SEES 503 - 4. Evaporation and Transpirationusers.metu.edu.tr/bertug/SEES503/SEES 503 - 4 Evaporation and... · EVAPORATION AND TRANSPIRATION. ... the consumptive use of water by plants

SEES 503 Sustainable Water Resources 1/30

Instructor

Assist. Prof. Dr. Bertuğ Akıntuğ

Civil Engineering ProgramMiddle East Technical University

Northern Cyprus Campus

SEES 503SEES 503SUSTAINABLE WATER RESOURCESSUSTAINABLE WATER RESOURCES

EVAPORATION EVAPORATION ANDAND

TRANSPIRATIONTRANSPIRATION


4. EVAPORATION AND EVAPOTRANSPIRATION 4. EVAPORATION AND EVAPOTRANSPIRATION

Introduction

Evaporation: The process through which water is transferred from the surface of the Earth to the atmosphere.

Evaporation is important in all areas of water resourcesBecause it affects

the capacity of the reservoir,the yield of river basin,the consumptive use of water by plants …. etc.

In this chapter,Evaporation: the water vaporization from the open water surfaces.Transpiration: the water loss from the plants, through the pores at the surface of their leaves.Evapotranspiration: Evaporation + Transpiration



OverviewFactors Effecting Evaporation

Meteorological factorsThe Nature of the Evaporating Surface

Measurement of Evaporation

Methods of Estimation of EvaporationWater Budget (Storage Equation) ApproachEnergy Budget MethodPanman (Combination) Method

Estimation of EvapotranspirationBlaney Criddle MethodThornthwaite Method



Factors Effecting Evaporation

Meteorological FactorsSolar radiation:

Evaporation is a process of energy exchange.Solar radiation supply the energy necessary for the liquid watermolecules to evaporate.

Relative humidity:As the humidity of air increase its ability to absorb more watervapor decreases, and the rate of evaporation becomes slower.

The temperature of air:Temperature increase saturation vapor pressure (increases saturation deficit), High temperature implies that there is energy available for evaporation.




Meteorological FactorsWind:

As the liquid water vaporizes from a water body, the air adjacent to this body will be saturated. For the continuation of evaporation, this saturated air should be removed. This is possible by wind.10% change in the wind speed 1-3% change in evaporation.

Atmospheric Pressure: An increase in atmospheric pressure prevents the movement of molecules out of water.




The Nature of the Evaporating SurfaceImportant factors affecting the evaporation from different surfaces:

Temperature of liquid water:(High liquid water temp.) (High molecular motion in the water)

(The number of molecules leaving the water body will be high)

Salinity:Adversely affects evaporation.1% increase in salt concentration 1% decrease in evaporation

Aerodynamic characteristics of the surface:Roughness, texture or size of the surface.

Reflection coefficient (albedo) of the surface:High albedo Low evaporation from the surface











Direct measurement of evaporation is possible using evaporation pan.The most widely used pan:

The standard U.S. Weather Bureau Class A pan.Daily measurements: height of water & volume of water / dayAnemometer to measure the windPrecipitation gauge nearby the pan.

Diameter: 122 cm (4 ft)Pan depth: 25.4 cm (10 in)Water depth: 20 cm (8 in)




Due to the heating of water by the walls of the pan, pan measurement should be corrected by a coefficient.

The pan coefficient may change from time to time or from one location to another.

In Turkey, it is constant (0.7).

http://commons.wikimedia.org/wiki/File:Evaporation_Pan.jpg




Installation of a panSunkenFloatingSurface

Other instruments:Wild EvaporimeterLivingston AtmometerPische Armometer

http://commons.wikimedia.org/wiki/File:Evaporation_Pan.jpg










Methods of Estimation of Evaporation

The estimation of evaporation from free water surfaces is necessary for mainly two purposes:

The determination of actual evaporation taking place from an existing reservoir for an optimum operation of the reservoir.The estimation of future evaporation from a reservoir to be constructed.

