HIDROLOGI Kuliah III (Rainfall Losses)

3. Rainfall LossesKind of losses:1. Interception, 2. Depression storage, 3. Evaporation, 4. Transpiration, 5. Evapotranspiration, 6. Infiltration.

3.1-2. Interception and depression storageKind of initial losses

1. Caught by leaf of vegetation →

interception → evaporate, stem-flow, through fall. 2. Depression storage →

evaporate,

infiltration, overflow, → depend on: soil type, soil moisture, catchment slope, surface condition.

3.3. EvaporationPerubahan air (liquid → masa air) menjadi uap air (gas) → temperatur, tekanan uap, angin, tekanan udara, berat satuan air, panas tersimpan dalam air → terjadi setiap saat. Diukur dengan Pan Evaporation (Class A EP, Collorado

Sunken EP)

E l

= c (ew

– ea

)ew

= tek uap jenuh, ea

= tek uap nyata.

Colorado Sunken Evaporation Pan

US Class A Evaporation Pan

3.4. TranspirationPelepasan air dari tubuh tanaman → air diambil dari akar dan dilepas melalui daun untuk kebutuhan hidup tanaman tersebut, → terjadi hanya pada siang hari. Faktor yang mempengaruhi: jenis tanaman (jenis

akar dan kerapatan daun), intensitas penyinaran, angin, tekanan uap air, temperatur.

3.5. EvapotranspirationPelepasan air dari tubuh air (evaporation) dan tanaman (transpiration) pada waktu yang bersamaan → consumptive use. Jenis: PET (Potential Evapotranspiration), AET (Actual Evapotranspiration). PET →jumlah air

tidak terbatas, AET → jumlah air terbatas. Istilah yang sangat terkait dengan ET: field capacity(kapasitas lapang) and permanent wilting point (titik layu)

Alat ukur ET1. Lysimeter → 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.

Evapotranspirometer

Lisimeter

iummary of equations for calculating evaporation*

(1) Energy balance method _

E, (10353112n (mm/day)Where

Rn = net radiation (W/mz)

(2) Aerodynamic method

Ea =B(eals _ ea) (mm/day)

u; is wind velocity (m/s) measured at height 22 (cm), and zu is from Table 2.8.2. Also,

T: air temperature (°C)

ea :Rheas in which R,, is the relative humidity (0 E R,, S 1).

Sumber: Chow et al, 1988