MK. PENGELOLAAN SDALH
SWRM:
SUSTAINABLE
WATER RESOURCES
MANAGEMENT
Smno.psdl.pdkl.ppsub2013
Ketika musim hujan tiba, tentunya inilah rahmat yang
dianugerahkan Allah SWT kepada bumi dan seisinya.
Sebagaimana firman-Nya :
“ Dan Dialah yang meniupkan angin sebagai pembawa
berita gembira sebelum kedatangan rahmat-Nya (hujan);
hingga apabila angin itu telah membawa awan mendung,
Kami halau ke suatu daerah yang tandus, lalu Kami
turunkan hujan di daerah itu, maka Kami keluarkan
dengan sebab hujan itu berbagai macam buah-buahan.
Seperti itulah Kami membangkitkan orang-orang yang
telah mati, mudah-mudahan kamu mengambil pelajaran”
(Al-Qur’an Surah Al-A’raf [7]: 57)
Perhatikan pula Surah Al-An’am (6) ayat 6 :
“ Apakah mereka tidak memperhatikan berapa banyaknya
generasi-generasi yang telah Kami binasakan sebelum
mereka, padahal (generasi itu), telah Kami teguhkan
kedudukan mereka di muka bumi, yaitu keteguhan yang
belum pernah Kami berikan kepadamu, dan Kami curahkan
hujan yang lebat atas mereka dan Kami jadikan sungaisungai mengalir di bawah mereka, kemudian Kami binasakan
mereka karena dosa mereka sendiri, dan kami ciptakan
sesudah mereka generasi yang lain”
Water resources are sources of water that are useful or
potentially useful to humans.
Uses of water include agricultural, industrial, household, recreational and
environmental activities. Virtually all of these human uses require fresh water.
Fresh water is a renewable resource, yet the world's supply of clean, fresh water
is steadily decreasing. Water demand already exceeds supply in many parts of
the world and as the world population continues to rise, so too does the water
demand.
Awareness of the global importance of preserving water for ecosystem services
has only recently emerged as, during the 20th century, more than half the
world’s wetlands have been lost along with their valuable environmental
services.
Biodiversity-rich freshwater ecosystems are currently declining faster than
marine or land ecosystems.
The framework for allocating water resources to water users (where such a
4
framework exists) is known as water rights.
Water management is the activity of planning, developing,
distributing and optimum use of water resources under
defined water polices and regulations.
Hal ini berarti bahwa:
Management of water treatment of drinking water, industrial water,
sewage or wastewater
Management of water resources
Management of flood protection
Management of irrigation
Management of the water table.
DAUR HIDROLOGI
CURAH HUJAN
intersepsi
VEGETASI
PERMUKAAN TANAH
Aliran permukaan
infiltrasi
AIR TANAH
banjir
Aliran bawah permukaan
perkolasi
CADANGAN
SALURAN
Aliran air bawah tanah
transpirasi
AIR BAWAH TANAH
evaporasi
EVAPOTRANSPIRASI
KEBOCORAN
LIMPASAN
6
CURAH HUJAN:
Air cair yg turun dari atmosfer
ke permukaan dinyatakan sbg
kedalaman air pd permukaan
mendatar (rainfall)
Presipitasi:
Banyaknya curahan pd
permukaan mendatar
selama sehari, sebulan
atau setahun, yg
digunakan unt menyatakan
curahan hatrian, bulanan
atau tahunan
7
Kedalaman perakaran
Soil water stored in deep
layers can be used by the
plants only when roots
penetrate to that depth.
The depth of root
penetration is primarily
dependent on the type of
crop, but also on the type
of soil.
The thicker the rootzone,
the more water available to
the plant.
HUJAN:
Curahan berupa air semua
ukuran, baik yg berbentuk
tetes yg bergaris tengah lebih
dari 0.5 mm maupun yg lebih
kecil (rain)
HUJAN es:
Curahan berupa bola kecil
atau butiran es yg bergaris
tengah antara 5 dan 50 mm,
kadang-kadang lebih,
jatuhnya secara terpisahpisah atau bergabung
menjadi gumpalan yg
bentuknya tak teratur (hail)
9
Effective rainfall (8) = (1) - (4) - (5) - (7)
In other words, the effective
rainfall (8) is the total
rainfall (1) minus runoff (4)
minus evaporation (5) and
minus deep percolation (7);
only the water retained in
the root zone (8) can be used
by the plants, and
represents what is called the
effective part of the
rainwater. effective
The term rainfall is used to
define this fraction of the
total amount of rainwater
useful for meeting the water
need of the crops.
www.fao.org/docrep/r4082e/r4082e05.htm
Shift in Thinking is Needed
from Blue Water to Green Water
Under the pressures of population growth, development
aspirations and a growing knowledge of the importance of
ecosystem support and services, water is increasingly
understood as a key factor in socio-economic development.
