Environmental Measurement Techniques

Surveillance

• Involves frequent standardized measurement and observation of the environment
• Used for warning and control

Moving Boat Method

• Measures discharge from a boat by traversing the stream along the measuring section
• Continuously measures velocity, depth, and distance travelled

Multi-annual Mean

• Arithmetic mean of data occurrences over multiple years
• Calculated for a specific date or period within the year
• Example: Multi-annual mean monthly discharge for April

Multi-annual Storage

• Volume of water that can be stored in a reservoir to cope with variations in inflow and demand over multiple years
• Synonymous with multi-annual storage capacity

Multiple Correlation

• Synonymous with multiple regression
• Interdependence of more than two variables

Multiple Correlation Coefficient

• First product-moment correlation between the actual values of the "dependent" variate in multiple regression and the values given by the regression equation
• Synonymous with correlation coefficient and covariance

Multiple Regression

• Regression of a "dependent" variate on more than one "independent" variate
• Synonymous with multiple correlation

Muskingum Method

• Approximate hydrological method of flood routing
• Based on the equation of continuity for a reach and a storage equation expressing the linear dependence of the water volume in the reach on the weighted inflow and outflow discharges

Natural Flow

• Flow in a stream under natural conditions

Normal Distribution

• Mathematically defined, symmetrical, bell-shaped, continuous probability distribution
• Traditionally assumed to represent random errors
• Synonymous with Gaussian distribution

Normal Hydrological Value

• Mean value of a hydrological characteristic taken over a period selected by consensus
• Mean over any longer period does not significantly differ

Accumulative Precipitation Gauge

• Precipitation gauge used at stations visited only at long-spaced time intervals

Active Basin Area

• Basin area excluding undrained parts

Actual Evaporation

• Quantity of water evaporated from an open water surface or from the ground

Actual Evapotranspiration

• Sum of the quantities of water evaporated from the soil and the plants when the ground is at its natural moisture content

Advection

• Process of transfer of air-mass properties by the velocity field of the atmosphere

Altitude

• Vertical distance of a level, a point or an object from mean sea-level

Annual Exceedence Series

• Series of values of independent events, such as floods, occurring above a base value
• Number of values in the series equals the number of years of the record

Annual Flood

• Highest peak discharge in a water year
• Flood that has been equalled or exceeded once each year on average

Annual Flow

• Total volume of water that flows during a year
• Usually refers to the outflow of a drainage area or river basin

Annual Maximum Series

• Extreme-value series with largest annual values

Annual Minimum Series

• Extreme-value series with smallest annual values

Annual Runoff

• Total volume of water that flows during a year
• Usually refers to the outflow of a drainage area or river basin

Annual Series

• Hydrological series with terms representing some characteristic or element for each of a series of years

Annual Storage

• Volume of water that can be stored in a reservoir to cope, partly or completely, with seasonal variations in inflow and demand

Annual Storage Capacity

• Volume of water that can be stored in a reservoir to cope, partly or completely, with seasonal variations in inflow and demand

Antecedent Precipitation Index

• Weighted summation of past daily precipitation amounts, used as an index of soil moisture

Elevation

• Vertical distance of a level, a point or an object considered as a point, measured from mean sea-level

Empirical Flood Formula

• Formula expressing peak discharge as a function of catchment area and other factors

Engineering Hydrology

• Branch of applied hydrology dealing with hydrological information intended for engineering applications (planning, design, operation, and maintenance)
• Example: Design and operation of engineering measures and structures

Envelope Curve

• Smooth curve representing the boundary within which all or most of the known data points are contained

Enveloping Curve

• Smooth curve representing the boundary within which all or most of the known data points are contained

Ephemeral Stream

• Stream which flows only in direct response to precipitation or the flow of an intermittent spring

Estuary

• Generally broad portion of a stream near its outlet

Evaporation

• Emission of water vapour by a free surface at a temperature below the boiling point
• Amount of water evaporated

Evaporimeter

• Instrument for measuring the amount of water evaporated into the atmosphere during a given time interval

