EVAPORATION (LIQUID CONVERSION INTO VAPOUR PRESSURE)

EVAPORATION (LIQUID CONVERSION INTO VAPOUR PRESSURE)

• 1.
  Evaporation
• 2.
  Evaporation • Terminology – Evaporation– process by which liquid water passes directly to the vapor phase – Transpiration - process by which liquid water passes from liquid to vapor through plant metabolism – Sublimation - process by which water passes directly from the solid phase to the vapor phase
• 3.
  Factors Influencing Evaporation •Energy supply for vaporization (latent heat) – Solar radiation • Transport of vapor away from evaporative surface – Wind velocity over surface – Specific humidity gradient above surface • Vegetated surfaces – Supply of moisture to the surface – Evapotranspiration (ET) • Potential Evapotranspiration (PET) – moisture supply is not limited nR E Net radiation Evaporation Air Flow u
• 4.
  Evaporation from aWater Surface • Simplest form of evaporation – From free liquid of permanently saturated surface
• 5.
  Evaporation from aPan • National Weather Service Class A type • Installed on a wooden platform in a grassy location • Filled with water to within 2.5 inches of the top • Evaporation rate is measured by manual readings or with an analog output evaporation gauge h Area, A CS w a AEm wv  dt dh E  nRsH Sensible heat to air Net radiation Vapor flow rate Heat conducted to ground G
• 6.
  Methods of EstimatingEvaporation • Energy Balance Method • Aerodynamic method • Combined method
• 7.
  Energy Method • CVcontains liquid and vapor phase water • Continuity - Liquid phase    CS w CV wv d dt d m dAV 0 dt dh Aw No flow of liquid water through CS AEm wv  E dt dh  hw a vm dt dh E  nRsH G
• 8.
  Energy Method • Continuity- Vapor phase   CS avw qAE dAV 0 Steady flow of air over water AEw    CS av CV avv qdq dt d m dAV   CS av w q A E dAV  1   CS avv qm dAV hw a vm dt dh E  nRsH G
• 9.
      CS u CV u dgzVe dgzVe dt d dt dW dt dH AV    )2/( )2/( 2 2 EnergyMethod • Energy Eq. 0 hw a vm dt dh E  nRsH G .,0;0 consthV    CV wu de dt d dt dH  GHR dt dH sn  GHR sn 
• 10.
  Energy Method • EnergyEq. for Water in CV Assume: 1. Constant temp of water in CV 2. Change of heat is change in internal energy of water evaporated hw a vm dt dh E  nRsH G vvml dt dH  GHR dt dH sn  GHRml snvv  AEm w  GHR Al E sn wv   1 Recall: wv n r l R E   Neglecting sensible and ground heat fluxes
• 11.
  Wind as aFactor in Evaporation • Wind has a major effect on evaporation, E – Wind removes vapor-laden air by convection – This Keeps boundary layer thin – Maintains a high rate of water transfer from liquid to vapor phase – Wind is also turbulent • Convective diffusion is several orders of magnitude larger than molecular diffusion
• 12.
  Aerodynamic Method • Includetransport of vapor away from water surface as function of: – Humidity gradient above surface – Wind speed across surface • Upward vapor flux • Upward momentum flux nR E Net radiation Evaporation Air Flow 12 21 zz qq K dz dq Km vv wa v wa     12 12 zz uu K dz du K mama        12 21 uuK qqK m m vvw   
• 13.
  Aerodynamic Method • Log-velocityprofile • Momentum flux nR E Net radiation Evaporation Air Flow    12 21 uuK qqK m m vvw           oZ Z ku u ln 1 *     2 12 12 ln         ZZ uuk a      2 12 12 2 ln 21 ZZK uuqqkK m m vvaw     Thornthwaite-Holzman Equation u Z
• 14.
  Aerodynamic Method • Oftenonly available at 1 elevation • Simplifying nR E Net radiation Evaporation Air Flow      2 12 12 2 ln 21 ZZK uuqqkK m m vvaw     uqv and     2 2 2 2 ln 622.0 o aasa ZZP ueek m     AEm w  aasa eeBE    2 2 2 2 ln 622.0 ow a ZZP uk B    2@pressurevapor Zea 
• 15.
  Combined Method • Evaporationis calculated by – Aerodynamic method • Energy supply is not limiting – Energy method • Vapor transport is not limiting • Normally, both are limiting, so use a combination method ar EEE         wv n r l R EE    aasa eeBEE  wv hp Kl pKC 622.0  2 )3.237( 4098 T e dT de ss   rEE    3.1 Priestly & Taylor
• 16.
  Example – Elev =2 m, – Press = 101.3 kPa, – Wind speed = 3 m/s, – Net Radiation = 200 W/m2, – Air Temp = 25 degC, – Rel. Humidity = 40%, • Use Combo Method to find Evaporation kJ/kg244110)25*36.22500( 237010501.2 3 6   x Txlv mm/day10.7 997*102441 200 3  xl R E wv n r 
• 17.
  Example (Cont.) – Elev= 2 m, – Press = 101.3 kPa, – Wind speed = 3 m/s, – Net Radiation = 200 W/m2, – Air Temp = 25 degC, – Rel. Humidity = 40%, • Use Combo Method to find Evaporation   mm/day45.7 )day1/s86400(*)m1/mm1000(*126731671054.4 11    xEa       sm/Pa1054.4 1032ln997*3.101 3*19.1*4.0*622.0 ln 622.0 11 24 2 2 2 2 2    x xZZP uk B ow a   Pa3167ase Pa12673167*4.0*  asha eRe
• 18.
  Example (Cont.) – Elev= 2 m, – Press = 101.3 kPa, – Wind speed = 3 m/s, – Net Radiation = 200 W/m2, – Air Temp = 25 degC, – Rel. Humidity = 40%, Pa/degC1.67 102441*622.0 103.101*1005 622.0 3 3  x x Kl pKC wv hp  • Use Combo Method to find Evaporation Pa/degC7.188 )253.237( 3167*4098 2    738.0    mm/day2.745.7*262.010.7*738.0       ar EEE    262.0   
• 19.
  Example – Net Radiation= 200 W/m2, – Air Temp = 25 degC, • Use Priestly-Taylor Method to find Evaporation rate for a water body rEE    3.1 Priestly & Taylor mm/day10.7rE 738.0    mm/day80.610.7*738.0*3.1 E
• 20.
  Evapotranspiration • Evapotranspiration – Combinationof evaporation from soil surface and transpiration from vegetation – Governing factors • Energy supply and vapor transport • Supply of moisture at evaporative surfaces – Reference crop • 8-15 cm of healthy growing green grass with abundant water – Combo Method works well if B is calibrated to local conditions
• 21.
  Potential Evapotranspiration • Multiplyreference crop ET by a Crop Coefficient and a Soil Coefficient rcs ETkkET  3.10.2 t;CoefficienCrop   c c k k 10 t;CoefficienSoil   s s k k ETActualET ETCropReferencerET http://www.ext.colostate.edu/pubs/crops/04707.html CORN 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 20 40 60 80 100 120 140 160 Time Since Planting (Days) CropCoefficient,kc