a gauging station.
runoffThe water draining out of a catchment area via a stream or river.  Runoff depth (in mm) refers to the volume of water which leaves a catchment over a period of time (day/month/year) divided by the catchment area.  Make sure you get your voulme in m3 and your catchment area in m2 before you start (area in m2 = area in km2 x 1 000 000), the convert into mm depth afterwards.
saturationCatchment state when voids within soils, drift or solid geology are full – i.e. there is no more capacity for water storage below ground.  The water table is the surface which separates saturated conditions from unsaturated.
SMDSoil Moisture Deficit, measured in mm.  This is a measure of how much storage is available in the catchment, caused by evaporative lossses and drainage.
snowmeltLike it says on the tin – the melt of snow!  But think carefully – the snow pack must first have become isothermal (0 C through the whole depth profile), then must become ripe (with some surface melt having drained through the profile and likely refrozen at depth); snowmelt will only begin to be noticeable at the catchment outlet once the snowpack can no longer hold any more liquid, guaranteeing that any further melt at the surface does translate to runoff from the areas of melt.
spateThe only Scots word in this glossary so far? Some of us think that a spate is a significant rise in the level and flow of a watercourse, but may fall short of a flood.
staff gaugeA water level gauge, normally graduated in 1m, 10 cm and 1 cm graduations.  Some hydrologists will try to read the water level by careful observation to the nearest 1 mm.
stageHydrologist-speak for water level, measured above a local arbitrary datum.  Good practice is to survey the gauge zero in the Ordnance Datum, which becomes very important in the case of damage to/loss of gauging equipment.
stage boardSee staff gauge.
stochasticConcerned with or relating to chance.
storageWater held in a catchment.  This is so important in hydrology – it’s the functioning of all these storages *together* which causes patterns of river flow to be different from the patterns of precipitation as catchment input.  The 5 storages are on vegetation, on the ground surface, in the soil, the groundwater and in the channel network (the rivers, strams and all other waterbodies which drain the water from the catchment).
surface runoffWater which flows over the ground surface to the nearest watercourse.  This is the quick component of catchment runoff.  Normally caused by come combination of saturation, heavy rainfall and snowmelt, this is the water which is going to be responsible for a flood, if one is going to occur.  How fast it flows, over what area and for how long all combine to control the rate of flow in the drainage network.
timeWe all know what time is, but it’s important to note that in many hydrometric organisations, official protocol is to always record hydrological observations to UTC (which is the same as GMT) – this applies to data loggers as well as field observations – with the intention of avoiding confusion between British Summer Time and GMT readings.  We should always be careful to record the time consistently, stating the unit of time measurement (e.g. GMT) just like we should always record measurements of length in metres/mm/km and units of mass in grams or kg, as appropriate.
time to peakThe time taken for a river to rise from its pre-spate condition to a peak. Time to peak is rarely the same between one event and the next, not least due to differences in antecedent conditions and event rainfall. Quite how we measure the time to peak is also important, but unimodal peaks (those with just a single peak) are normally recommended.
transpirationLoss of water by the internal processes of plants of all kinds – from grasses through to trees.  Are lichens plants?  I suppose they use water too.  Plants lose surplus water through microscopic openings on the under-sides of their leaves after it has been used for the basic functioning of the plant.  Tissue building of the plant also counts as a use of water included within transpiration losses.
water tableThe upper surface of saturation – below or at the ground surface.
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