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REFERENCE EVAPOTRANSPIRASTION ( ETo ). Penman Method

REFERENCE EVAPOTRANSPIRASTION ( ETo )

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REFERENCE EVAPOTRANSPIRASTION (ETo)

Penman Method

Climatic data required are : mean teperature (0C), mean relative humidity (RH in %), total windrum ( U in km/day at2 m height) and mean actual sunshine duration (n in hour/day) or mean radiation (Rs or Rn equivalent evaporation in mm/day). Also measured or estimated data in mean maximum relative humidity (RHmax in %) and mean daytime windspeed (Uday in m/sec at 2 m height) must be available.

Eto = c [ W . Rn + ( 1 â€“ W) . F (U). (ea â€“ ed)]

Where ;

(ea â€“ ed) = vapour pressure deficit i.e. the difference between saturation vapour pressure (ea) at Tmean in mbar (Table 9) and actual vapour pressure (ed) in mbar where ed = ea.RH/100

f (U) =wind function of f(U) = 0.27 (1 + U/100) with U in km/day measured at 2 m height

Rn = total net radiation in mm/day or Rn = 0.75Rs â€“ Rnl where Rs is obtained from Rs = (0.25+0.5 n/N)Ra. Ra is extra-terrestrial radiation in mm/day (table 10), n is mean actual sunshine duration in hour/day and N is maximum possible sunshine duration in hour/day (Table 11). Rnl is net longwave radiation in mm/day and is function of temperature, f(T), of actual vapour pressure, f(ed) and sunshine duration, f(n/N), or Rnl = f(T).f(n/N).f(ed) (Table 12, 13, 14)

W =temperature and altitude dependent weighting factor (table 15)

c =adjustment factor for ratio Uday/Unight, for Rhmax and for Rs (Table 16)

Example :

Given : Location 30 oN ; altitude 95 m; July ; Tmean 28.5 oC; Rhmean 55% ; Umean 232 km/day ; n mean 11.5 hour/day ; (Rhmax 80%, Uday 3 m/sec, Uday/Unight 1.5).

Calculation :

ea T = 28.5 oCTable 938.9 mbar

ed ea . RH/100calc21.4 mbar

ea â€“ edcalc17.5 mbar

f(U) 0.27(1+U/100) ; U = 232 km/daycalc0.9

Ra30 oN, JulyTable 1016.8 mbar

N30 oN, JulyTable 1113.9 hour/day

Rs(0.25+0.50 n/N)Racalc11.2 mm/day

Rnlf(T).f(ed).f(n/N)Table 12,13,141.8 mm/day

Rn0.75Rs â€“ Rnlcalc6.6 mm/day

WT = 28.5 oC ; 95mTable 150.77

cRhmax 80% ; Rs 11.2; Uday/Unight 1.5Table 161.01

ETo c[W.Rn + (1-W).f(U).(ea-ed)]calc8.8 mm/day

2. Radiation method

Climate data required are mean temperature (T in oC) and mean actual sunshine duration (n in hour/day) or mean incoming shortware radiation (Rs in mm/day) equivalent evaporation). Estimated values of mean relative humidity (RH in %) nd mean daytime windspeed (Uday in m/sec at 2 m height)

ETo = c (W . Rs)

Where :

Rs= measured mean incoming shortware radiation in mm/day or is obtained from Rs = (0.25 + 0.50 n/N) Ra where Ra is extra â€“ terrestrial radiation in mm/day (Table 10), N is maximum possible sunshine duration in hour/day (table 11) and n is measured mean actual sunshine duration in hour/day

W= temperature and altitude dependent weighting factor (Table 15)

c = adjustment factor made graphically on W. Rs using estimated values of Rhmean and Udaytime (figure 1).

Example :

Given : Location 30 oN, altitude 95 m; July; Tmean 28.5 oC; n mean 11.5 hour/day; Uday mean = moderate (2 to 5m/sec); RHmean = medium (about 55%)

Calculation :

Ra30 oN ; JulyTable 10 16.8 mm/day

N30 oN ; JulyTable 11 13.9 mm/day

Rs(0.25 + 0.50 n/N) Racalc 11.2 mm/day

WT = 28.5 oC ; 95mTable 15 0.77

W.Rscalc 8.6 mm/day

EToRH 55 %; U = 2 to 5 m/secFig.1 8.4 mm/day

3. Pan Evaporation Method

Data required are mean pan evaporation (Epan in mm/day), estimated values mean relative humidity (RH in %) and mean windrum (U in km/day at 2 m height) and information on whether the pan is surrounded by a cropped or dry fallow area.

ETo = kpan. Epan

Where :

Epan = evaporation in mm/day from an unscreened class A evappotarion pan

k pan = pan coeeficient (Table 17)

Example :

Given : July ; E class A pan 11.3 mm/day; Rhmean = medium ; Umean = moderate; pan surronded by cropped area of several hectares.

Calculation :

kpan RH = medium; U = moderate; croppedTable 17 0.75

ETokpan. Epancalc 8.5 mm/day

Contoh :

Pan Evaporimeterberupabakataupanci yang mempunyaiukurantertentu (tinggi Â± 2,5 dm dan diameter Â± 12,2 dm) yang terbuatdaribahan plat besiatauseng, yang didalamnyaterdapat STILL WELL dengandilengkapijarumpenunjuk, gambar.

Keterangan :

Bak/Panci

Still Well

JarumPenunjuk

BetonPenyagga

1

2

3

4

Cara Pengamatan :

- Bilatidakterjadihujan, angkapenguapandapatlangsungdiukurdengancarapenambahan air setinggijarumpenunjukdimanasebelumnya (waktupagi) air selalutepatpadajarumpenunjuk
- Bilaterjadihujantapibelumsampaimelampuijarumpenunjuk, makapenambahan air tetapdiberikanhinggatepatdipermukaanjarum. Dan angkapenguapan : jumlahcurahhujanditembahpenambahan air.
- Jika air melampuibatasjarum, makapenguapansamadengancurahhujandikurangidenganpengurangan air hinggabatasjarumpenunjuk
- Bilahujansangatlebatsehingga air tumpahdaribak, makapenguapantidakbisadihitung, sehinggaperludigunakanalat-alat lain sepertiPiche.

Infromation required on crops is :

- The date of sowing
- The length of total growing season, including
- The duration of initial stage (germination to 10 percent ground cover)
- The duration of crop development stage (from 10 percent to 80 percent ground cover)
- The duration of the mid-season stage (from 80 percent ground cover to start of ripening)
- The duration of late season stage (fro start of ripening to harvest)
General informatio n on crop development stages is given for different crops in Part B. For crop coefficients see Table 18. climate data required for the selection of kc values are windspeed and humidity.

Example :

Given : Maize; planted 1 May; harvested 31 August; initial stage is 20 days ; development stage 35 days; mid-season stage 40 days; late season stage 28 days; windspeed is light to moderate; minimum mean relative humidity is low.

Calculation : From Part B, Maize, or from Table 18:

kc May = 0.4; June = 0.75; July = 1.15; August = 0.85

Data required are monthly mean ETo in mm/day and the kc values for the given crop over each 30 or 10-day periode. Maximum evapotranspiration

ETm = kc. Eto

Example

Given : Climate data collected at station located within an irrigated area :

MayJuneJulyAugust

ETo mm/day 8.99.48.87.6

kc0.40.751.150.85

Calculation

ETm mm/day3.67.110.16.5

mm/month110212314201840