hydrology and fluvial geomorphology Flashcards
aeration zone
the upper zone of a permeable rock, above the water table, in which rock pore spaces are partly filled with water, partly with air.
annual hydrograph
graph showing average river flows over the year (river regime).
aquiclude
a relatively impermeable rock layer which excludes water. any geological formation that absorbs and holds water but does not transmit it at a sufficient rate to supply springs, wells, etc.
aquifer
literally “water-bearing” rock (economically important); a rock layer which stores water and allows it to pass freely through; e.g. limestone (permeable via its joints) and chalk (porous).
artesian basin
a structural basin (usually a syncline) of sedimentary rocks contained between aquicludes, which allows water to flow through the rock. If it meets the ground surface water flows up under hydrostatic pressure, e.g. the London Basin in which water falls on the Chilterns and North Downs, flows laterally through the porous chalk, and emerges at the surface in central London in artesian wells.
bankfull discharge
the condition when a river is full up to the top of its banks; any further increase in discharge will lead to flooding (overbank stage).
baseflow
the flow of water through saturated permeable, or porous, rocks deep underground towards (into) a river, under the influence of gravity
Bourne
an intermittent or seasonal stream, usually found in chalk areas, the length of which varies according to the height of the water table.
confluence
the point at which two rivers meet.
discharge
the volume of water flowing in a river channel, measured in “cumecs” (cubic metres per second).
drainage basin
the main area unit in hydrology; an area outlined by its watershed and which is drained by a river and its tributaries, e.g. the Thames basin: the area drained by the River Thames and its tributaries.
drainage density
a measure of the texture of a drainage system, expressed as the ratio of the total length of all stream channels within a drainage basin to the total area of the basin. DD = L/A km/km2.
ephemeral stream
a temporary stream, often seasonal in nature, which flows only after torrential rainfall or when exceptionally high groundwater conditions occur.
evaporation
the physical process by which a liquid is changed into a gas by molecular transfer; e.g. water to water vapour.
evapotranspiration
the combined loss of moisture from the earth’s surface by means of direct evaporation (physical process) and transpiration (biological process) from the soil and vegetation.
falling limb
the declining curve on a hydrograph. The shape of this curve reflects both the catchment area water stores and the shape and relief of the drainage basin.
flood hydrograph
a graph showing the response of river flow to a single rainfall event.
groundwater
water which occupies pores, joints and other cavities in rocks.
hydrographic
graphical representation of the discharge (volume of water) in a river, plotted against time. A unit hydrograph shows the variation of discharge (y-axis) against time (x-axis) following a single rainfall event.
hydrological cycle
the cycle of water through the earth-ocean-atmosphere system.
hydrosphere
one of the three components of the earth’s natural system, the others being the atmosphere and the lithosphere; it includes the oceans, ice-caps, lakes, rivers and lesser stores of water/ ice.
impermeable
soils or rocks which will not allow water to pass through them. This may be because they are impervious or because they are non-porous.
impervious
rocks or soils which will not allow water to pass through them because they lack joints or cracks.
infiltration
the process by which water on the surface moves into the soil store.
infiltration excess overland flow
water flowing on the surface as a result of intense rain exceeding the soil’s infiltration capacity (Hortonian overland flow).
infiltration capacity
the maximum rate at which water can enter the soil if enough water is freely available.
infiltration rate
the rate at which water enters the earth’s surface at a given place at a given time. Measured in millimetres per minute, or mm per hour
interception
the process by which vegetation prevent rain from reaching the surface directly; the intercepted water is temporarily held as interception storage, it then either falls to the ground by processes such as leafdrip and stemflow, or is evaporated back into the atmosphere.
interflow
a semi-deep downslope flow of water in the unsaturated (aerated) zone above the saturated region where baseflow takes place.
interfluve
literally “between the rivers”; the dividing line between two river basins; otherwise known as the watershed.
lag time
the time interval between maximum precipitation and maximum river discharge on a hydrograph.
leafdrip
the process by which water, which has been intercepted by vegetation leaves, falls to the earth’s surface.
non porous
rocks which contain no pore spaces and therefore cannot absorb water, although water may be able to pass through rock joints.
overbank stage
the stage at which the flow of a river cannot be contained within its normal channel, and it overflows onto the floodplain.
overland flow
the movement of water over the earth’s surface; also known as surface runoff; it arises from rainfall (or snowmelt) which has not been evaporated, nor infiltrated into the soil.
percolation
the process by which surface water moves downwards through pores and joints in the soil and rocks to deeper stores, e.g. groundwater store.
permeability
the ease with which water (and air) can pass through soil or rocks; this normally occurs via pore-spaces between the soil grains/ rock minerals, or via joints and bedding planes in rocks.
pervious
soils or rocks which allow water to pass through them via joints, cracks and fissures.
pipeflow
a rapid movement of water in subterranean channels (pipes) in the soil, sometimes following worm burrows.
porous
soils or rocks which allow water to be stored and to pass through the gaps (“pore spaces”) between their grains.
porosity
the volume of pore spaces within a rock or soil; expressed as the ratio of the volume of pore spaces to the total volume of the material; it varies from 25 – 35% for alluvial gravels, to 0.001% for slate.
potential evapotranspiration
the maximum amount of evapotranspiration that can take place from a fully vegetated surface, given a soil maintained at field capacity (see ecosystems).
ppt
the deposition of water (in solid or liquid form) from the atmosphere onto the earth’s surface; it includes rain, drizzle, snow and hail.
recharge
the process by which water is added to an aquifer or to soil, particularly following a dry season or drought.
recurrence interval
a statistical average relating to the timing between events of a given size (e.g. floods). The bigger the event, the longer the recurrence interval between them.
resurgent stream
a stream that emerges from underground, usually at the base of a limestone outcrop.
rising limb
the initial rising curve on a flood hydrograph. The steepness of the curve depends upon the intensity of the rainfall and the character of the basin.
river regime
the annual pattern of river flow, shown as a graph of discharge (y-axis) against time (x-axis). Can be shown as average monthly discharges, or as the variations in actual discharge over the hydrological year (October to October in the UK).
runoff
waterflow generated by a rainfall event (or, in some regions, snowmelt); discharge is usually converted to mm (spread over the drainage basin) to be comparable with precipitation.
saturated overland flow
overland flow in the parts of the basin where soils are saturated so that water cannot soak into the soil (compare infiltration excess overland flow)
saturation zone
the zone within a rock (or soil) which is fully saturated with water, i.e. water has replaced air in all of the pore and joint spaces.
stemflow
the flow of water, which has previously been intercepted by vegetation, along the branches and down the main trunk to the ground surface.
surface run off
water flowing over the earth’s surface; also called overland flow and may include saturated overland flow and infiltration excess overland flow.
throughfall
precipitation which falls through vegetation, but isn’t intercepted.
throughflow
the downslope lateral movement of water, through the soil matrix, roughly parallel to the surface; similar to interflow, but at shallower depth.
transpiration
the loss of water vapour from the inside of plant leaves back to the atmosphere, via pores (stomata) on the surface of the leaves.
transmissivity
the rate at which water can move through an aquifer.
transpiration
the movement of water from the soil store to the atmosphere as a result of biological processes in plants.
underbank stage
the stage which occurs when a river’s discharge is less than bankfull.
water balance graph
a graph showing the relative balance between precipitation and potential evapotranspiration through the year.
watershed
the line dividing adjacent drainage basins formed by joining the highest points between one basin and the next.
watertable
the variable upper limit of the saturation zone in permeable rock; the top of the groundwater.