This will require a broadening of the global water debate from
its current concentration on managing blue water resources
in rivers, lakes and aquifers, and its current focus on the
provision of potable water, the financing of such provision,
and whether more water for irrigation can solve the world’s
food challenge.
11
Green water is a significant water resource, much larger volume-wise than the water
replenishing streams, lakes and aquifers (blue water).
12
A conceptual breakthrough that allowed an integrated land-water
approach came at a UN Food and Agriculture Organization seminar in
January 1993, when
the concept of green water was proposed for soil moisture .
According to this concept, rainfall constitutes the basic water resource
and is partitioned between “green” water, which is consumed by
vegetation, and “blue” water, which constitutes water in rivers and
aquifers, accessible for societal use.
Green water is important to terrestrial ecosystems. It is involved in
(rainfed) plant production and, therefore, in the production of food,
fuelwood, biofuels, timber, and forests. Because changes in plant cover
alter the partitioning between green and blue water resources, this plant
cover change is a key phenomenon in deforestation and reforestation.
Blue water, on the other hand, is the base for the household,
municipality, industry, and irrigated agriculture water supply; a carrier of
solutes and silt through the water systems; and the habitat for aquatic
13
ecosystems.
GREEN = BLUE BALANCE
14
The reality of the big
picture is that in a drainage
basin
perspective, the rainfall
over an area is the water
resource.
Part of the water is
consumed in terrestrial
ecosystems by vegetation
and evaporation from moist
surfaces (green water fl
ow), while the surplus
recharges aquifers and
rivers (blue water)
becoming available for
societal use and aquatic
ecosystems.
Naturally, indicated, the
green-blue balance is
determined by the local
hydroclimate.
15
GREEN WATER MANAGEMENT
The water necessary to produce the food required for an
expanding human population is usually discussed only as an
issue of blue water for irrigation (the water we use from rivers
and quifers).
Most food production is from rain fed farming.
This is critical not least in hunger and poverty stricken areas
with rapid population growth, areas that depend not on blue
water but on green water from infi ltrated rain (the soil
moistures used by plants and returned as vapour flow).
A shift in water thinking which considers soil moisture is
essential in order to find realistic and sustainable options to
16
feed the world of tomorrow.
Rain is the global water resource.
How well we capture and manage it will
determine if we can feed the planet’s 9 billion
inhabitants by 2050.
Green water is
the soil moisture,
exhaled during plant
growth as vapour
flow from land to
the atmosphere.
Blue water is the liquid
water in rivers and
aquifers.
The narrow focus on blue water as
the only water resource leads us to
believe that agriculture uses 70%
of the world’s freshwater, industry
20% and domestic use 10%. In
reality, this is not the case.
17
A sustainable water future
needs to incorporate the green water-consuming
systems, which are generally much larger and which
provide life support to humans and nature.
The bulk of future freshwater
needs for food production will
have to come from green
water management.
This will affect downstream
water availability.
Of the world’s poor,
70% live in rural
areas and often
depend on rainfallbased sources of
income.
The planet will need an additional 5,600 km3/yr
of water to feed itself by 2050. The most
optimistic irrigation projections show that no
more than 800 km3/yr could be contributed by
blue water by expansion and effi ciency
improvement of irrigation.
The future conflicts of
interest will be over land
use-water use, water
quantity-quality,
upstream-downstream
availability, and humansecosystems.
Climate change is a strong driving
force for lessening society’s largescale dependence on fossil fuels
through increased use of renewable
energy, though such a move will
increase consumptive water use for
biomass-based fuel alternatives.
19
A sustainable water future needs to incorporate the water
from infiltrated rain and the water-consuming vegetation
systems which provide life support to humans and nature:
Huge amounts of water are
needed to feed humanity,
and today nearly three time
more water is used in rain
fed agriculture than in
irrigated agriculture, with a
total global consumption of
7,000 km3/year.
In short, 50 to 100 times more
water is needed to produce
food for one person than the
amount needed on a
household domestic
consumption level.
Upstream land use and water
management determines
the volumes, patterns of fl
ow and quality of water for
downstream use, making
upstream land use for
forestry, rainfed farming and
grazing (all of which
consumes
freshwater) a determinant of
blue water availability
downstream.
