5 hydrology quantities-measures_instruments_activities

Hydrology:
Measurements, Quantities,
Instruments

Jackson Pollok, Free Form, 1949, Moma

Gianbattista Toller

Sunday, September 12, 2010

Hydro: quantities, instruments, and activities

Objectives:

• To describe the measurements that are relevant in
characterising hydrometeorological events
• To summarily describe the measuring instruments and list
the principles upon which they function

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Main hydrological quantities of
hydrological interest:

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1. Temperature

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1. Temperature
2. Humidity

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1. Temperature
2. Humidity
3. Precipitation

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1. Temperature
2. Humidity
3. Precipitation
4. Radiation

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1. Temperature
2. Humidity
3. Precipitation
4. Radiation
5. Wind

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1. Temperature
2. Humidity
3. Precipitation
4. Radiation
5. Wind
6. Pressure

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1. Temperature
2. Humidity
3. Precipitation
4. Radiation
5. Wind
6. Pressure
7. Wetting

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1. Temperature
2. Humidity
3. Precipitation
4. Radiation
5. Wind
6. Pressure
7. Wetting
8. Evapotranspiration

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Energy and Temperature
• A body possesses energy either when it is moving (kinetic
energy) or when, though static itself, it can (potential
energy), through adequate means, start moving or cause
another body to move.
• In a gas, the temperature is linked to the average velocity of
its particles (atoms or molecules).
• In a crystal, the temperature is linked to the kinetic energy of
the vibrations of its atoms around their mean position.
• In accordance with the principles of thermodynamics, two
bodies with different temperatures that are placed in contact
with one another will reach the same temperature.
• The Thermometer is the instrument that is used to measure
temperature.
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Main measurement scales of temperature
• Relative
– celsius [°C] 0°C=triple point of H2O; 100°C boiling
point of H2O at 1013mB
– fahrenheit [°F] 0°F=temperature of a mix of ice and
NH3Cl; 97°F=average temperature of the human body
– t[°C]=5*(t[°F]-32)/9

• Absolute
– kelvin [K] T[K]=273.16+t[°C]
– rankine [R] T[R]=459.67+t[°F]
– In the International System (SI) temperature is measured in
°C e K
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Some physical phenomena that are
used to measure temperature

• Dilatation

• Electrical conductivity

• Seebeck effect

• Emission of electromagnetic waves

• Pyroelectricity (variations in temperature electrically
polarise certain crystals)

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Dilatation

• Dilatation: the volume of a body varies in function to
temperature (generally an increase in temperature
causes an increase in volume, the notable exception
being water around 0 °C )

• Gas thermometer (PV=nRT)

• Liquid thermometer (Hg, alcohol)

• Bimetallic thermometer

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Thermo-Hygrograph

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Electrical Conductivity
• Electrical Conductivity: temperature affects the movement of
electrical charges.
• In metals (Pt, Fe, Cu) the conductivity reduces with raising
temperatures:

• platinum resistance thermometers (Pt100, Pt1000)
• In semiconductors (Ge, Si) the conductivity generally increases
with increases in temperature:

• diode thermometer, transistor, NTC thermistor (negative
temperature coefficient)

• NOTE: the PTC thermistor (positive temperature
coefficient) behaves like a metal

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Pt100 Surface Temperature

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Seebeck Effect

• Seebeck Effect: (after the German physicist Johann
Thomas S. 1770-1831) if the junctions between different
conductors are at different temperatures a current loop
is created.

• Thermocouple thermometer

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Seebeck Effect

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Emission of Electromagnetic Wave
• Stefan-Boltzmann law: the energy irradiated by a body grows
with the fourth power of it absolute temperature and with its
relative emissivity
• For a black body: W [Wm-2]=σT4; σ=5.67032E-8 [Wm2K4] =
Stefan-Boltzmann constant; T[K]
• Wien’s displacement law: the frequency at which the
intensity of radiation is maximum increases with increases in
absolute temperature
• T*λmax=2.898E-3 [mK]; T[K]; λ[m]
• λ[m] * ν[s-1] = c [m s-1]; speed of light (in a vacuum = 300
000 km/s)
•Instruments: bolometer, thermal imaging cameras
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Thermal Imaging Camera

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Humidity

• Indicates the quantity of water contained in a body
• In agrometeorology the interest is particularly on:
• Air
• Soil
• Parts of a plant

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Mixtures and Phases
• Mixture: a set of two or more substances that, though
intimately mixed together, still conserve their chemical
properties unaltered.

• Phase: in chemistry, a homogeneous part of a system
that is demarcated by a physically defined separation
surface.

