hydrology - eng.uowasit.edu.iq · ali abdul-hussein abed 1 evaporation evaporation: - is the net...

Asst. Lec. Ali Abdul-Hussein Abed

Fourth year

HYDROLOGY Evaporation

Ali Abdul-Hussein Abed

1

Evaporation

Evaporation: - is the net rete of vapor transport to the atmosphere.

The change in state requires an exchange of approximately 600 Cal for each gram of

water evaporated

Factors affecting Evaporation:

1- Wind.

2- Relative humidly

3- Temperature.

4- Solar radiation.

5- Atmosphere pressure.

6- Water quality.

7- Vapor pressure.

Transpiration:-

That portion (largest) of water which is absorbed by the root system to plants and

leaves the plant tissues.

Measurement of transpiration:-

It is measured by placing one or more potted plants a close container and register

the increase in moisture content of the space the instrument is called phytometer.

Evapotranspiration:-

Evapotranspiration =evaporation of moister water, soil, vegetation that may cover

the land and other surface + transaction from plant.

Consumptive use (CU): - total evaporation from an area + water used directly in

building plant tissue.

Potential Evapotranspiration: - The water loss which will occur if at no time there is

deficiency of water in the soil for the use of vegetation.

Irrigation water requirement (water duty):-

Cu + expected precipitation + delivery losses.


2

Estimating of Evaporation form water surface (Reservoirs, lakes,

river channel, etc.)

A- Pan Evaporation

Class A

Class A pan is most widely used evaporation pan.

It is of unpainted galvanized iron 122-cm in dimeter by 25.4 cm deep put in wood

from to let air circulate beneath the pan.

It is filled to depth of 20 cm and refilled when the depth has fallen to 18 cm.

Water – surface level is measured daily with a hook gage in stilling well the

difference in water level indicates evaporation.

Young pan

The Young pan is (61) cm in diameter and (91.5) cm deep and is covered with (6)

mm hardware cloth, the screen modifies the pan coefficient to near unity.

Colorado pan

The Colorado pan is (91.5 cm) square and (46 cm) deep .

Floating pan

Floating pan is 90cm diameter and 45 cm deep.

The evaporation from a pan floating in a lake more nearly approximates evaporation

from the lake than that from an on-shore installation.

Pan coefficient

The pan evaporation data have to be corrected to obtain the actual evaporation from

water surfaces of lakes and reservoirs, i.e., by multiplying by a coefficient called pan

coefficient and is defined as

Pan coefficient = (Lake evaporation/ Pan evaporation).

And the experimental values for pan coefficients range from 0.67 to 0.82 with an

average of 0.7

E (mm/day) = EP (mm/day) * Pan coef.


3

B- Water –budget determinations of reservoir evaporation:

Evaporation can be measured from the continuity equation:

gO -O -S2) + I + P-E= (S1

E: Evaporation

S1,S2: Storage before and after the period

I: Surface inflow

O: Surface out flow

: Subsurface seepage gO

P: precipitation

This is simple in theory but difficulties arise in the measurements of ( P.I.O.S2).

Subsurface seepage is most difficult to measure because it has to be measured from

measurements of ground water levels.

C- Empirical Equations for evaporation

There are several equation for estimation of evaporation rate, most of that equations

depend on Dalton’s law that shown below.

E = C (ew – ea) F (u)

Where,

E = daily evaporation (mm/day)

ew = saturated vapour pressure at the temperature of water (mm mercury) table( )

ea = vapour pressure of the air (about 2 m above) (mm mercury)

F (u) = wind speed function

C, = constant


4

1- Meyer's formula

E = K (ew – ea) (1 + U9/16)….. ea= RH% * ew

E = daily evaporation (mm/day)

U9= wind speed (km/hr.) at height 9 m above ground.

K= 0.36 for large reservoirs and lakes, 0.5 for small reservoir.