Water is of central
importance in other
sectors: industry
production, forestry
and fibre
production,
fisheries, etc.
A sustainable water future needs to incorporate the water
from infiltrated rain and the water-consuming vegetation
systems which provide life support to humans and nature:
Huge volumes of blue
water fl ows are
required to
sustain aquatic
ecological functions in
rivers, lakes,
riparian zones and
estuaries.
The largest freshwater
consumption is required
to sustain biomass growth in
terrestrial ecosystems,
supporting key ecological
functions such as biodiversity,
carbon sequestration and antidesertifi cation.
Water supply for various
sectors of society is getting
increasingly complicated
as water contamination
escalates, and awareness
grows among water users
of the links between
upstream polluters of
water with
downstream water users.
Key Recommendations
1. Raise awareness of the distinction between blue water in rivers and aquifers
and green water in the soil.
2. Accept in scientifi c, management, political and other circles the fundamental
fact that there is not enoughblue water left to meet competing food, water and
environment needs for the future in large regions.
3. At the same time realise that proper management of the green water in the soil
represents a large potential for global food production.
4. Analyse the linkages between global trade regimes and different strategies to
attain national food security.
5. Introduce a green water dimension and incorporate land-use into IWRM and
adequate governance activities.
6. Further clarify the linkages between global poverty, hunger and shortage of
green and/or blue water.
7. Raise awareness of the improvements possible in the livelihoods of
communities – particularly those in water-scarce regions – through a
broadened approach to water.
8. Further clarify the linkages between rain fed agriculture and both green and
blue water.
22
Integrated Water
Resources
Management
Integrated Water Resources
Management (IWRM) has been
defined by the Technical
Committee of the Global Water
Partnership (GWP) as "a process
which promotes the coordinated
development and management of
water, land and related
resources, in order to maximize
the resultant economic and
social welfare in an equitable
manner without compromising
the sustainability of vital
ecosystems."
IWRM approaches involve
applying knowledge from
various disciplines as well
as the insights from diverse
stakeholders to devise and
implement efficient,
equitable and sustainable
solutions to water and
development problems.
Integrated Water Resources Management
IWRM is a comprehensive, participatory planning and
implementation tool for managing and developing water
resources in a way that balances social and economic needs,
and that ensures the protection of ecosystems for future
generations.
Water’s many different uses—for agriculture, for healthy
ecosystems, for people and livelihoods—demands coordinated
action.
An IWRM approach is an open, flexible process, bringing together
decision-makers across the various sectors that impact water
resources, and bringing all stakeholders to the table to set
policy and make sound, balanced decisions in response to
specific water challenges faced.
It has been agreed to consider water as an 'finite and
economic commodity , in order to emphasize on its
scarcity in the :
1. Fresh water is a finite and vulnerable resource, essential to
sustain life, development and the environment.
2. Water development and management should be based on a
participatory approach, involving users, planners and
policy makers at all levels.
3. Women play a central part in the provision, management
and safeguarding of water.
4. Water has an economic value in all its competing uses and
should be recognized as an economic good, taking into
account of affordability and equity criteria.
Green and blue water flow domains for human life support, distinguished in direct
functions (direct social and economic support) and indirect functions (water for
ecosystem support).
Sumber Daya Air
(SDA) mempunyai sifat
mengalir dan dinamis
serta berinteraksi dengan
sistem sumber daya lain
dari berbagai sektor
dengan berbagai
kepentingan dari para
pemilik kepentingan
sehingga membentuk
suatu sistem yang
disebut sistem wilayah
sungai yang tak jarang
sangat kompleks,
contohnya saja sistem
wilayah sungai Brantas.
DAS adalah
kesatuan terkecil
dari pengelolaan
air, aspek
pengelolaannya
meliputi:
Daerah tangkapan
hujan
Kuantitas air
Kualitas air
Pengendalian
banjir
Jasa Lingkungan
DAS
Prasarana
pengairan 28
CALCULATING A MONTHLY WATER BUDGET
29
NERACA AIR:
Keseimbangan air
masuk dan air ke
luar di suatu daerah
hidrologi yg
ditetapkan, misalnya
cekungan, danau,
dan lainnya, dengan
diperhitungkan
bahwa tempat
menyimpan tidak
berubah (water
balance)
30
31
Infiltrasi dari segi hidrologi
penting, karena hal ini menandai
peralihan dari air permukaan
yang bergerak cepat ke air tanah
yang bergerak lambat dan air
tanah.