• Esempi:
• Monophasic mixture: dry (and filtered) air.
• Polyphasic mixture: clouds, soil
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Gas Vapour Saturation
• Gas: a set of atoms or molecules in a particular state of
aggregation that allows them to move freely and leave
without limits.

• Critical temperature: temperature at and above which a
gas cannot be liquefied by compression.

• Vapour: gas below critical temperature.

• Saturated vapour: vapour in equilibrium with its
condensed phase (solid and/or liquid).

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Air Humidity

• Absolute Humidity [kgm-3] = quantity of water contained in
a cube metre of free air

• Relative Humidity [%] = RH% = ratio of the quantity of
water in 1m3 free air and the quantity that would be
contained in 1m3 of air at pressure of water vapour in
equilibrium (i.e. at “saturation” pressure) at the same
temperature

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used to measure air humidity
• Dilatation

• Adsorption (solids), absorption (liquids) with variations
in:
• weight
• electric conductivity
• dielectric constant

• Absorption of electromagnetic radiation

• Emission of electromagnetic radiation

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Dilatation

• Dilatation: the volume of certain animal fibres varies
(usually increases) as a function of relative humidity

• Hair hygrometer

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Hair Hygrometer

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Adsorption with Variance of the
Dielectric Constant
• Adsorption: the number of water molecules adsorbed by a
membrane of suitable polymer grows as a function of the
relative humidity

• Placed between to gold contact points (frame), the polymer
membrane behaves like an electric capacitor

• Because of its high dielectric constant (80), the adsorbed
water has a significant effect on the capacitance of the
capacitor
– Electronic hygrometer
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Capacitive Sensors

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Regnault’s Psychrometer

• It is made up of two identical thermometers placed close to
one another

• One thermometer is used normally (dry bulb; ta)

• The bulb of the other is thermometer is wrapped in a cloth
soaked with distilled water (wet bulb; tb) and subjected to an
air flow with a velocity of 3÷8 m/s

• When RH%=100%, ta=tb; tb drops in drops in function of RH%

• Regnault’s formula calculates RH% on the basis of ta and tb
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Asmann Psychrometer

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Soil Moisture

• Uwet w/w [kg/kg] = weight over weight; kg of water in one kg
of wet soil

• Udry w/w [kg/kg] = weight over weight; kg of water in one kg
of dry soil

• Uv v/v [m3/m3] = volume over volume; m3 of water in one m3
of soil as it is; it is the most useful measure in irrigation;
often it is expressed in [mm/m] or [L/m3]

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used to measure soil moisture

• Weight

• Interfacial tension

• Electric conductivity

• Thermal conductivity

• Dielectric constant

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Weight
• Weight: the water in a certain volume of soil is weighed

– Gravimetric method: the dry weight(24 hours in an oven
at 105°C) of the soil sample is subtracted from the fresh
weight of the same sample (it is a destructive method).

– Weighing lysimeter: a potted plant is periodically
weighed; it measures variations, not absolute values

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Interfacial Tension
• Interfacial tension: water (or more precisely, the circulating
solution) is held in the ground mainly by capillary action.
– tensiometer: a container with de-aerated water is placed
in contact with soil by means of a porous ceramic layer.
Water leaves the container through the ceramic layer
creating a depression that that can be measured with a
vacuometer. The flow of water ceases when the internal
and external tensions are in equiibrium.

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Tensiometer

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Electric Conductivity

• Electric conductivity: the variation of the quantity of
water contained in a porous medium in the ground
causes variations in the electric conductivity

– Boyoucous resistance meter: two electrodes are
inserted in a block of porous gypsum

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Electric Conductivity

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Thermal Conductivity

• Thermal Conductivity: the variation of the quantity of
water contained in the ground causes variations in its
thermal diffusivity

– diffusivity-based sensors: they are composed of a
heating element and a thermometer

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Thermal Conductivity

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Dielectric Constant
• Dielectric constant: the variation of the quantity of water
(dielectric constant=80) contained in the ground (dielectric
constant=3-4) causes significant variations in the total
dielectric constant of the system

– Electric capacitance: an electric capacitor that uses the
soil as a dielectric

– Propagation speed: the speed of an electromagnetic
wave decreases in correspondence of an increase of the
dielectric constant of the medium through which it
propagates (TDR,TDT)
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Dielectric Constant

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Precipitation
• The term refers to the water that leaves the atmosphere
and is deposited on the ground
• Liquid phase
– rain
– dew
• Solid phase
– snow
– hail
– frost

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used to measure precipitation

• Weight & Volume

• Absorption of electromagnetic waves

• Reflection of electromagnetic waves

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Weight & Volume
• Weight: Precipitation is collected in a container and
weighed
– Tipping bucket rain gauge

– Weighing precipitation gauge

• Volume: the precipitation is collected in a graduated
container
– standard rain gauge

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Rain Gauge

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Absorption of electromagnetic
radiation
• Absorption of electromagnetic radiation: the passage of
every drop, flake or hailstone momentarily interrupts a beam
of light.

