TL 2130 Hidrologi
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Transcript of TL 2130 Hidrologi
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TL 2130 Hidrologi
Infiltrasi (Infiltration)
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Infiltration
• General– Process of water
penetrating from ground into soil
– Factors affecting• Condition of soil surface,
vegetative cover, soil properties, hydraulic conductivity, antecedent soil moisture
– Four zones• Saturated, transmission,
wetting, and wetting front
depth
Wetting Zone
TransmissionZone
Transition ZoneSaturation Zone
Wetting Front
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Definitions
solid
Pore withair
Pore withwater
Element of soil, V(Saturated)
Element of soil, V(Unsaturated)n0content;moisturenS
V
V
S0;saturationV
VS
porosityV
Vn
waterofvolumeV
solidsofvolumeV
poresofvolumeV
elementofvolumegrossV
w
v
w
v
w
s
v
1
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• Lapisan Tak Jenuh (Unsaturated zone)= lapisan yang tidak seluruh pori-pori mikro tanah terisi air
• Lapisan jenuh air (saturated zone) = lap yang seluruh pori-pori tanah terendam air Gaya gravitasi dan gaya kapiler
• Porositas (porosity)= prosentase bagian suatu material yang berupa pori-pori terhadap volume totalnya
• Porositas total= vol. pori—pori dalam satu material (berhubungan dan terisolir)
vol. keseluruhan dari material
• Porositas efektif=vol. pori-pori bersambungan atau berhubungan
volume keseluruhan
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• Kapasitas lapang (field capacity/specific retention)= besarnya kandungan air maximum yang dapat ditahan oleh tanah terhadap gaya tarik gravitasi- kerikil : 1 – 10%
- pasir : 5 – 20% - clay : 25 - 50%
Tumbuhan mengisap air dari sumber ini.• Koefisien layu/kelembaban kritik (wilting point)=
kandungan air pada tanah, dengan tumbuh-tumbuhan dalam keadaan layu permanen dan akan mati bila air tidak ditambahkan
• Abstraksi awal (initial abstraction)= jumlah intersepsi dan penampungan cekungan yang harus dipenuhi lebih dahulu, sebelum terjadinya limpasan hujan (overland flow)
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• Soil moisture content= nilai prosentase kebasahan/lembab tanahSMC = kehilangan berat pada waktu tanah mengering
berat tanah dalam keadaan basah
SMC = berat tanah dalam keadaan basah – berat tanah kering
berat tanah dalam keadaan basah
• Available soil moisture (lembab tanah yang tersedia)= selisih antara nilai kebasahan tanah dalam keadaan kapasitas lapangan dan kebasahan tanah dalam keadaan titik layu permanen. Hal ini menggambarkan kebasahan yang dapat disimpan dalam tanah untuk menunjang kelanjutan hidup dari tanaman.
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• Infiltrasi= proses meresapnya air/proses pelaluan air, ke dalam tanah melalui permukaan tanah. Kebalikan: mata air, perembesan (seepage)
• Perkolasi= pergerakan air di dalam tanah melalui soil moisture zone (lingkungan sejumlah kecil air diantara sela-sela tanah yang menyebabkan kebasahan tanah) pada unsaturated zone, sampai mencapai muka air tanah pada saturated zone
Infiltrasi vs perkolasi
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• Kapasitas infiltrasi (infiltration capacity= fp)= kec. infiltrasi maks. yang bisa terjadi kap. pada awal hujan besar mengecil bila profil tanah sudah jenuh (1-2 jam hujan).
• Kecepatan infiltrasi (infiltration rate= fa)= kec. infiltrasi yang terjadi sesungguhnya) Dipengaruhi oleh intensitas hujan dan kapasitas infiltrasifa<fp bila i < fpfa=fp bila i ≥ fp
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• Infiltration rate– Rate at which water enters the soil at the surface
(in/hr or cm/hr)
• Cumulative infiltration– Accumulated depth of water infiltrating during given
time period
t
dftF0
)()(
)(tf
dt
tdFtf
)()(
Infiltrasi
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• Kapasitas perkolasi (percolation capacity=Pp)= kecepatan perkolasi maksimumJumlah perkolasi (mm)= jumlah infiltrasi yang terjadi (mm) – jumlah air yang diperlukan untuk pengisian kelembaban tanah (soil moisture) (mm)
• Kecepatan perkolasi (percolation rate=Pa)= kecepatan perkolasi yang sesungguhnya terjadi. Nilai ini dipengaruhi oleh kecepatan infiltrasi dan kapasitas perkolasi.