Kapasitas infiltrasi suatu tanah
dipengaruhi oleh sifat-sifat
fisiknya dan derajat
kemampatannya, kandungan air
dan permebilitas lapisan bawah
permukaan, nisbi air, dan iklim
mikro tanah.
Air yang berinfiltrasi pada sutu
tanah hutan karena pengaruh
gravitasi dan daya tarik kapiler
atau disebabkan juga oleh
tekanan dari pukulan air hujan
pada permukaan tanah
32
Ada 2 faktor pengaruh
utama infiltrasi air
hujan yaitu :
Faktor yang
mempengaruhi air
untuk tinggal di
suatu tempat
sehingga air
mendapat
kesempatan untuk
berinfiltrasi.
Faktor yang
mempengaruhi
proses masuknya air
33
ke dalam tanah.
Sprinkler Irrigation
Many types microsprinklers, solid set,
aluminum pipe
a. Advantages: use
less water, more precise
amounts of water can be
applied, less run off (tail
water), may be used on
slightly hilly land
b. Disadvantages:
expensive (installation,
labor, filters,
maintenance), salt
buildup
34
KURVA ENERGI - LENGAS TANAH
Tegangan air menurun secara gradual dengan meningkatnya kadar air
tanah.
Tanah liat menahan air lebih banyak dibanding tanah pasir pada nilai
tegangan air yang sama
Tanah yang Strukturnya baik mempunyai total pori lebih banyak, shg
mampu menahan air lebih banyak
Pori medium dan mikro lebih kuat menahan air dp pori makro
KANDUNGAN
AIR DAN
TEGANGAN
Tegangan air tanah, Bar
10.000
Liat
Lempung
Pasir
0.01
10
Kadar air tanah, %
70
35
Soil moisture
content
The soil moisture
content indicates
the amount of water
present in the soil.
It is commonly
expressed as the
amount of water (in
mm of water depth)
present in a depth of
one metre of soil.
For example: when
an amount of water
(in mm of water
depth) of 150 mm is
present in a depth of
one metre of soil,
the soil moisture
content is 150
mm/m
36
A very general and simplified soil
profile can be described as follows:
a.The plough layer (20 to 30 cm thick):
is rich in organic matter and contains
many live roots. This layer is subject to
land preparation (e.g. ploughing,
harrowing etc.) and often has a dark
colour (brown to black).
b. The deep plough layer: contains
much less organic matter and live roots.
This layer is hardly affected by normal
land preparation activities. The colour is
lighter, often grey, and sometimes
mottled with yellowish or reddish spots.
c. The subsoil layer: hardly any organic
matter or live roots are to be found. This
layer is not very important for plant
growth as only a few roots will reach it.
d. The parent rock layer: consists of
rock, from the degradation of which the
soil was formed. This rock is
sometimes called parent material.
37
Gerakan
Air Tanah
Tidak Jenuh
Gerakan tidak jenuh = gejala kapilaritas = air bergerak dari muka air
tanah ke atas melalui pori mikro.
Gaya adhesi dan kohesi bekerja aktif pada kolom air (dalam pri mikro),
ujung kolom air berbentuk cekung.
Perbedaan tegangan air tanah akan menentukan arah gerakan air tanah
secara tidak jenuh.
Air bergerak dari daerah dengan tegangan rendah (kadar air tinggi)
ke daerah yang tegangannya tinggi (kadar air rendah, kering).
Gerakan air ini dapat terjadi ke segala arah dan berlangsung secara
terus-menerus.
Pelapisan tanah berpengaruh terhadap gerakan air tanah.
Lapisan keras atau lapisan kedap air memperlambat gerakan air
Lapisan berpasir menjadi penghalang bagi gerakan air dari lapisan
yg bertekstur halus.
Gerakan air dlm lapisan berpasir sgt lambat pd tegangan
38
Air hujan dan irigasi memasuki tanah, menggantikan udara
dalam pori makro - medium - mikro. Selanjutnya air bergerak
ke bawah melalui proses gerakan jenuh dibawah pengaruh gaya
gravitasi dan kapiler.
Gerakan air jenuh ke arah bawah ini berlangsung terus selama
cukup air dan tidak ada lapisan penghalang
Gerakan Jenuh
(Perkolasi)
Lempung berpasir
cm
Lempung berliat
0
15 mnt
4 jam
30
60
90
1 jam
24 jam
120
24 jam
48 jam
150
30 cm
60 cm
Jarak dari tengah-tengah saluran, cm
39
Part of the water
applied to the soil
surface drains
below the rootzone
and feeds deeper
soil layers which
are permanently
saturated;
the top of the
saturated layer is
called groundwater
table or sometimes
just water table
40
A perched groundwater
layer can be found on top
of an impermeable layer
rather close to the
surface (20 to 100 cm).
