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TRANSCRIPT
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Prof.dr.ir. Hubert H.G. Savenije
GWC 2: Evapora-on
CTB3300WCx: Introduc2on to Water and Climate
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Importance of Evapora-on
§ Generally the largest outgoing flux § Par;cularly in dry climates
§ Is o?en seen as a ‘loss’ (Sudd) § But is an important supplier of con;nental precipita;on (moisture recycling)
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Types of Evapora-on
§ Direct evapora;on (physical process) § Open water evapora;on Eo § Soil evapora;on Es § Intercep;on evapora;on Ei § Sublima;on of snow or ice Esnow
§ Transpira;on ET (bio-‐physical process) § Total evapora;on E = Eo+Es+Ei+Esnow+ET
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Evapora-on or ‘evapotranspira-on’
Avoid to use the term ‘Evapotranspira3on’
§ ‘Evapotranspira;on’ is opaque jargon for bulk evapora-on, masking that we do not know its composi;on à Use (total) evapora2on instead
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Poten-al Evapora-on
Poten3al evapora3on, Ep § Would occur if there is no shortage of water, § Or other factors that may limit transpira;on (temperature, solar radia2on, humidity).
Actual evapora-on, E § occurs if these stress factors are accounted for
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Average (annual) evapora-on
E = P −Q
EP= 1− Q
P= 1−CR
is the mean annual runoff [mm/a]
is the mean annual precipita;on [mm/a]
is the mean annual evapora;on [mm/a]
QPE
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Actual evapora-on
E ≤ Ep
E ≤ P
Energy constraint
Moisture constraint
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Budyko Curve
EP= 1− exp −
EpP
⎛
⎝⎜
⎞
⎠⎟
⎛
⎝⎜
⎞
⎠⎟ = 1−CR
§ If P → 0, E = P § If P → ∞, E = Ep Budyko (1920-‐2001)
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0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
1,00
0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 4,00 4,50 5,00
E/P
Epot/P = Aridity index
Congo
Yangtze
Yellow River
Brahmaputra/Ganges
Lena
Parana
Amazon
Mississippi
Mackenzie
Danube
Volga
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Meteorological factors affec-ng evapora-on
§ Energy balance § Radia;on § Humidity § Aerodynamic resistance
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Radia-on Balance
BCN RRrR −−= )1(RN : Net short wave radiation [W/m2] RC : Incoming short wave radiation RB : Outgoing long wave radiation r : Albedo or whiteness
Surface Albedo (r)
Open water 0.06
Grass 0.24
Bare soil 0.10 – 0.30
Fresh snow 0.90
Short wave Long wave
RA
RB rRC RC
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Radiometer
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Sunshine Recorder
§ RC can be determined empirically by the theore;cal sun hours and n/N
§ n/N is the ra;o of recorded sun hours to the theore;cal (poten;al) sun hours
Netherlands RC = (0.20 + 0.48 n/N)RA Average RC = (0.25 + 0.50 n/N)RA New Delhi RC = (0.31 + 0.60 n/N)RA Singapore RC = (0.21 + 0.48 n/N)RA
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Short wave radia3on expressed in terms of evapora3on RA/λ in kg m-2day-1
Maximum amount of sun hours per day N
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Outgoing Long wave radia-on
§ RB is calculated through an empirical equa;on
le? middle right
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Humidity es = 0.61exp
19.9ta273+ ta
⎛⎝⎜
⎞⎠⎟
s = desd t
= 5430es273+ ta( )2
ea (ta ) = es (tw )−γ (ta − tw )
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Psychrometer
ta is the dry bulb temperature tw is the wet bulb temperature es(tw) is the saturation pressure at the wet bulb temperature γ is the psychrometer constant (0.066 kPa/oC h is the relative humidity
ea (ta ) = es (tw )−γ (ta − tw )
h = ea (t)es (t)
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Energy balance
EN
S R H A Et
ρλΔ
= − − −Δ
[Wm-‐2]
( ) (1 )N C BR H r R R HEρλ ρλ
− − − −= = [m/d]
Assume: ΔS/Δt=0, A=0 } On daily basis !!