• the number and duration of these interruptions allow one to
establish the type and dimension of every object counted

– disdrometer

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Reflection of electromagnetic
radiation
• Reflection of electromagnetic radiation: a beam of
electromagnetic radiation of appropriate wavelength is
projected towards the precipitation area.

• The intensity of the reflected radiation allows one to
estimate the type and intensity of the precipitation.

– Weather radar

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Radiation

• The term indicates the energy that is exchanged in the
form of electromagnetic waves

• Global radiation [Wm-2]: electromagnetic power arriving
onto a flat surface

• Net radiation [Wm-2]: (Incoming radiation – Outgoing/
emitted radiation) in relation to a flat surface

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used to measure radiation

• Dilatation

• Seebeck effect

• Photovoltaic effect

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Dilatation

• Dilatation: the volume of a body varies (generally increases)
as a function of temperature

– Pyroheliometer: blackened bimetallic thermometer,
protected with a glass dome.

– Actinometer: mercury thermometer with a blackened
bulb

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Seebeck Effect

• Seebeck effect: if the junctions between different
conductors connected in a circuit are at different
temperatures a current loop is created.

– Thermocouple radiometer: thermocouples in series
with the warm junction in contact with a blackened
surface protected by a glass dome

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Thermopile Pyranometer

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Photovoltaic Effect

• Voltaic Effect: a properly constructed P-N junction
diode generates an electric current directly proportional
to the intensity of radiation absorbed

– Diode-based radiometer (generally silicon)
– Visible radiation cameras

– Thermal imaging camera

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Wind
• The term indicates the movement of the air

• Wind is a vectorial quantity composed of:

– Intensity [m/s] [km/h] [knots]

– Direction [sexagesimal degrees]

• NB: the wind direction refers to its PROVENANCE

• In current agrometeorological practice, the vertical
component of the wind is not taken into account

• Generally clockwise polar coordinates are used with N=0°
and S=180°

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used to measure the wind
• Differential pressure

• Stagnation pressure (Pitot tube)

• Venturi effect (vane anemometer)

• Speed of sound

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Differential Pressure
• Differential pressure: pressure applied differently to the sides
of an object tends to make the object rotate
– Anemoscope: a flag free to rotate around a vertical axis so
that the forces acting on its two surfaces are in equilibrium
– Cup anemometer: three hemispherical cups placed at the
vertices of an equilateral triangle with a vertical axis of
rotation passing through the centre of the triangle. Wind
thrust is maximum on the concavity and minimum on the
convexity from. The rotational velocity is proportional to
the windspeed.

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Windspeed and
Direction

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Speed of Sound
• Speed of sound: an ultrasound emitter is placed at a certain
distance from a sensor.

• The travel time of a sound impulse is lesser with favourable
wind and greater in a headwind

• measurements are taken along two or three orthogonal axes

• Windspeed and direction are measured (as well as air
temperature)

• Sonic anemometer

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Sonic Anemometer

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Atmospheric Pressure

• It represents the weight of the “column” of atmosphere on
a given point

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used to measure pressure

• Hydrostatic equilibrium

• Elasticity

• Piezoelectric effect

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Hydrostatic Equilibrium

• Hydrostatic Equilibrium: Hydrostatic equilibrium
between two fluids with different densities (mercury-air)

– Torricelli Barometer

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Mercury Barometer

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Elasticity

• Elasticity: Deformation of an elastic container in which
a vacuum has been created

– Aneroid barometer

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Aneroid Barometer

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Piezoelectric Effect

• Piezoelectric effect: the deformation of a piezoelectric
crystal induces a difference in electrical potential across
its faces

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Piezoelectric Sensors

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Evapotranspiration
• Evaporation (E): water evaporated from non-living materials
(ground, lakes, etc)

• Transpiration (T): water evaporated from living materials
(leaves, fruits, stems)

• Evapotranspiration (ET): is the sum of E and T

• The biggest part of transpired water is used to keep the
plant hydrated and fresh, only a small percentage of water is
used for photosynthesis

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How to measure and estimate ET

• Water balance of the soil

• Formulae: ET=f (Temperature,RH%,Wind, Radiation)

• Thermal conductivity

• Eddy Correlation

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Thank you for your attention!

G.Ulrici - Uomo dope aver lavorato alle slides , 2000 ?

Gianbattista Toller


5 hydrology quantities-measures_instruments_activities

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