It covers usually a limited
area.
The top of the perched
water layer is called the
perched groundwater
table.
The impermeable layer
separates the perched
groundwater layer from
the more deeply located
groundwater table
41
PERKOLASI
Jumlah air perkolasi
Faktor yg berpengaruh:
1. Jumlah air yang ditambahkan
2. Kemampuan infiltrasi permukaan tanah
3. Daya hantar air horison tanah
4. Jumlah air yg ditahan profil tanah pd kondisi
kapasitas lapang
Keempat faktor di atas ditentukan oleh struktur dan tekstur tanah
Tanah berpasir punya kapasitas ilfiltrasi dan daya hantar air sangat tinggi,
kemampuan menahan air rendah, shg perkolasinya mudah dan cepat
Tanah tekstur halus, umumnya perkolasinya rendah dan sangat
beragam; faktor lain yg berpengaruh:
1. Bahan liat koloidal dpt menyumbat pori mikro & medium
2. Liat tipe 2:1 yang mengembang-mengkerut sangat berperan
42
LAJU
GERAKAN
AIR TANAH
Kecepatan gerakan air dlm tanah dipengaruhi oleh dua faktor:
1. Daya dari air yang bergerak
2. Hantaran hidraulik = Hantaran kapiler = daya hantar
i = k.f
dimana i = volume air yang bergerak; f = daya air yg bergerak dan k =
konstante.
Daya air yg bergerak = daya penggerak, ditentukan oleh dua faktor:
1. Gaya gravitasi, berpengaruh thd gerak ke bawah
2. Selisih tegangan air tanah, ke semua arah
Gerakan air semakin cepat kalau perbedaan tegangan semakin tinggi.
Hantaran hidraulik ditentukan oleh bbrp faktor:
1. Ukuran pori tanah
2. Besarnya tegangan untuk menahan air
Pada gerakan jenuh, tegangan airnya rendah, shg hantaran hidraulik berbanding
lurus dengan ukuran pori
Pd tanah pasir, penurunan daya hantar lebih jelas kalau terjadi penurunan kandungan
air tanah
Lapisan pasir dlm profil tanah akan menjadi penghalang gerakan air tidak jenuh
43
RETENSI AIR
TANAH
KAPASITAS RETENSI MAKSIMUM adalah:
Kondisi tanah pada saat semua pori terisi penuh air, tanah jenuh air, dan
tegangan matrik adalah nol.
KAPASITAS LAPANG: air telah meninggalkan pori makro, mori makro
berisi udara, pori mikro masih berisi air; tegangan matrik 0.1 - 0.2 bar;
pergerakan air terjadi pd pori mikro/ kapiler
KOEFISIEN LAYU: siang hari tanaman layu dan malam hari segar kembali,
lama-lama tanaman layu siang dan malam; tegangan matrik 15 bar.
Air tanah hanya mengisi pori mikro yang terkecil saja, sebagian besar air tidak
tersedia bagi tanaman.
Titik layu permanen, bila tanaman tidak dapat segar kembali
KOEFISIEN HIGROSKOPIS
Molekul air terikat pada permukaan partikel koloid tanah, terikat kuat
sehingga tidak berupa cairan, dan hanya dapat bergerak dlm bentuk uap air,
tegangan matrik-nya sekitar 31 bar.
Tanah yg kaya bahan koloid akan mampu menahan air higroskopis lebih
banyak dp tanah yg miskin bahan koloidal.
