MODUL #3-EVAPOTRANSPIRASI HIDROLOGI DASAR GEL 1301 – 4 SKS
JURUSAN GEOGRAFI LINGKUNGAN
FAKULTAS GEOGRAFI UNIVERSITAS GADJAH MADA SEMESTER GENAP 2008
KLASIFIKASI IKLIM
Empat faktor penggerak cuaca/iklim Dinamika atmosfer Radiasi dari matahari Air di udara → kelembapan dan hujan Rotasi bumi dan topografi
Hal yang perlu diperhatikan
Tujuan klasifikasi iklim dibuat untuk : pertanian, kelautan, penerbangan dll Luas cakupan wilayah klasifikasi iklim : makro, meso, dan mikro. Latar belakang pembuatan klasifikasi iklim
Tipe Iklim di Indonesia
Koppen digunakan untuk iklim pada tumbuhan/vegetasi Schmidth-Ferguson digunakan untuk iklim kehutanan dan perkebunan. Oldeman digunakan untuk iklim lahan pertanian pangan.
A Climates (Humid Equatorial / Tropical) 36% of the Earth's total surface Coolest month must be warmer than 64.4 degrees F or 18 degrees C and Precipitation exceeds evaporation
Tipe : Af: all months receive >2.4 in. precip.(Tropical Rain Forest) Am: a short dry season with at least 1 month < 2.4 in. precip. (Tropical Monsoon) Aw: summer wet, winter dry (Tropical Savanna)
B Climates (Arid and Semi-Arid) 35% of Earth's land surface Evaporation exceeds precipitation
Tipe : BW: desert / precipitation < 1/2 evaporation BS: semi arid / steppe / precipitation > 1/2 evaporation
C Climates (Humid Temperate) 27% of Earth's total surface area 55 % of world's population Warmest month > 50 degrees F Coldest month > 32 degrees F but < 64.4 degrees F
Tipe : Cf: no dry season / all months > 1.2 in. precip. Cw: winter dry period Cs: Dry summer / "Mediterranean"
D Climates (Humid Cold) 21% of Earth's land surface (7% total surface) warmest month > 50 deg F coldest month < 32 deg F great variability in temperature snow climates only in mountains in the southern hemisphere
Tipe : Df: no dry period Dw: dry winter
E Climates (Polar Climates) warmest month below 50 degrees F cold, ice climates tundra and ice cap
H Climates (Undifferentiated Highland) mountainous areas
Data dasar : hujan Q = Rasio jumlah bulan kering/jumlah bulan basah Kriteria Mohr 0 ≤ Q < 0,143 sangat basah 0,143 ≤ Q < 0,333 basah 0,333 ≤ Q < 0,600 agak basah 0,600 ≤ Q < 1,000 sedang 1,000 ≤ Q < 1,670 agak kering 1,670 ≤ Q < 3,000 kering 3,000 ≤ Q < 7,000 kering Q ≥ 7,000 luar biasa kering
Iklim A: lebih dari 9 bulan basah berturutan (penanaman tanaman pangan sepanjang tahun) Iklim B: 7-9 bulan basah berturutan (dua kali tanam setahun) Iklim C: 5-6 bulan basah berturutan (satu kali tanam dalam setahun) Iklim D: 3-4 bulan basah berturutan (satu kali tanam dalam setahun) Iklim E: kurang dari 3 bulan basah berturutan
11-12 bulan basah berturutan (penanaman tanaman pangan sepanjang tahun) 9-10 bulan basah berturutan (perencanaan teliti untuk penanaman sepanjang tahun) 6-8 bulan basah berturutan (dua kali tanam dalam setahun) 3-5 bulan basah berturutan (satu kali tanam dalam setahun) Kurang dari 3 bulan basah berturutan (tidak sesuai untuk tanaman pangan tanpa penambahan sumber air )
Pengertian Evapotranspirasi
Evapotranspiration is the combined effect of evaporation of water from moist soil and transpiration of water by a growing crop (Kijne, 1974) Potential Evapotranspiration (Ep), is the maximum amount of vapour which could be transferred from an area to atmosphere under the existing meteorological condition Actual Evapotranspiration (Ea), is the maximum amount of vapour which could be transferred from an area to atmosphere which depends not only on existing meteorological condition, but also on the availability of water to meet atmospheric demand and, in the case of vegetation, its ability to extract moisture from the soil
Often, scientists distinguish between two different aspects of evapotranspiration: potential evapotranspiration and actual evapotranspiration. Potential evapotranspiration or PE is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply. Actual evapotranspiration or AE is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration. Scientists consider these two types of evapotranspiration for the practical purpose of water resource management. Around the world humans are involved in the production of a variety of plant crops. Many of these crops grow in environments that are naturally short of water. As a result, irrigation is used to supplement the crop's water needs. Managers of these crops can determine how much supplemental water is needed to achieve maximum productivity by estimating potential and actual evapotranspiration. Estimates of these values are then used in the following equation: crop water need = potential evapotranspiration - actual evapotranspiration
The following factors are extremely important in estimating potential evapotranspiration: •Potential evapotranspiration requires energy for the evaporation process. The major source of this energy is from the sun. The amount of energy received from the sun accounts for 80 % of the variation in potential evapotranspiration. •Wind is the second most important factor influencing potential evapotranspiration. Wind enables water molecules to be removed from the ground surface by a process known as eddy diffusion. •The rate of evapotranspiration is associated to the gradient of vapor pressure between the ground surface and the layer of atmosphere receiving the evaporated water.
Faktor-faktor yang berpengaruh (Ward, 1967, Priciples of Hydrology) 1.
2.
3.
Faktor-faktor meteorologi: Radiasi matahari, Suhu udara dan permukaan, Kelembaban, Angin, Tekanan Barometer Faktor-faktor geografi: Kualitas air, Jeluk tubuh air, Ukuran dan bentuk permukaan air. Faktor-faktor lainnya: Kandungan lengas tanah, Karakteristik kapiler tanah, Jeluk muka airtanah, Warna tanah, Tipe kerapatan dan tingginya vegetasi, Ketersediaan air.
insolasi
Suhu udara
The Hydrologic Cycle
Infiltration = Groundwater System
Runoff = Surface Water System Runoff = Precipitation - Evapotranspiration
Perhitungan/Penaksiran Evapotranspirasi
Metoda Neraca P + I + Gi = Ea + O + Go + ∆S Rumus Penman: Eo = [10{(R∆)/L} + ðE2] / [∆+ð] Rumus-rumus Empiris: (Nomograf) a. Thornthwaite: Ep* = 16 [(10 T)/(I)]a b. Blaney and Criddle: U = Kb F; U = ΣKb f ; f = TP/100 c. Turc and Langbein: Ea = P / [ 0,9 + { (P2) / {L(T)}2}] (1/2)
Perhitungan Evapotranspirasi
Tugas II (Kelompok)
Tugas II-a: Gunakan salah satu data Tugas I dari kelompok anda dan tentukan apa tipe iklim dari DASnya (Koppen, Schmidt-Fergusson, Oldeman) Tugas II-b: pilih salah satu Metoda: Penman, Blaney-Criddle atau Thornthwaite Cari data untuk menghitung Evaporasi atau Evapotranspirasi suatu wilayah, boleh melalui internet atau hasil penelitian dan gunakan pula Nomograph yang ada untuk membandingkan hasil perhitungannya Jawaban diserahkan tanggal 19 Maret 2008.
