Glossary
Glossary
A
Active storage:
the volume in a reservoir that is between the minimum operating level and the full supply level
Annual exceedence probability (AEP):
the likelihood of a flood of given size or larger in any year1
Annual flood series:
a list of the largest flood in each water year that has occurred in the period of record at a stream gauging station
Areal actual evapotranspiration (AAET):
the evapotranspiration that takes place, under the condition of existing water supply, from a large area
Areal potential evapotranspiration (APET):
the evapotranspiration that would take place, under the condition of unlimited water supply, from a large area
Areal reduction factors (ARFs):
these relate the design rainfall estimated for a point to the design rainfall for a catchment
Attenuation:
the reduction in the magnitude of a flood peak as it moves down a river
Average annual damage:
the average cost of flood damage per year
Average recurrence interval (ARI):
a statistical estimate of the average period in years between the occurrence of a flood of a given size or larger; for example a 100-year ARI flood will occur on average once every 100 years
B
Back loaded storm:
a storm in which most of the rain occurs towards the end of the storm
Baseflow:
the slow component of catchment runoff that occurs in response to rainfall; it is usually a result of groundwater discharge to a stream
Baseflow index:
the ratio of the baseflow volume to the total volume of stream flow
Bayesian statistics:
statistical methods that allow the incorporation of prior information and new evidence and observations to provide efficient estimates of parameters or tests of hypotheses
Black box model:
rainfall-runoff models which are based on mathematical or statistical relationships that do not attempt to represent the physics of runoff processes
C
Calibrate:
use measured data to estimate the parameters of a rainfall-runoff model
Carry over storage:
a reservoir that is operated so that water may be stored for several years before being released to supply a demand (also called over-year storage)
Catchment:
all the upstream land and water surface area that drains to a specified location on a stream2
Concentrated storage:
an approach to routing which uses conceptual storages to represent the hydrologic affect of stream reaches on a flood hydrograph
Conceptual model:
a rainfall-runoff model that includes a simplified representation of the physics of runoff processes in a catchment
Conceptual storage:
a hypothetical reservoir that is specified to produce the same routing effect on a flood hydrograph (attenuation and translation) as occurs in real stream reach
Confidence limits:
the upper and lower bounds of a confidence interval that express the uncertainty in a parameter or quantile
Continuing loss:
the amount of rainfall that is lost and does not become quickflow, for the period of a storm after the initial loss has been satisfied
Critical storm duration:
the duration of a rainfall event that causes maximum flow at a catchment outlet for a given ARI
Cumec:
an abbreviation of cubic metres per second, a unit of discharge
D
Data Drill:
a source of meteorological data on a 3 arcminute (~ 5 km x 5 km) grid that covers the whole of Australia
Depression storage:
the volume of water that is held in puddles or small depressions at the soil Surface
Distributed model:
a mathematical representation of the spatial arrangement of rainfall-runoff processes in a catchment, see lumped model
Distributed storage:
an approach to routing that allows for storage throughout a stream reach
Double mass curve:
a plot of the cumulative totals of rainfall at a gauge against average cumulative totals of rainfall from several nearby gauges
Draft:
the average volume of water that a specific system can supply to a set of demands over a specified time period and at a specified level of reliability; also known as yield
Dry-bulb temperature:
the air temperature measured with a standard mercury-in-glass thermometer; see also wet-bulb temperature
Duration:
the length of time of a hydrologic event
E
Evaporation:
the transfer of water vapour from a water surface to the air at temperatures below the boiling point of water
Evapotranspiration:
the transfer of water, as vapour, from near the Earth's surface to the air; it
includes evaporation from streams and lakes, sublimation of frost, ice and snow, transpiration from plants, and transfer of water vapour directly from a soil surface
F
Flood frequency:
the annual exceedence probability or average recurrence interval of a flood
Front loaded storm:
a storm in which most of the rain occurs towards the start
Full supply level (FSL):