44
Perubahan status air dalam tanah, mulai dari kondisi
jenuh hingga titik layu
Status Air
Tanah
Jenuh
Kap. Lapang
Padatan
Titik layu
Pori
100g
air
40g
100g
20g
udara
100g
10 g
udara
100g
8g
udara
tanah jenuh air
kapasitas lapang
koefisien layu
koefisien higroskopis
45
Agihan air
dalam tanah
Berdasarkan tegangan air tanah dapat dibedakan menjadi tiga
bagian: Air bebas, kapiler dan higroskopis
Koef. Higroskopis
kurang lebih 31 atm
Kap. Lapang
kurang lebih 1/3 atm
Jml ruang pori
Lapisan olah
Air higroskopik
Hampir tdk
menunjukkan
sifat cairan
Air Kapiler
Peka thd gerakan
kapiler, laju penyesuaian meningkat dg meningkatnya kelembaban tanah
Ruang diisi udara
Biasanya jenuh uap air
Setelah hujan lebat
sebagian diisi air,
tetapi air cepat hilang krn gravitasi
bumi
Lapisan bawah tanah
Karena pemadatan ruang
pori berkurang
Strata bawah (jenuh air)
Kolom tanah
Jumlah ruang pori
46
Soil water
•infiltration & percolation
•permeability
•porosity
•zone of aeration
•soil water storage
•plant uptake &
transpiration
•evaporation
•throughflow
•water table
•zone of saturation
•groundwater flow
•aquifer
47
SUPLAI AIR
ke TANAMAN
Dua proses yg memungkinkan akar tanaman mampu menyerap air dlm
jumlah banyak, yaitu:
1. Gerakan kapiler air tanah mendekati permukaan akar penyerap
2. Pertumbuhan akar ke arah zone tanah yang mengandung air
LAJU GERAKAN KAPILER
Bulu akar
menyerap
air
Gerakan
kapiler
2.5 cm
sagt penting
Jumlah
air tanah
berkurang
Laju gerakan
tgt perbedaan
tegangan dan daya
hantar pori tanah
Tegangan
air tanah
meningkat
Terjadi
gerakan kapiler
air menuju
bulu akar
Terjadi
perbedaan
Tegangan
dg air tanah di
sekitarnya
LAJU PERPANJANGAN AKAR
Selama masa pertumbuhan tanaman, akar tanaman tumbuh memanjang dengan
cepat, sehingga luas permukaan akar juga tumbuh terus.
Jumlah luas permukaan akar penyerap yang bersentuhan langsung dengan
sebagian kecil air tanah (yaitu sekitar 1-2%)
48
HADANGAN HUJAN OLEH TUMBUHAN
Tajuk tumbuhan mampu menangkap sejumlah air hujan,
sebagian air ini diuapkan kembali ke atmosfer.
Vegetasi hutan di daerah iklim basah mampu menguapkan
kembali air hujan yg ditangkapnya hingga 25%, dan hanya 5%
yg mencapai tanah melalui cabang dan batangnya.
KEHILANGAN
UAP AIR
DARI TANAH
Awan hujan
presipitasi
Pembentukan Awan
transpirasi
evaporasi
Run off
infiltrasi
Tanah permukaan
perkolasi
Batuan
Groundwater
Sungai - laut
49
Hadangan
hujan oleh
tanaman
semusim
Sekitar 5 - 25% dari curah hujan dihadang tanaman dan
dikembalikan ke atmosfer.
Besarnya tergantung pada kesuburan tanaman dan stadia
pertumbuhan tanaman .
Dari curah hujan 375 mm, hanya sekitar 300-350 mm yang
mencapai tanah.
Hadangan curah hujan oleh jagung dan kedelai
Keadaan hujan
Persen dari curah hujan total untuk:
Jagung
Kedelai
Langsung ke tanah
Melalui batang
70.3
22.8
65.0
20.4
Jumlah di tanah
Yang tinggal di atmosfer
93.1
6.9
85.4
14.6
Sumber: J.L.Haynes, 1940.
50
Lingkaran
Tanah-AirTanaman
LTAT mrpk sistem dinamik dan terpadu dimana air mengalir
dari tempat dengan tegangan rendah menuju tempat dengan
tegangan air tinggi.
Hilang melalui stomata
daun (transpirasi)
Air kembali ke
atmosfer
(evapo-transpirasi)
Air dikembalikan ke
tanah melalui hujan
dan irigasi
Penguapan
Serapan bulu akar
51
EVAPOTRANSPIRASI
Kehilangan uap air dari tanah:
1. EVAPORASI: penguapan air dari permukaan tanah
2. TRANSPIRASI: Penguapan air dari permukaan tanaman
3. EVAPOTRANSPIRASI = Evaporasi + Transpirasi
Laju penguapan air tgt pd perbedaan potensial air = selisih tekanan uap
air = perbedaan antara tekanan uap air pd permukaan daun (atau
permukaan tanah) dengan atmosfer
Faktor Iklim dan Tanah:
1. Energi Penyinaran
2. Tekanan uap air di atmosfer
3. Suhu
4. Angin
5. Persediaan air tanah
Air tanah
Jagung
Tinggi
Sedang
17.7
12.7
Evapotranspirasi (cm:
Medicago sativa
24.4
20.5
Sumber: Kelly, 1957.