Example: - Water Lake with area of 300 hectare, average temperature =25 c,

RH=50%, wind speed at 3 m height =25 km/hr., Saturation vapour pressure =17.5

mmhg.assume (k=1/7) .

Req: - 1- Average daily water Evaporation. 2- Weekly Evaporation losses.

Sol/

es

eRH %

Hgmm

e

.5.175.0

Hgmme .75.8

)2(.NoStep متر من سطح االرض 9ايجاد سرعة الرياح على ارتفاع

)2.........(........................................* 7

1

hCUh

7

1

3*/25 mChrkm

37.21C

7

1

9*37.219 U

hrkmU /25.299

35.0K هكتار تعتبر من المساحات الكبيرة ولهذا سوف نستخدم الثابت االتي 033منطقة ذات مساحه

)4(.NoStep ايجاد قيمة معدل التبخر اليومي من سطح البحيرة

)1...(....................).........16

1)(( 9UeesKEt

)16

/25.291)(75.85.17(35.0)/(

hrkmmmHgmmHgdaymmEt

)/(66.8 daymmEt

)2(..NoREQحجم الماء المفقود بالتبخر خالل اسبوع ايجاد


5

)(*10000)( 2 hectarAreamArea

)(300*10000)( 2 hectarmArea

2610*0.3 mArea

tAreaEtVolume **

week

day

day

mmmVolume 7*66.8*10*0.3 26

week

mVolume

3

181860

2- Jensen- Haise's formula

)4......(..............................)( RSTxTaCTEtp

CT=temperature coefficient.

Ta= mean temperature in EF.

TX =the intercept of the temperature axis

Rs= measured global solar radiation in Cal/cm2/day

)5.......(..............................*21

1

CHCCCT

)6...(..........).........305

*2(381 0 ELCC

)7(..................................................6.72 0CC

)8.....(........................................12

50

ee

mbarCH

)9.......(....................)*69.035.0( RaSRS

)10........(..................................................N

nS

)11.........(..........550

)12(14.05.2EL

eeTX


6

)12......(....................min*51.0595 T

)13()........./(.10

)/( daylangleyEtpdaymmEtp

Example: - Catchment area with elevation 250 meter USL with lat.36 degree north,

its meteorological data given in table below

Req. 1:- Dailey evapotranspiration in (mm\day) for month of jan.by using Jension –

Hais method.

From table (1) t min=2.5, t max=12.8

From table (2), and according to temperature in table (1)

, the saturation vapor pressure as show below:-

[email protected] cTambare

[email protected] cTambare

12 eee

32.78.14 e

mbare 48.7

)8.....(........................................12

50

ee

mbarCH

32.78.14

50

mbarCH

68.6CH

)6...(..........).........305

*2(381 0 ELCC

)305

250*2(381 0CC

36.361C

6.72 C

)5.......(..............................*21

1

CHCCCT

68.6*6.736.36

1

CT

114.0CT


7

)3(.NoStep نوجد قيمة الحرارة عند التقاطع مع المحور الممثل للحرارة من خالل المعادلة

)11.(....................550

)12(14.05.2EL

eeTX

550

250)32.78.14(14.05.2 TX

00.4TX

From table (3) solar radiation

daycm

calRa

*34535@

2

daycm

calRa

*2.12

5

6020

daycm

calRa

*8.3322.12345

2

)5(.NoStep لى اقصى ساعات اضاءة ا ساعات االضاءة الفعلية ما بيننوجد النسبة

N

nS

10

7.4S

47.0S

)6(.NoStep نوجد مقدار االشعاع الشمسي الساقط النهائيRS من خالل المعادلة

.......()....................)*69.035.0( RaSRS

8.332)47.0*69.035.0( RS

daycm

calRS

*4.224

2

)7(.NoStep نوجددددد مقدددددار التبخددددر والنددددتح الكلددددي مددددن خددددالل معادلددددة جنسددددن هددددي االتيددددة بوحدددددات

)/( daylangley

......()....................)( RSTxTaCTEtp

4.244*)0.40.7(011.0 Etp

daylagEtp /4052.7


8

)8(.NoStep نجدددول مقددددار التبخدددر والندددتح الكلدددي مدددن خدددالل معادلدددة جنسدددن هدددي االتيدددة مدددن وحددددات

)/( daylangley الى وحدات)/( daymmEtp

......()....................*51.0595 Tmean

00.7*51.0595

43.591

())........./(.10

)/( daylangleyEtpdaymmEtp

)/(405.7*43.591

10)/( daylangleydaymmEtp

)/(475.0 daymmEtp .