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Penman (1948)
§ Open water evapora;on based on the energy balance, § but making use of empirical rela;ons § 4 standard meteorological variables:
§ air temperature § rela2ve humidity § wind velocity § net radia2on
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Penman Formula
p aN s a
ao
csR e er
Es
ρ
ρλ ρλ
γ
⎛ ⎞−+⎜ ⎟
⎝ ⎠=+
RN net radiation at the Earth surface [J day-1 m-2] λ heat of evaporation (λ = 2.45 MJ/kg ) [J kg-1] s slope of the saturation pressure curve [kPa K-1] cp specific heat of air (1004 J kg-1 K-1) [J kg-1 K-1] ρa density of air (1.205 kg/m3) [kg m-3] ρ density of water (1000 kg/m3) [kg m-3] ea actual vapour pressure of the air at 2 m elevation [kPa] es saturation vapour pressure for the temp. at 2 m elevation [kPa] γ psychrometer constant (γ = 0.066 kPa/oC) [kPa K-1] ra aerodynamic resistance [day m-1]
[m/d] ra =
2450.54u2 + 0.5( )
186400
[d/m]
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Penman-‐Monteith
1
p aN s a
aa
c
a
csR e er
Ersr
ρ
ρλ ρλ
γ
⎛ ⎞−+⎜ ⎟
⎝ ⎠=⎛ ⎞
+ +⎜ ⎟⎝ ⎠
[m/d]
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Crop resistance rc
§ Provides a constraint on the transpira;on of vegeta;on § Depends on the opening of stomata in leaves, as a func;on of:
§ Soil moisture availability § Rela2ve humidity § Sunlight § Temperature
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Evapora-on of the World
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Direct measurement of evapora-on
§ Water balance: § Evapora;on pan § Lysimeter § Shallow Lysimeter
E = P − QA− dSd t
[L/T]
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Pan evapora-on
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Lysimeter
Soil
Soil saturated with water
Pump
Floater
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Intercep-on measurement Precipita3on
Canopy intercep3on
Forest floor intercep3on
throughfall
stemflow
Infiltra3on
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Shallow Lysimeter
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Shallow Lysimeter
dSupperdt
+dSlower
dt= P − E − Q
A
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The evapora-on tower
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Further reading
Mohamed, Y. A., van den Hurk, B. J. J. M., Savenije, H. H. G., and Bas;aanssen, W. G. M., 2005. Hydroclimatology of the Nile: Results from a regional climate model. Hydrol. and Earth Syst. Sc., 9: 263-‐27. hjp://www.hydrol-‐earth-‐syst-‐sci.net/9/263/2005/hess-‐9-‐263-‐2005.html Wang-‐Erlandsson, L., R. van der Ent, L. Gordon and H.H.G. Savenije. 2014 Contras;ng roles of intercep;on and transpira;on in the hydrological cycle – Part 1: Simple Terrestrial Evapora;on to Atmosphere Model, Earth Syst. Dynam. Discuss., 5, 203-‐279. hjp://www.earth-‐syst-‐dynam-‐discuss.net/5/203/2014/esdd-‐5-‐203-‐2014.html
Gerrits, A.M.J., H.H.G. Savenije, L. Hoffmann and L. Pfister, 2007. New technique to measure forest floor intercep;on – an applica;on in a beech forest in Luxembourg, Hydrol. and Earth Syst. Sc., 11, 695–701.hjp://www.hydrol-‐earth-‐syst-‐sci.net/11/695/2007/hess-‐11-‐695-‐2007.html Euser, T., W. M. J. Luxemburg, C. S. Everson, M. G. Mengistu, A. D. Clulow, and W. G. M. Bas;aanssen, 2014. A new method to measure Bowen ra;os using high-‐resolu;on ver;cal dry and wet bulb temperature profiles, Hydrol. Earth Syst. Sci., 18, 2021-‐2032. hjp://www.hydrol-‐earth-‐syst-‐sci.net/18/2021/2014/hess-‐18-‐2021-‐2014.html
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Prof.dr.ir. Hubert H.G. Savenije
GWC 2: Evapora-on
CTB3300WCx: Introduc2on to Water and Climate
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Prof.dr.ir. Hubert H.G. Savenije
GWC 2: Evapora-on
CTB3300WCx: Introduc2on to Water and Climate
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Ques-ons
1. Why is actual evapora2on smaller than poten2al evapora2on?
2. Why is average annual evapora2on less than average annual precipita2on?
3. Is evapora2on also less than precipita2on on a daily basis?