Evapotranspiration

• Quantity of water transferred from the soil to the atmosphere by evaporation and plant transpiration

Expected Value

• Best approximation of the true value
• Usually the mean of several or many computations, estimates, or measurements

Experimental Basin

• Basin where natural conditions are deliberately modified to study the effects of these modifications on the hydrological cycle

Experimental Watershed

• Synonymous with experimental basin

Extreme Rainfall

• Statistical (physical) upper limit of precipitation for a given duration over a particular basin

Extreme Value Distribution

• Probability distribution of the largest (smallest) observation in a sample
• Common distributions: FrŽchet, Gumbel and Weibull

Extreme Value Series

• Hydrological series including the largest or smallest values, with each value selected from an equal time interval in the record

Rate of Flow

• Volume of water flowing through a river (or channel) cross-section in unit time

Rating Curve

• Curve showing the relation between stage and discharge of a stream at a hydrometic station
• If digitized, it is a rating table

Rational Method

• Formula estimating the peak rate of storm runoff
• Product of the catchment area, a peak rate of rainfall, and a runoff coefficient

Recession

• Period of decreasing discharge, as indicated by the falling limb of a hydrograph starting from the peak

Recording Rain Gauge

• Instrument recording the depth of water from precipitation over time

Recurrence Interval

• Long-term average interval of time or number of years within which an event will be equalled or exceeded (e.g. flood peak discharge)

Regional Analysis

• Extension of the results of the analysis of point data to a region

Regression Coefficient

• Coefficient of an independent variate in a regression equation
• For a first degree linear regression, the slope of the regression line

Regression Curve

• Diagrammatic exposition of a regression equation, generally referring to a linear regression
• Synonymous with regression line

Remote Sensing

• Measurement or acquisition of information on some property of an object or phenomenon by a recording device that is not in physical contact with the object under study

Representative Basin

• Basin permitting the study of the hydrological cycle in a characteristic natural region by the simultaneous observation of climatic and hydrometric data
• Basin where intensive hydrological studies are conducted under relatively unchanged conditions
• Basin in which hydrological stations are installed to obtain measurements representing a broad area

Reservoir Routing

• Technique used to compute the movement and change of shape of a flood wave moving through a river reach or a reservoir

Response Function

• Mathematical representation of the behaviour of a (hydrological) system due to a specified input

Simulation

• Reproduction or representation of an event or sequence of related events in a model

Skewness

• Asymmetry in a frequency or probability distribution

Spillway Design Flood

• Flood used for the design of a dam spillway
• Maximum flood flow that could be passed without damage or serious threat to the stability of engineering structures

Staff Gauge

• Graduated scale used to indicate the level of the water surface in a stream channel, reservoir, lake, etc.

Stage

• Vertical distance of the water surface of a stream, lake, reservoir (or groundwater observation well) relative to a gauge datum

Stage Gauge

• Graduated scale used to indicate the level of the water surface in a stream channel, reservoir, lake, etc.

Stage Hydrograph

• Graph showing the variation in time of some hydrological data (stage, discharge, velocity, sediment load, etc.)
• Hydrograph is mostly used for stage or discharge

Stage-Discharge Relation

• Relation between stage and discharge at a river cross-section
• May be expressed as a curve, table, or equation(s)

Standard Deviation

• Widely used measure of dispersion of a frequency distribution or of a set of values
• Positive square root of the variance

Standard Error

• Positive square root of the variance of the sampling distribution of a statistic

Standard Error of Estimate

• Expression for the standard deviation of the observed values about a regression line
• An estimate of the variation likely to be encountered in making predictions from the regression equation

Standard Project Flood

• Discharge that may be expected from the most severe combination of meteorological and hydrological conditions considered reasonably characteristic of the geographical region involved
• Excludes extremely rare combinations

Standardized Variate

• Value of a variate reduced to standardized form (zero mean, unit variance) by subtracting the population or sample mean and then dividing by the population or sample standard deviation.

Stationary Process

• A stochastic process whose probability distribution is completely independent of time
• Also referred to as stationary in the wide sense (or to the second order) if the mean and variance are independent of time