3- Hargreaves's formula

.....()....................)78.17(0135.0. RSTaEtg

.()........................................)(1.0 2

1

SRaRS

..()...............................2.1)/( EtgdaymmEtp

Penman's formula -4

.()..............................**

.

EaQnPET

()..........).........*9.01.0(*)092.056.0(*)*)(1( 4

N

neaTa

N

nbaRaQn

.()..............................273)()( 0 CTkelvenT

....()..........).........)(160

1(35.0 2 eaewU

Ea

E=evaporation (mm/day).

γ = factor depends on atmosphere pressure. (0.49 mmhg/C0)

Qn= net radiation , mm /day.


9

Ra = the solar radiation ,mm/day.

Ea= function of wind speed and deficit in the saturation.

∆= slope vapour pressure (mmhg /C0) table (1)

α = albedo (0.05 for water).table(4)

cos*29.0a a = constant depend on lat.

b= constant (0.52)

n/N = the ration between actual sunshine hours and possible sunshine hours

= Stefan-Boltzmann constant (2*10-9mm/day)

Ta = mean temp. of air (kelvin)

ew = saturated vapour pressure at the temperature of water (mm mercury) ,table (1)

ea = actual vapour pressure of the air (mm mercury)

U2= mean wind speed (km/day)

Example/

Water reservoir has area 250 ha. at latitude 2804// .with elevation 230 above sea

level ,the meteorological data for January at that area as show below :-

Mean temp. = 19 C0, relative humidity = 75%, average of actual sunshine=9 hrs.

Wind speed at height 2m above ground level= 85 km/hr.

Compute the Potential evaporation by using penman formula.

Sol/ 0/0.1 Cmm mmHgew 5.16From table (4)

daywatermmRa /..501.95),From table (

hrN ...716.10 …… From table (6)

The ration between actual sunshine hours and possible sunshine hours

N

nS

716.10

9S

84.0S

The actual vapour pressure of the air (mm mercury)

ew

eaRH %

Hgmm

e

.5.16%75.0 ……….. Hgmme .38.12

Find constant (a) that depend on latitude


10

cos*29.0a

//0428cos*29.0a

2559.0a

b=0.52, = Stefan-Boltzmann constant (2*10-9mm/day)

Ta = mean temp. of air (kelvin)

.()..............................273)()( 0 CTkelvenT

27319)( kelvenT

kelvenT 292

From table (7) ……..α= 0.25

()..........).........*9.01.0(*)092.056.0(*)*)(1( 4

N

neaTa

N

nbaRaQn

)89.0*9.01.0(*)38.12092.056.0(613.14)82.0*52.02559.0)(25.01(506.9 Qn

99.1Qn mm/day

....()..........).........)(160

1(35.0 2 eaewU

Ea

)38.125.16)(160

851(35.0 Ea

208.2Ea mm/day

Apply the quationPenmans .

γ = factor depends on atmosphere pressure. (0.49 mmhg/C0)

.()..............................**

.

EaQnPET

149.0

)208.2*49.0)99.1*1(.

PET

daymmPET /06.2.


