Prof.dr.ir.  Hubert  H.G.  Savenije  

GWC  2:  Evapora-on  

CTB3300WCx:  Introduc2on  to  Water  and  Climate  

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)  

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

 

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  

   

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

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

Actual  evapora-on  

E ≤ Ep

E ≤ P

Energy  constraint  

Moisture  constraint  

Budyko  Curve  

EP= 1− exp −

EpP

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟ = 1−CR

§  If  P → 0, E = P §  If  P → ∞, E = Ep Budyko  (1920-‐2001)  

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  

Meteorological  factors  affec-ng  evapora-on  

§  Energy  balance  §  Radia;on  §  Humidity  §  Aerodynamic  resistance  

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  

Radiometer  

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

Short  wave  radia3on  expressed  in  terms  of  evapora3on    RA/λ in  kg m-2day-1  

Maximum  amount  of  sun  hours  per  day  N    

Outgoing  Long  wave  radia-on  

§ RB  is  calculated  through  an  empirical  equa;on  

le?   middle   right  

Humidity  es = 0.61exp

19.9ta273+ ta

⎛⎝⎜

⎞⎠⎟

s = desd t

= 5430es273+ ta( )2

ea (ta ) = es (tw )−γ (ta − tw )

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)

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  !!  

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  

 

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]

Penman-‐Monteith  

1

p aN s a

aa

c

a

csR e er

Ersr

ρ

ρλ ρλ

γ

⎛ ⎞−+⎜ ⎟

⎝ ⎠=⎛ ⎞

+ +⎜ ⎟⎝ ⎠

[m/d]  

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  

 

Evapora-on  of  the  World  

Direct  measurement  of  evapora-on  

§  Water  balance:  §  Evapora;on  pan  §  Lysimeter  §  Shallow  Lysimeter    

E = P − QA− dSd t

[L/T]  

Pan  evapora-on  

Lysimeter  

Soil  

Soil  saturated  with  water  

Pump  

Floater  

Intercep-on  measurement  Precipita3on  

Canopy  intercep3on  

Forest  floor    intercep3on  

throughfall  

stemflow  

Infiltra3on  

Shallow  Lysimeter  

Shallow  Lysimeter  

dSupperdt

+dSlower

dt= P − E − Q

A

The  evapora-on  tower  

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      

Prof.dr.ir.  Hubert  H.G.  Savenije  

GWC  2:  Evapora-on  

CTB3300WCx:  Introduc2on  to  Water  and  Climate  

Prof.dr.ir.  Hubert  H.G.  Savenije  

GWC  2:  Evapora-on  

CTB3300WCx:  Introduc2on  to  Water  and  Climate  

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?