hidrologi kuliah iii (rainfall losses)
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hidrologiTRANSCRIPT
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3. Rainfall Losses
Kind of losses:
1. Interception,
2. Depression storage,
3. Evaporation,
4. Transpiration,
5. Evapotranspiration,
6. Infiltration.
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3.1-2. Interception and
depression storageKind of initial losses
1. Caught by leaf of vegetation àinterception à evaporate, stem-flow, through fall.
2. Depression storage à evaporate, infiltration, overflow, à depend on: soil type, soil moisture, catchment slope, surface condition.
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3.3. Evaporation
Perubahan air (liquid à masa air) menjadi uap
air (gas) à temperatur, tekanan uap, angin, tekanan udara, berat satuan air, panas tersimpan
dalam air à terjadi setiap saat.
Diukur dengan Pan Evaporation (Class A EP,
Collorado Sunken EP)
El = c (ew – ea)
ew = tek uap jenuh, ea = tek uap nyata.
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US Class A Evaporation Pan
Colorado Sunken Evaporation Pan
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3.4. Transpiration
Pelepasan air dari tubuh tanaman à air diambil dari akar dan dilepas melalui daun
untuk kebutuhan hidup tanaman tersebut, àterjadi hanya pada siang hari.
Faktor yang mempengaruhi:
jenis tanaman (jenis akar dan kerapatan
daun), intensitas penyinaran, angin, tekanan
uap air, temperatur.
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3.5. Evapotranspiration
Pelepasan air dari tubuh air (evaporation) dan tanaman (transpiration) pada waktu yang bersamaan à consumptive use.
Jenis: PET (Potential Evapotranspiration), AET (Actual Evapotranspiration).
PET àjumlah air tidak terbatas, AET à jumlah air terbatas.
Istilah yang sangat terkait dengan ET: field capacity (kapasitas lapang) and permanent wilting point (titik layu)
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Alat ukur ET
1. Lysimeter à tangki khusus yang berisi tanah dan jenis tanaman tertentu.
2. Field Plots:
ET = P + IR – RO –Gs ±ΔS
P = precipitation, IR = Irrigation input, RO =
surface runoff, Gs = groundwater loss, ΔS =
increase in soil storage.
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Evapotranspirometer
Lisimeter
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Sumber: Chow et al, 1988
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Empirical Formula
1. Blaney-Criddle,
Et = 2,54 KF,
F = Σ phŤf/100
2. Thornthwaite.
Et = 1,6 La(10T/It)a
Rincian lebih lanjut lihat K Subramanya, 1986,
Engineering Hydrology, page 66-68
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3.6. Infiltration
Peresapan air kebawah permukaan tanah
Dua Parameter pokok dalam peresapan:
1. Kecepatan resapan air dalam tanah à jenis tanah, kepadatan tanah, intensitas hujan, dan lama hujan,
2. Jumlah air yang dapat ditampung (diikat oleh masa tanah) à field capacity.
Alat Ukur: Infiltrometer, Floding Type (Field Plot)
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Kapasitas Infiltrasi (fc)
f = fc à bila i > fc, f = i à bila i < fc,
f = infiltrasi (mm/jam)
Faktor yang mempengaruhi fc: Karakter tanah, permukaan, dan aliran dalam tanah.
ft = fc + (fo – fc)e-Kt (Hortonian’s formula)
à 0 ≤ t ≤ td
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Notasi:
ft = kapasitas infiltrasi pada saat t,
fc = infiltrasi tanah jenuh,
fo = infiltrasi awal,
e = bilangan eksponensial,
K = koefisien pengatusan,
t = waktu,
td = durasi hujan.
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Sumber: Subramanya, 1986
Variation of infiltration capacity
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Terapan
Rainfall losses (interception, depression
storage, evaporation, evapotranspiration,
infiltration)à not effective (excess) rainfall
Effective Rainfall (Re) = Total rainfall (R) –
Rainfall losses (Rl).
c = Re/R = (R – Rl)/R
c = runoff coefficient.
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Ф index and W index
Ф = (R – Re)/te
W = (R – Re – Ia)/te
te = duration of excess rainfall
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Sumber: Subramanya, 1986
Ø Index
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Latihan 2.
Hujan selama 4 jam berturutan sebesar 8, 24, 18,
dan 10 mm. Total volume limpasan pada DAS
adalah 1,5 juta m3. Bila luas DAS adalah 42 km2,
hitung hujan efektif dan indeks Ф. Identifikasikan
pemanfaatan lahan pada DAS tersebut bila
koefisien limpasan untuk: hutan = 0,15 – 0,35; kebun
= 0,35 - 0,52; sawah/ladang = 0,52 – 0,66;
hunian/pemukiman = 0,67 – 0,87; dan lahan
perkerasan = 0,88 – 0,98.
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Latihan 3.
Sebuah waduk mempunyai luas permukaan rerata
20 km2 selama bulan April 2011. Aliran masuk pada
bulan tersebut adalah 10 m3/detik, dan aliran keluar
15 m3/detik, hujan yang terjadi pada bulan tersebut
adalah 24 cm, sedangkan perubahan tampungan
yang terjadi adalah 18 juta m3. Bila rembesan dan
bocoran adalah 2,4 cm, hitung penguapan pada
waduk tersebut.