CE 374 K Hydrology. Evaporation. Daene C. McKinney

Transcription

1 C 374 Hydrology aporation Daene C. Mcinney

2 aporation Terminology aporation: liquid ater pae directly to the apor phae Tranpiration: liquid ater pae from liquid to apor through plant metabolim Sublimation: ater pae directly from the olid phae to the apor phae

3 Factor Influencing aporation nergy upply for aporization (latent heat) Solar radiation Tranport of apor aay from eaporatie urface Wind elocity oer urface Specific humidity gradient aboe urface Vegetated urface Supply of moiture to the urface apotranpiration (T) Potential apotranpiration (PT) moiture upply i not limited

4 aporation from a Water Surface Senible heat to air H Net radiation R n Vapor flo rate m& ρ A ρ a National Weather Serice Cla A type Intalled on a ooden platform in a gray location Filled ith ater to ithin.5 inche of the top aporation rate i meaured by manual reading or ith an analog output eaporation gauge CS dh Area, A ρ G Heat conducted to ground h

5 Method of timating aporation nergy method Aerodynamic method Combined method

6 nergy Method Continuity of Liquid Phae H R n m& ρ a m& d ρ d + CV ρ V CS da dh ρ G h dh ρ A dh 0 No flo of liquid ater through CS m& ρ A

7 nergy Method () Vapor Phae - Continuity m& d q ρ d + CV a q CS ρ V a da H R n m& 0 ρ a m& q CS ρ V da a ρ A Steady flo of air oer ater dh ρ h ρ A q CS ρ V a da G ρ A q CS ρ V a da

8 nergy Method (3) nergy q. H R n m& dh 0 dw d ( e + ( e + V / + gz) ρd CV u u + V / + gz) ρv r d CS A r 0; V 0, h cont. dh ρ a ρ h dh d e u ρ d CV R n H G G dh R n H G

9 nergy Method (4) nergy q. for Water in CV dh Rn H G Aume:. Contant temp of ater in CV. Change of heat i change in internal energy of eaporated ater dh l m & H R n dh m& ρ a ρ G h l m& R n H G Recall: m& ρ A l ρ A ( R H G) n r Rn l ρ Neglecting enible and ground heat fluxe

10 Aerodynamic Method Include tranport of apor aay from ater urface a function of: Humidity gradient aboe urface Wind peed acro urface Upard apor flux dq q m& ρ a ρa dz z Upard momentum flux du τ ρam ρam dz u z u z q z m& τ R n Air Flo aporation Net radiation ( q q ) m ( u u )

11 m& Aerodynamic Method () τ ( q q ) m ( u u ) R n Net radiation Log-elocity profile u τ ρ a Z ln k Z Momentum flux m& τ k ρ a ρ k ln a m o ( u u ) ( Z Z ) ( q q )( ) u u [ ln( Z Z )] Thornthaite-Holzman quation Air Flo aporation Too many ariable! Often only kno q and u at eleation

12 Aerodynamic Method (3) Simplify R n Net radiation a B ( e e) B Pρ 0.6k ρ u [ ln( Z Z )] a o Air Flo aporation e apor preure e at. apor preure

13 Combined Method aporation could be calculated by Aerodynamic method: hen energy upply i not limiting nergy method: hen apor tranport i not limiting Normally, both are limiting, o ue a combination method

14 Combined Method (Cont.) Combining nergy balance Aerodynamic Method Combined Method Δ γ r + a Δ + γ Δ + γ γ C p r 0.6l R n l ρ a h p B ( e e) 4098e Δ ( T ) Well uited to mall area ith detailed data Net Radiation Air Temperature Humidity Wind Speed Air Preure. 3 Δ Δ + γ r Prietly & Taylor

15 xample Ue Combo Method to find aporation z m P 0.3 kpa u 3 m/ R n 00 W/m T 5 degc R h 40% l r.50x T (500.36*5) x0 R 00 n l ρ x * mm/day 3 44 kj/kg

16 xample (Cont.) Ue Combo Method to find aporation z m P 0.3 kpa u 3 m/ R n 00 W/m T 5 degc 0.6k ρ u 0.6*0.4 *.9*3 4.54x0 RB h 40% a Pρ [ ln( )] ( 4 ) Z Zo 0.3*997 ln 3x0 e e 367Pa R h * e 0.4* Pa [ ] m/pa a 4.54x0 7.45mm/day ( ) *(000 mm /m)*(86400 /day)

17 xample (Cont.) Ue Combo Method to find aporation z m P 0.3 kpa u 3 m/ R n 00 W/m T 5 degc R h 40% C ph p 005*0.3x0 γ 0.6l 3 0.6* 44x0 4098e Δ ( T ) Δ Δ + γ *367 ( ) 3 γ 0.6 Δ + γ 67.Pa/degC 88.7 Pa/degC Δ γ r + Δ + γ Δ + γ a 0.738* * mm/day

18 xample Ue Prietly-Taylor Method to find aporation rate for a ater body z m P 0.3 kpa u 3 m/ R n 00 W/m T 5 degc R h 40%. 3 Δ Δ + γ r Prietly & Taylor Δ r 7.0 mm/day Δ + γ.3*0.738* mm/day

19 apotranpiration apotranpiration Combination of eaporation from oil urface and tranpiration from egetation Goerning factor nergy upply and apor tranport Supply of moiture at eaporatie urface Reference crop 8-5 cm of healthy groing green gra ith abundant ater Combo Method ork ell if B i calibrated to local condition

20 Potential apotranpiration Multiply reference crop T by a Crop Coefficient and a Soil Coefficient T Actual T T r Reference Crop T k c Crop Coefficient; 0. k k 0 k c T k.3 Soil Coefficient; k c T r Crop Coefficient, kc CORN Time Since Planting (Day)

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22 Combined Method aporation could be calculated by Aerodynamic method: hen energy upply i not limiting nergy method: hen apor tranport i not limiting Normally, both are limiting, o ue a combination method Senible heat flux i difficult to etimate ( ) Aume it i proportional to the apor heat flux H β lm& Where β Boen ratio nergy balance equation (G0) Rn H G lρ A R l m& + β n ( ) ( )

23 Combined Method () Tranport equation for heat and apor dz dq H ρ a dz dt C m h p ρ a & ( ) ) ( h p q q T T C m H & ( ) ) (.6 0 e e l T T p C h p β ( ) m l H & β p e q 6 0. h p l p C 0.6 γ ( ) ( ) e e T T γ β h

24 Recall Vapor Preure Vapor Preure R h e e Saturate apor preure for gien temperature e e Saturation Vapor Preure Vapor Preure De Point temp, Td Meaured temp, T Temperature 7.7T e 6exp T Δ de dt 4098e ( T )