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1. Karakteristik hujan: lama hujan, intensitas hujan terjadi pengurangan kapasitas infiltrasi secara konstan, karena:
- pemadatan permukaan tanah
- pembengkakan tanah liat dan humus
- penyumbatan pori-pori oleh partikel kecil
- terjeratnya gelembung-gelembung udara
Faktor-faktor yang mempengaruhi kapasitas infiltrasi
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2. Kondisi permukaan tanah; ada/tidak ada tanaman
3. Karakteristik tanah; tekstur dan strukturTekstur= aspek geometris dari partikel komponen suatu batuan, termasuk ukuran, bentuk dan aturan susunan butir-butir tanahStruktur= perbedaan-perbedaan dari bidang perlapisan yang normal (paralel atau sejajar) proses sedimentasi, tektonik
4. Pengerjaan tanah5. Kondisi klimatologi6. Nilai lembab tanah (soil moisture)7. Kedalaman air yang tertahan di permukaan
tanah (D) serta ketebalan lapisan tak jenuh (L)
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Soil Particle Sizes
Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system.
Name of soil separate
Diameter limits (mm)
Very coarse sand* 2.00 - 1.00
Coarse sand 1.00 - 0.50
Medium sand 0.50 - 0.25
Fine sand 0.25 - 0.10
Very fine sand 0.10 - 0.05
Silt 0.05 - 0.002
Clay less than 0.002
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Soil Texture Triangle
Source: USDA SoilSurvey Manual Chapter 3
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Soil Water Content
TotalVol
VolWater
Soil Water Content
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Infiltration Methods
• Horton and Phillips – Infiltration models developed as approximate
solutions of an exact theory (Richard’s Equation)
• Green – Ampt– Infiltration model developed from an
approximate theory to an exact solution
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Hortonian Infiltration
• Recall Richard’s Equation– Assume K and D are
constants, not a function of or z
• Solve for moisture diffusion at surface
Kz
Dzt
z
K
zD
t
2
2
02
2
zD
t
ktcc effftf )()( 0
f0 initial infiltration rate, fc is constant rate and k is decay constant
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Hortonian Infiltration
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2
Time
Infi
ltra
tio
n r
ate,
f
k1
k3
k2
k1 < k2 < k3
fc
f0
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Philips Equation• Recall Richard’s
Equation– Assume K and D are
functions of , not z
• Solution– Two terms represent
effects of • Suction head• Gravity head
• S – Sorptivity– Function of soil suction
potential– Found from experiment
Kz
Dzt
KtSttF 2/1)(
KSttf 2/1
2
1)(
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Green – Ampt Infiltration
Wetted Zone
Wetting Front
Ponded Water
Ground Surface
Dry Soil
0h
L
n
i
z
LLtF i )()(
dt
dL
dt
dFf
Kz
Kf
fz
hKqz
MoistureSoilInitial
Front WettingtoDepth
i
L
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Green – Ampt Infiltration (Cont.)
• Apply finite difference to the derivative, between – Ground surface– Wetting front
Kz
Kf
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L
i
z0,0 z
fLz ,
KL
KKz
KKz
Kff
0
0
F
L
LtF )(
1
FKf
f
Kz
Kf
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1
LK
dt
dL f
1
FKf
f
dt
dLf
Green – Ampt Infiltration (Cont.)
LtF )(
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L
i
z
L
dLdLdt
K
f
f
CLLtK
ff
)ln(
Integrate
Evaluate the constant of integration
)ln( ffC
0@0 tL
)ln(L
LKtf
ff
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Green – Ampt Infiltration (Cont.)
)ln(L
LKtf
ff
)1ln(f
fF
KtF
1
FKf
f
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L
i
z
Nonlinear equation, requiring iterative solution.
ψ is wetting front soil suction head; θ is porosity; K is Hydraulic conductivity; F is the total volume already infiltrated
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Soil Parameters
• Green-Ampt model requires – Hydraulic conductivity, Porosity, Wetting Front
Suction Head– Brooks and Corey
Soil Class Porosity Effective Porosity
Wetting Front
Suction Head
Hydraulic Conductivity
n e K (cm) (cm/h) Sand 0.437 0.417 4.95 11.78 Loam 0.463 0.434 9.89 0.34 Clay 0.475 0.385 31.63 0.03
re n
ees )1(
e
res
Effective saturation
Effective porosity
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Ponding time
• Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)
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Ponding Time
• Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate)
if ptiF *
1
FKf
f
1
* p
f
tiKi
)( KiiKt
fp
Potential Infiltration
Actual Infiltration
Rainfall
Accumulated Rainfall
Infiltration
Time
Time
Infi
ltra
tion
rate
, f
Cu
mu
lati
ve
Infi
ltra
tion
, F
i
pt
pp tiF *
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Example
• Silty-Loam soil, 30% effective saturation, rainfall 5 cm/hr intensity 30.0
/65.0
7.16
486.0
e
e
s
hrcmK
cm
340.0)486.0)(3.01()1( ees
340.0*7.16
hr17.0))(65.00.5(0.5
68.565.0
)(
KiKiiKt
fp
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Infiltrometer
Single ring infiltrometerSingle ring infiltrometer
Double ring infiltrometerDouble ring infiltrometer
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