REFERENCE EVAPOTRANSPIRASTION (ETo)

1. Penman MethodClimatic 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 – W) . F (U). (ea – ed)]

Where ;(ea – 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 – 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 oC Table 9 38.9 mbared ea . RH/100 calc 21.4 mbarea – ed calc 17.5 mbarf(U) 0.27(1+U/100) ; U = 232 km/day calc 0.9Ra 30 oN, July Table 10 16.8 mbarN 30 oN, July Table 11 13.9 hour/dayRs (0.25+0.50 n/N)Ra calc 11.2 mm/dayRnl f(T).f(ed).f(n/N) Table 12,13,14 1.8 mm/dayRn 0.75Rs – Rnl calc 6.6 mm/dayW T = 28.5 oC ; 95m Table 15 0.77c Rhmax 80% ; Rs 11.2; Uday/Unight 1.5 Table 16 1.01ETo c[W.Rn + (1-W).f(U).(ea-ed)] calc 8.8 mm/day

2. Radiation methodClimate 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 – 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 :Ra 30 oN ; July Table 10 16.8 mm/dayN 30 oN ; July Table 11 13.9 mm/dayRs (0.25 + 0.50 n/N) Ra calc 11.2 mm/dayW T = 28.5 oC ; 95m Table 15 0.77 W.Rs calc 8.6 mm/dayETo RH 55 %; U = 2 to 5 m/sec Fig.1 8.4 mm/day

3. Pan Evaporation MethodData 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 pank 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; cropped Table 17 0.75ETo kpan. Epan calc 8.5

mm/day

Pengukuran Evaporasi secara LangsungContoh :Pan Evaporimeter berupa bak atau panci yang mempunyai ukuran tertentu (tinggi ± 2,5 dm dan diameter ± 12,2 dm) yang terbuat dari bahan plat besi atau seng, yang didalamnya terdapat STILL WELL dengan dilengkapi jarum penunjuk, gambar.

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Keterangan :1. Bak/Panci2. Still Well3. Jarum Penunjuk4. Beton Penyagga

Cara Pengamatan :a. Bila tidak terjadi hujan, angka penguapan dapat langsung

diukur dengan cara penambahan air setinggi jarum penunjuk dimana sebelumnya (waktu pagi) air selalu tepat pada jarum penunjuk

b. Bila terjadi hujan tapi belum sampai melampui jarum penunjuk, maka penambahan air tetap diberikan hingga tepat di permukaan jarum. Dan angka penguapan : jumlah curah hujan di tembah penambahan air.

c. Jika air melampui batas jarum, maka penguapan sama dengan curah hujan dikurangi dengan pengurangan air hingga batas jarum penunjuk

d. Bila hujan sangat lebat sehingga air tumpah dari bak, maka penguapan tidak bisa dihitung, sehingga perlu digunakan alat-alat lain seperti Piche.

CROP COEFFICIENT (kc)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

MAXIMUM EVAPOTRANSPIRATION (ETm)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. EtoExampleGiven : Climate data collected at station located within an irrigated area :

May June July AugustETo mm/day 8.9 9.4 8.8 7.6kc 0.4 0.75 1.15 0.85CalculationETm mm/day 3.6 7.1 10.1 6.5 mm/month 110 212 314 201 840