5 hydrology quantities-measures_instruments_activities

Hydrology: Measurements, Quantities,

Instruments

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Gianbattista Toller

Sunday, September 12, 2010

Gianbattista Toller

Hydro: quantities, instruments, and activities

Objectives:

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• 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


Gianbattista Toller


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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Gianbattista Toller



1. Temperature

2. Humidity

3. Precipitation

4. Radiation

5. Wind

6. Pressure

7. Wetting

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Gianbattista Toller



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 13


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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: 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).

Gas Vapour Saturation

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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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• 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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Some physical phenomena that are used to measure air humidity


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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 24


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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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• Weight

• Interfacial tension

• Electric conductivity

• Thermal conductivity

• Dielectric constant

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Some physical phenomena that are used to measure soil moisture


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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: 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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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)35


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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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• Weight & Volume

• Absorption of electromagnetic waves

• Reflection of electromagnetic waves

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Some physical phenomena that are used to measure precipitation


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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: 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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Reflection of electromagnetic radiation


Gianbattista Toller


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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• Dilatation

• Seebeck effect

• Photovoltaic effect

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Some physical phenomena that are used to measure radiation


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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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• Differential pressure

• Stagnation pressure (Pitot tube)

• Venturi effect (vane anemometer)

• Speed of sound

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Some physical phenomena that are used to measure the wind


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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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• Hydrostatic equilibrium

• Elasticity

• Piezoelectric effect

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Some physical phenomena that are used to measure pressure


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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!

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