hydrology and fluvial geomorphology

Question 1

aeration zone

Answer 1

the upper zone of a permeable rock, above the water table, in which rock pore spaces are partly filled with water, partly with air.

Question 2

annual hydrograph

Answer 2

graph showing average river flows over the year (river regime).

Question 3

aquiclude

Answer 3

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.

Question 4

aquifer

Answer 4

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).

Question 5

artesian basin

Answer 5

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.

Question 6

bankfull discharge

Answer 6

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).

Question 7

baseflow

Answer 7

the flow of water through saturated permeable, or porous, rocks deep underground towards (into) a river, under the influence of gravity

Question 8

Bourne

Answer 8

an intermittent or seasonal stream, usually found in chalk areas, the length of which varies according to the height of the water table.

Question 9

confluence

Answer 9

the point at which two rivers meet.

Question 10

discharge

Answer 10

the volume of water flowing in a river channel, measured in “cumecs” (cubic metres per second).

Question 11

drainage basin

Answer 11

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.

Question 12

drainage density

Answer 12

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.

Question 13

ephemeral stream

Answer 13

a temporary stream, often seasonal in nature, which flows only after torrential rainfall or when exceptionally high groundwater conditions occur.

Question 14

evaporation

Answer 14

the physical process by which a liquid is changed into a gas by molecular transfer; e.g. water to water vapour.

Question 15

evapotranspiration

Answer 15

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.

Question 16

falling limb

Answer 16

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.

Question 17

flood hydrograph

Answer 17

a graph showing the response of river flow to a single rainfall event.

Question 18

groundwater

Answer 18

water which occupies pores, joints and other cavities in rocks.

Question 19

hydrographic

Answer 19

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.

Question 20

hydrological cycle

Answer 20

the cycle of water through the earth-ocean-atmosphere system.

Question 21

hydrosphere

Answer 21

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.

Question 22

impermeable

Answer 22

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.

Question 23

impervious

Answer 23

rocks or soils which will not allow water to pass through them because they lack joints or cracks.

Question 24

infiltration

Answer 24

the process by which water on the surface moves into the soil store.

Question 25

infiltration excess overland flow

Answer 25

water flowing on the surface as a result of intense rain exceeding the soil’s infiltration capacity (Hortonian overland flow).

Question 26

infiltration capacity

Answer 26

the maximum rate at which water can enter the soil if enough water is freely available.

Question 27

infiltration rate

Answer 27

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

Question 28

interception

Answer 28

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.

Question 29

interflow

Answer 29

a semi-deep downslope flow of water in the unsaturated (aerated) zone above the saturated region where baseflow takes place.

Question 30

interfluve

Answer 30

literally “between the rivers”; the dividing line between two river basins; otherwise known as the watershed.

Question 31

lag time

Answer 31

the time interval between maximum precipitation and maximum river discharge on a hydrograph.

Question 32

leafdrip

Answer 32

the process by which water, which has been intercepted by vegetation leaves, falls to the earth’s surface.

Question 33

non porous

Answer 33

rocks which contain no pore spaces and therefore cannot absorb water, although water may be able to pass through rock joints.

Question 34

overbank stage

Answer 34

the stage at which the flow of a river cannot be contained within its normal channel, and it overflows onto the floodplain.

Question 35

overland flow

Answer 35

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.

Question 36

percolation

Answer 36

the process by which surface water moves downwards through pores and joints in the soil and rocks to deeper stores, e.g. groundwater store.

Question 37

permeability

Answer 37

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.

Question 38

pervious

Answer 38

soils or rocks which allow water to pass through them via joints, cracks and fissures.

Question 39

pipeflow

Answer 39

a rapid movement of water in subterranean channels (pipes) in the soil, sometimes following worm burrows.

Question 40

porous

Answer 40

soils or rocks which allow water to be stored and to pass through the gaps (“pore spaces”) between their grains.

Question 41

porosity

Answer 41

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.

Question 42

potential evapotranspiration

Answer 42

the maximum amount of evapotranspiration that can take place from a fully vegetated surface, given a soil maintained at field capacity (see ecosystems).

Question 43

ppt

Answer 43

the deposition of water (in solid or liquid form) from the atmosphere onto the earth’s surface; it includes rain, drizzle, snow and hail.

Question 44

recharge

Answer 44

the process by which water is added to an aquifer or to soil, particularly following a dry season or drought.

Question 45

recurrence interval

Answer 45

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.

Question 46

resurgent stream

Answer 46

a stream that emerges from underground, usually at the base of a limestone outcrop.

Question 47

rising limb

Answer 47

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.

Question 48

river regime

Answer 48

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).

Question 49

runoff

Answer 49

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.

Question 50

saturated overland flow

Answer 50

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)

Question 51

saturation zone

Answer 51

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.

Question 52

stemflow

Answer 52

the flow of water, which has previously been intercepted by vegetation, along the branches and down the main trunk to the ground surface.

Question 53

surface run off

Answer 53

water flowing over the earth’s surface; also called overland flow and may include saturated overland flow and infiltration excess overland flow.

Question 54

throughfall

Answer 54

precipitation which falls through vegetation, but isn’t intercepted.

Question 55

throughflow

Answer 55

the downslope lateral movement of water, through the soil matrix, roughly parallel to the surface; similar to interflow, but at shallower depth.

Question 56

transpiration

Answer 56

the loss of water vapour from the inside of plant leaves back to the atmosphere, via pores (stomata) on the surface of the leaves.

Question 57

transmissivity

Answer 57

the rate at which water can move through an aquifer.

Question 58

transpiration

Answer 58

the movement of water from the soil store to the atmosphere as a result of biological processes in plants.

Question 59

underbank stage

Answer 59

the stage which occurs when a river’s discharge is less than bankfull.

Question 60

water balance graph

Answer 60

a graph showing the relative balance between precipitation and potential evapotranspiration through the year.

Question 61

watershed

Answer 61

the line dividing adjacent drainage basins formed by joining the highest points between one basin and the next.

Question 62

watertable

Answer 62

the variable upper limit of the saturation zone in permeable rock; the top of the groundwater.