52
WATER BALANCE
Gains: precipitation
Soil moisture
storage
Losses: utilization and
evapotranspiration
actual evapotranspiration (AE)
potential evapotranspiration (PE)
Simple water balance
•moisture abundant
environments
•P > PE and therefore AE = PE
•moisture limited environments
•P < PE and therefore AE < PE
•seasonal moisture
environments
53
Ketersediaan
Air Tanah vs
Evapotranspirasi
Ketersediaan air di daerah perakaran sangat
menentukan besarnya evapotranspirasi.
Kedalaman daerah perakaran tanaman 50 - 60
cm.
Air tanah pada lapisan olah mengalami
pengurangan karena evaporasi permukaan
Air tanah pd lapisan bawah mengalami
pengurangan karena diserap akar tanaman
Kedalaman tanah (cm) Evapotranspirasi (cm):
Jagung Padang Rumput Hutan
0 - 17.5
17.5 - 180.0
24.25
20.75
23.45
21.17
23.27
22.25
Sumber: Dreibelbis dan Amerman, 1965.
54
Soil moisture balance
diagrams
(Thornthwaite and
Mather - 1955) - soil
depth 50 cm.
55
Soil moisture balance
diagrams
(Thornthwaite and
Mather - 1955) - soil
depth 300 cm.
56
PEMAKAIAN
KONSUMTIF
(PK)
Pemakaian Konsumtif merupakan jumlah kehilangan air melalui
evaporasi dan transpirasi.
Lazim digunakan sebagai ukuran dari seluruh air yg hilang dari tanaman
melalui evapotranspirasi
Ini merupakan angka-praktis untuk keperluan pengairan
Dua faktor penting yg menentukan PK adalah:
1. KEDALAMAN PERAKARAN TANAMAN
2. FASE PERTUMBUHAN TANAMAN
PK dapat berkisar 30 - 215 cm atau lebih:
1. Daerah basah - semi arid dg irigasi: 37.5 - 75 cm.
2. Daerah panas dan kering dg irigasi: 50 - 125 cm.
EVAPORASI vs TRANSPIRASI
Faktor yg berpengaruh adalah:
1. Perbandingan luas tutupan tanaman thd luas tanah
2. Efisiensi pemakaian air berbagai tanaman
3. Perbandingan waktu tanaman berada di lapangan
4. Keadaan iklim
Di daerah basah : EVAPORASI  TRANSPIRASI
Di daerah kering:
1. EVAPORASI  70 - 75 % dari seluruh hujan yg jatuh
2. TRANSPIRASI  20 - 25%
3. RUN OFF  5%
57
WUE : Water
Use Efficiency
WUE  Produksi tanaman yg dapat dicapai dari pemakaian sejumlah air
tersedia
WUE dapat dinyatakan sbg:
1. Pemakaian konsumtif (dalam kg) setiap kg jaringan tanaman yg
dihasilkan
2. Transpirasi (dalam kg) setiap kg jaringan tanaman yg dihasilkan
……… NISBAH TRANSPIRASI
Jumlah air yg diperlukan untuk menghasilkan 1 kg
bahan kering tanaman
NISBAH TRANSPIRASI
Untuk tanaman di daerah humid: 200 - 500, di daerah arid duakalinya
Tanaman
Nisbah Transpirasi
Beans
Jagung
Peas
Kentang
209 - 282 - 736
233 - 271 - 368
259 - 416 - 788
385 - 636
Sumber: Lyon, Buckman dan Brady, 1952.
58
Pengendalian
Penguapan
MULSA & PENGELOLAAN
Mulsa adalah bahan yg dipakai pd permukaan tanah untuk mengurangi
penguapan air atau untuk menekan pertumbuhan gulma.
Lazimnya mulsa spt itu digunakan untuk tanaman yang tidak
memerlukan pengolahan tanah tambahan
MULSA KERTAS & PLASTIK
Bahan mulsa dihamparkan di permukaan tanah, diikat spy tdk terbang, dan tanaman
tumbuh melalui lubang-lubang yg telah disiapkan
Selama tanah tertutup mulsa, air tanah dapat diawetkan dan pertumbuhan gulma
dikendalikan
MULSA SISA TANAMAN
Bahan mulsa berasal dari sisa tanaman yg ditanam sebelumnya, misalnya jerami padi,
jagung, dan lainnya
Bahan mulsa dipotong-potong dan disebarkan di permukaan tanah
Cara WALIK DAMI sebelum penanaman kedelai gadu setelah padi sawah
MULSA TANAH  Pengolahan tanah
Efektivitas mulsa tanah dalam konservasi air-tanah (mengendalikan evaporasi) masih
diperdebatkan, hasil-hasil penelitian masih snagat beragam
59
Mulching is an alternative to turf
around trees, and its use
eliminates potential competition.