11

البيانات المناخية لمحطة ارصاد جوية( 1)جدول رقم

تفاصيل بيانات االرصاد

الجوية

الشهر

2كانون

.Jan

شباط

.Feb

اذار

.Mar

نيسان

.Apr

ايار

.May

حزيران

.Jun

تموز

.July

اب

.Aug

ايلول

.Sep

1تشرين

Oct

2تشرين

.Nov

1كانون

.Dec

اقصى ساعات اضاءة

(day

hr) 13 10.9 11.9 13.0 13.9 14.5 14.3 13.4 12.4 11.3 10.2 9.7

ساعات االضاءة الفعلية

(day

hr) 4.7 5.9 6.7 8.1 10.0 12.5 12.5 12.0 10.8 8.4 6.5 4.9

إشعاع الموجة القصير اعلى

طبقة االتموسفير

(2*cmday

Cal) 413 545 693 843 941 983 961 887 755 569 466 381

إشعاع الموجة القصير القادم

ن ) المخم2*cmday

Cal) 204 289 375 482 583 679 670 628 525 376 270 195

معدل درجة الحرارة

(0C) 7 8.9 12.3 17.4 24.1 30.5 33.9 33.1 27.9 20.7 13.9 8.0

درجة الحرارة العظمى

(0C) 12.8 15.3 19.0 25.4 32.9 39.6 43.4 43.0 38.7 31.2 22.4 15.0

درجة الحرارة الصغرى

(0C) 2.5 3.5 6.3 10.2 15 19.5 22.9 21.8 15.6 11.4 7 3.2

معدل الرطوبة النسبية

)%( 82 76 71 64 48 31 28 30 37 47 67 79

ضغط البخار

(mbar) 8.3 8.6 10 12.2 13.0 12.1 13.5 13.6 12.3 11.2 10.2 8.7

عجز ضغط البخار

(mbar) 1.8 2.7 4.2 7.2 15.6 30.1 38.3 35.2 23.4 12.3 5.1 2.3

متر 13السرعة على ارتفاع

(sec

m) 2.44 2.66 2.75 2.86 3.06 3.33 3.21 2.97 2.71 2.25 1.91 2.13

التبخر

(month

mm) 62 85 130 189 295 429 564 518 345 192 105 65


12

بين قيم ضغط البخار المشبع فوق الماء لدرجات الحرارة العظمى والصغرى من نوجد الفرق ما

( 2) جدول رقم


13

من خالل الجداول االتية و حسب خطوط العرضRaنوجد مقدار االشعاع الشمسي الساقط

( قيمة االشعاع الشمسي حسب خطوط العرض 0 جدول )