a nominated maximum level of the water surface of a reservoir in normal operations that may be exceeded when water passes over a spillway
G
Gauging station:
a location on a stream where routine measurements of stage and discharge are made
H
Historical floods:
floods that occurred before the start of the stream gauging record at a site
Hydrograph:
a graph of stream flow against time
Hyetograph:
a graph of rainfall (or rainfall intensity) against time
Hypsometric methods:
methods that use the relationship between elevation and rainfall depth to
improve the estimation of rainfall away from the measured values at gauges
I
Infiltration capacity:
the maximum rate at which water can enter the surface of a soil
Infiltration excess runoff:
caused when the rainfall intensity is greater than the infiltration capacity of a
soil (also called Hortonian overland flow)
Infiltration rate:
the rate at which water enters the soil surface
Initial loss:
the rainfall at the start of a storm required to wet a catchment before quickflow commences
Interception loss:
the amount of rainfall that is stored and then evaporated because of its interaction with vegetation; see interception store
Interception store:
the amount of rain that can be retained on vegetation and therefore prevented from contributing to runoff; see interception loss
Interflow:
the lateral movement of water through soil (also referred to as subsurface lateral flow or throughflow)
Irradiance:
the rate of energy received per unit area; the SI unit is W m-2 or J s-1 m-2
Isohyets:
contours of equal rainfall depth
L
Level pool routing:
see reservoir routing
Longwave radiation:
infrared radiation, around 10 μm wavelength, generated by soil, water or the
Earth (see Box 3.2)
Lumped model:
a model that ignores the spatial arrangement of rainfall-runoff processes in a catchment; these processes are grouped (or lumped) together; see distributed model
M
Macropores:
large passages in soil that can be formed by roots or earthworms, and can be important conduits for water flow when the soil is saturated
Master charts:
maps that show rainfall intensity for a range of standard durations and frequencies
Minimum operating level:
the lower limit of normal operations for a reservoir; water that is held below the minimum operating level cannot normally be released because of either physical constraints or poor quality
N
Net radiation:
the energy from incoming shortwave radiation, less what is reflected from the surface, and less the amount of energy radiating from the surface as longwave radiation
O
Objective function:
a measure of model performance or how well a model can represent reality
Oktas:
a measurement of cloud cover, and denotes the number of eights of the sky covered by cloud
P
Parameter optimisation:
the process of finding the optimal values of parameters for a rainfall-runoff model
Partial series:
a list of all the independent floods that have occurred in the period of record at a stream gauging station that are above a specified threshold value (also referred to as the peak over threshold series)
Patched Point data:
meteorological data from 4650 meteorological stations in Australia; all gaps in
the record at these stations have been infilled
Peak over threshold:
see partial series
Physically based model:
a mathematical representation of at least some of the processes involved in
the conversion of rainfall to runoff; see conceptual model
Piezometer:
a vertical pipe placed in the ground used to measure the elevation of the water table
Plotting position:
an estimate of the exceedence probability of a past flood event
Pluviograph:
a recording rain gauge that can measure rainfall during short time periods
Point potential evapotranspiration (PPET):
the evapotranspiration that would take place, under the condition of unlimited water supply, from an area so small that the local transfer of water vapour does not alter local airmass properties
Porosity:
a measure of the volume of pore space in a soil
Precipitation:
the general term for condensation and deposition of water vapour from the air, and which includes rain, hail, sleet, snow, and dew
Probabilistic rational method:
a technique that relates rainfall intensity for a given frequency, with the design flood magnitude of the same frequency
Psychrometer:
a device for measuring the wet-bulb and dry-bulb temperature of the air
Q
Quantile:
the flood magnitude that corresponds to a particular flood frequency
Quickflow:
the rapid component of catchment runoff that occurs in response to rainfall
R
Radiant exposure:
the energy received per unit area, that is the time integral of the irradiance, with units J m-2
Rainfall burst:
an intense period of rain within a storm, also called the storm-core
Rainfall excess:
the portion of rainfall that contributes to streamflow
Rainfall intensity:
a measure of the heaviness of rain, measured in mm h-1
Ramsar wetland:
a wetland of international significance under the Ramsar convention-Ramsar is a city in Iran where the convention was negotiated
Rational method:
a technique to estimate a design flood magnitude for a given frequency, and based on catchment characteristics and rainfall intensity
Recession:
the decline in streamflow that occurs after the passage of a flow peak
Regulated river:
a river where flows are affected by dams or diversions
Relative humidity:
the ratio of ambient vapour pressure to saturation vapour pressure
Reservoir routing:
the mathematical description of the passage of a flood wave through a reservoir; also known as level pool routing or storage routing
Return flow:
water that has travelled downslope in surface soils before exfiltrating through the surface
Runoff routing:
a technique for modelling the generation and movement of flood hydrographs in a catchment
Run-on:
water contributed to a point by areas upslope of that point
S
Saturated hydraulic conductivity:
the rate of water movement through saturated soil
Saturation excess runoff:
runoff due to rainfall on saturated soil so water cannot infiltrate but instead runs off
Saturation vapour pressure:
the maximum moisture that can be held in air at a given temperature
Shortwave radiation:
solar radiation with wavelength about 0.5 μm (see Box 3.2)
Simulate:
use a rainfall-runoff model to estimate output, usually runoff
Soil moisture content:
the volume (or depth) of water contained in a given volume (or depth) of soil
Spill:
the uncontrolled flow of water over a reservoir spillway
Stage:
the elevation of the water surface in a stream measured to some convenient datum
Stationary:
the mean and variance of the data do not change with time
Stemflow:
the volume of water, originating as rainfall, that runs down tree trunks or plant stems
Storage routing:
see reservoir routing
Storm:
in hydrology any rainfall event; in common usage a period of extreme weather with intense rain and strong wind
Storm-core:
an intense period of rain within a longer storm, also called a rainfall burst
Sublimation:
the transformation of solid water (frost, ice and snow) directly to water vapour
Subsurface lateral flow:
see interflow
T
Temporal pattern:
the time sequence of rainfall intensity
The cap:
a limit on the diversion of water from streams in the Murray-Darling Basin to the volume of water that would have been diverted under 1993-94 levels of development
Thiessen method:
the rainfall at any point in the catchment is taken to be the rain falling at the
nearest gauge to that point; Thiessen polygons result from allocating portions of a catchment to their nearest gauge
Throughfall:
rain that passes through a vegetation canopy, including water that drips from leaves
Throughflow:
see interflow
Translation:
the change in time that a flow peak occurs as it moves down a river
Transmission loss:
the loss of water from streamflow when water infiltrates into the streambed or
floodplain
Transpiration:
the release of water vapour by plants through their leaves
U
Unregulated river:
a river or stream where flows are unimpeded-not affected by dams or diversions
V
Vadose zone:
the unsaturated soil layer between the soil surface and the water table
Validation:
the testing of a rainfall-runoff model on data that were not used for calibration
Vapour pressure:
the partial pressure of water vapour in air
Vapour pressure deficit:
the difference between the vapour pressure and the saturation vapour pressure
Variable source areas:
saturated areas of a catchment that contribute runoff; the size of these source
areas can change during storms or seasonally
W
Warm up period:
the time taken before a rainfall-runoff model output does not depend on initial
conditions
Water balance:
an account of all the water in some specified system; all inflows and outflows must be measured, along with the change in the volume of water held in storage
Water table:
the elevation where groundwater is at atmospheric pressure
Water year:
a 12-month period that is defined for the analysis of hydrologic events (see Box 5.3)
Wet-bulb depression:
the difference between the wet-bulb and dry-bulb temperatures
Wet-bulb temperature:
a measurement using a standard thermometer wrapped in wet cotton cloth; see also dry-bulb temperature
Wind run:
wind velocity, usually in units of km d-1
Within year storage:
a reservoir operated so that it will fill and spill in most years
Y
Yield:
see draft