Methods to estimate evaporation:Water Budget (Storage Equation) ApproachEnergy Budget MethodMass Transfer MethodEmpirical EquationsPenman Method




Water Budget (Storage Equation) ApproachContinuity Equation:

E = (∆S + P + Qs) – (Qo + Qss)

where ∆S : change in storage (S2-S1)P: precipitationQs: surface inflowQo: surface outflowQss: subsurface outflow (seepage)

∆t: week, month or year.




Energy Budget MethodQN - Qh – Qe = Qθ – Qv

where QN : net radiation absorbed by the water bodyQh : sensible heat transferQe : energy used for evaporationQθ : increase in energy stored in the water bodyQv : advected energy of inflow and outflow

1 langley/day = 1 cal/cm2




Energy Budget Method

where ρ : density of water (gr/cm3).Le : latent heat of vaporization (cal/gr)R : ratio of heat loss by conduction to that by evaporation

( )RLQQQE

e

vN

+−+

=1ρ

θ




Energy Budget Method

where γ : Psychrometer constant = 0.66 mb/°C.Ts : water surface temperature in °C.Ta : air temperature in °C.es : saturation vapor pressure at surface water temperature in mb.ea : vapor pressure of the air in mb.

as

as

eeTTR

−−

= γ

The ENERGY BUDGET METHOD is the most accurate method but requires collection of detailed atmospheric data.




Penman (Combination) MethodUsing energy budget and mass transport methods, Penman (1956) proposed a new equation which gives a good estimation of evaporation from lakes for daily to monthly periods:

where E0 : evaporation from open water surface (mm/day)Qn : net amount of radiation remaining at the free water surface

(g.cal/cm2/day = 59 mm/day)Ea : evaporation due to mass transfer of vapor, (mm/day)∆ : gradient of saturation vapor pressure at air temperature t (ºC).

∆ is determined as shown in the following figure.: Psychrometer constant (=0.66 mb/ºC or 0.49 mm Hg / ºC)

∆++∆

=γ

γ an EQE0

γ




Panman (Combination) Method




Panman (Combination) MethodThe values of Qn and Ea are determined from the following

equations:




Panman (Combination) MethodThe definition of the terms are as follows:












Panman (Combination) MethodPenman’s Method requires:

Mean air temperature,Relative humidity,Wind velocity, andDuration of sunshine.

Penman’s method commonly used in the world and in Turkey and it gives reasonably good results.










Estimation of Evapotranspiration

Consumptive Use

Evapotranspiration Water used in the plants for their growth

Consumptive use is a function of:climate, soil moisture, soil type, land management method and the type of vegetation.

Climate factor: temperature, precipitation, humidity, wind and latitude of the area, that all affect the length of growing season.




Evapotranspiration Measurement

The actual measurement is almost impossible,

Using large tanks (Ø 5 m or bigger) filled with the same type of soil and vegetation with the surrounding area.

It is called evapotranspirometer (lysimeters).

Potential Evapotranspiration:Blaney Criddle MethodPanman Method, andThornthwaite Method




Blaney Criddle MethodGives good estimation of Potential Evapotranspiration in arid and semi-arid zonesConsumptive Use

where U: Potential Evapotranspiration (mm/month)k: k1x k2 monthly crop coefficientk1: Seasonal coefficientk2: Monthly coefficient as % of k1f: Climatic factor

fkU 4.25=

ptf100

328.1 += t: mean monthly temperature (ºC)

p: monthly daytime hours as a percentage of the annual value










Thornthwaite MethodMonthly Evapotranspiration (cm)

where t: Mean monthly temperature (ºC)TE: Thornthwaite’s temperature efficiency index

a

TEtET

=106.1

514.112

1 5∑=

=

i

itTE

a: coefficienta=0.49239+0.01792 TE

SEES 503 - 4. Evaporation and Transpirationusers.metu.edu.tr/bertug/SEES503/SEES 503 - 4 Evaporation and... · EVAPORATION AND TRANSPIRATION. ... the consumptive use of water by plants

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