الشهر

خط

العرض

شماال

كانون

2

.Jan

شباط

.Feb

اذار

.Mar

نيسان

.Apr

ايار

.May

حزيران

.Jun

تموز

.July

اب

.Aug

ايلول

.Sep

تشرين

1

Oct

تشرين

2

.Nov

كانون

1

.Dec

03 58 152 319 533 671 763 690 539 377 197 87 35

55 100 219 377 558 690 780 706 577 430 252 133 74

53 155 290 429 617 716 790 729 616 480 313 193 126

55 216 365 477 650 729 797 748 648 527 371 260 190

53 284 432 529 677 742 800 755 674 567 426 323 248

05 345 496 568 700 742 800 761 697 603 474 380 313

03 403 549 600 713 742 793 735 703 637 519 437 371

25 455 599 629 720 742 780 745 703 660 561 486 423

23 500 634 652 720 726 760 729 697 680 597 537 474

15 545 673 671 713 706 733 706 684 697 623 580 519

13 584 701 681 707 684 700 681 665 707 648 617 565

5 623 722 690 700 652 663 645 645 710 663 650 606

3 652 740 694 680 623 627 616 623 707 684 680 619


14

شهرال

خط

العرض

جنوبا

2كانون

.Jan

شباط

.Feb

اذار

.Mar

نيسان

.Apr

ايار

.May

حزيران

.Jun

تموز

.July

اب

.Aug

ايلول

.Sep

تشرين

1

Oct

تشرين

2

.Nov

1انون ك

.Dec

5 648 758 690 663 590 587 577 590 693 690 727 627

01 710 772 681 640 571 543 526 558 680 690 727 710

05 729 779 665 610 516 497 497 519 657 687 747 739

01 748 779 645 573 474 447 445 481 630 677 753 761

05 761 779 629 533 419 400 406 439 600 665 767 777

01 771 772 600 497 384 353 358 390 567 648 767 793

05 774 754 568 453 335 300 310 342 530 629 767 806

01 774 729 529 407 281 243 261 290 477 603 760 813

05 774 704 490 357 229 183 203 235 447 571 747 813

51 761 669 445 307 174 127 148 177 400 535 727 806

55 748 630 397 250 123 77 97 123 343 497 707 794

01 729 588 345 187 77 33 52 74 283 455 700 787

----------------------------------------------------------------------------------------------


15

ted vapour pressure and slope vapour pressure at theTable (4), Satura

Temperature of water

درجة الحرارة

)درجة مئوية(

ضغط البخار

()..( Hgofmmew) )/( 0cmm

1 4.58 0.3

5 6.54 0.45

7.5 7.78 0.54

10 9.21 0.6

12.5 10.87 0.71

15 12.79 0.8

17.5 15.0 0.95

0.91

20 17.54 1.05

22.5 20.44 1.24

25 23.76 1.4

27.5 27.54 1.61

30 31.82 1.85

32.5 36.68 2.07

35 42.81 2.35

37.5 48.36 2.62

40 55.32 2.95

45 71.2 3.66


16

,mm/daySolar Radiation(Ra) )5Table (

ضخط العر

شماال

الشهر

كانون

0

.Jan

شباط

.Feb

اذار

.Mar

نيسان

.Apr

ايار

.May

حزيران

.Jun

تموز

.July

اب

.Aug

ايلول

.Sep

تشرين

0

Oct

تشرين

0

.Nov

كانون

0

.Dec

0 14.5 15.0 15.2 14.7 13.9 13.4 13.5 14.2 14.9 15.0 14.6 14.3

10 12.8 13.9 14.8 15.2 15.0 14.8 14.8 15.0 14.9 14.1 13.1 12.4

20 10.8 12.3 13.9 15.2 15.7 15.8 15.7 15.3 14.4 12.9 11.2 10.3

//0428

30 8.5 10.5 12.7 14.8 16.0 16.5 16.2 15.3 13.5 11.3 9.1 7.9

40 6.0 8.3 11.0 13.9 15.9 16.7 16.3 14.8 12.2 9.3 6.7 5.4

50 3.6 5.9 9.1 12.7 15.4 16.7 16.1 13.9 10.5 7.1 4.3 3.0

Table (6), mean daylight hours (N) for different latitude

MONTH

خط

العرض

شماال

كانون

0

.Jan

شباط

.Feb

اذار

.Mar

نيسان

.Apr

ايار

.May

حزيران

.Jun

تموز

.July

اب

.Aug

ايلول

.Sep

تشرين

0

Oct

تشرين

0

.Nov

كانون

0

.Dec

0 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1

10 11.6 11.8 12.1 12.4 12.6 12.7 12.6 12.4 12.9 11.9 11.7 11.5


17

20 11.1 11.5 12.0 12.6 13.1 13.3 13.2 12.8 12.3 11.7 11.2 10.9

//0428

30 10.4 11.1 12.0 12.9 13.7 14.1 13.9 13.2 12.4 11.5 10.6 10.2

40 9.6 10.7 11.9 13.2 14.4 15.0 14.7 13.8 12.5 11.2 10 9.4

50 8.6 10.1 11.8 13.8 15.4 16.4 16.0 14.5 12.7 10.8 9.1 8.1

Table (7) albedo value

α Surface

0.25-0.15 Green vegetation cover

0.45-0.05 Uncovered land

0.05 Water surface

0.9-0.45 Snow

hydrology - eng.uowasit.edu.iq · ali abdul-hussein abed 1 evaporation evaporation: - is the net...

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