A 2- to 4-inch layer of wood chips,
bark, or other organic material
over the soil under the drip line is
recommended because it :
Helps retain soil moisture
Helps reduce weeds and controls
grass;
Increases soil fertility when mulch
decomposes;
Improves appearance;
Protects the trunk from injuries
caused by mowing equipment and
trimmers that often result in
serious tree damage or death;
Improves soil structure (better
aeration, temperature, and
moisture conditions)
60
Olah Tanah vs
Penguapan Air
Tanah
Alasan pengolahan tanah:
1. Mempertahankan kondisi fisika tanah yg memuaskan
2. Membunuh gulma
3. Mengawetkan air tanah.
Pengendalian Penguapan vs Pemberantasan Gulma
Perlakuan
Hasil jagung (t/ha)
Tanah dibajak dg persiapan yg baik
1. Dibebaskan dari gulma
2. Gulma dibiarkan tumbuh
3. Tiga kali pengolahan dangkal
Persiapan Buruk
4. Dibebaskan dari gulma
Kadar air tanah (%)
hingga kedalaman 1 m
2.9
0.4
2.5
22.3
21.8
21.9
2.0
23.1
Sumber: Mosier dan Gutafson, 1915.
Pengolahan tanah yg dapat mengendalikan gulma dan memperbaiki kondisi fisik tanah akan
berdampak positif thd produksi tanaman
Pengolahan tanah yg berlebihan dapat merusak akar tanaman dan merangsang evaporasi,
shg merugikan tanaman
61
Soil Moisture and Groundwater
62
Stream Flow
63
Idealisasi Aquifer
64
Urban Runoff
Urban runoff is defined as a
stream flow or the sum of
surface runoff and subsurface
runoff. Surface runoff occurs
when the surface storage and
the soil become saturated,
infiltration ceases and
subsequent rainfall becomes
surface runoff. Subsurface
runoff is rainwater that
infiltrates the surface and
flows much more slowly on its
way to a stream than surface
runoff.
Rainfall and the soil conditions
are the direct causes of urban
runoff. Rainfall can take one of
several routes once it reaches
the earth’s surface.
65
Urban Runoff
Rainwater can be absorbed by the
soil on the land surface,
intercepted by vegetation, directly
impounded in many different
surface features from small
depressions to large lakes and
oceans, or infiltrated through the
surface and subsurface soils into
the groundwater.
Another route taken by falling
precipitation is runoff. Soil
characteristics in a watershed
have a direct effect on the rainfallrunoff process and these include
soil layer thickness, permeability,
infiltration rate, and the degree of
moisture in the soil before the
rain event.
The greater the permeability of
the soil, or the ability to infiltrate
rainfall to its lower strata, the less
remaining to become runoff
66
Surface runoff occurs relatively
rapidly in the urban watershed,
since storage and infiltration
capacity have been reduced to
practically zero. Much of the
surface consists of impervious
materials such as concrete or
asphalt
Structures which add large
amounts of impermeable areas to
the watershed in general increase
slopes and considerably diminish
the water storage capability.
The increased volumes and flow
rates of runoff produced by urban
watersheds have a number of
harmful effects including flooding
and stream erosion
www.dnr.state.md.us/.../impervious.html
67
The vegetative cover found
naturally in rural areas directly
effects the rainfall-runoff process
and is an important measure in
many runoff estimation
techniques.
Vegetation characteristics include
various types, canopies, and
densities, the extent of coverage,
the degree of residue or natural
litter at the base, and the degree of
surface roughness.
The water flow velocity of runoff
over a smooth, impermeable
surface such as a road or parking
lot is about ten times faster than
over a vegetated surface.
www.britannica.com/EBchecked/topic-art/552611...
68
Urbanization alters the hydrologic
regime of surface waters by
changing the way water cycles
through a drainage basin. In a
natural setting, precipitation is
intercepted or delayed by the forest
canopy and ground cover.
Vegetation, depressions on the land,
and soils provide extensive storage
capacity for precipitation.
Water exceeding this capacity
travels via shallow subsurface flow
and groundwater and eventually
discharges gradually to surface
water bodies.
In a forested undisturbed
watershed, direct surface runoff
occurs rarely or not at all because
precipitation intensities do not
exceed soil infiltration rates
69
The water quality of urban runoff
varies with its source and location. In
urban runoff, most pollutants occur
as solids or are associated with soil or
other natural particulates. This
condition differs among the specific
pollutants.
These pollutants come from many
different sources. These sources
include: transportation, industrial
activities, decaying vegetation, soil
erosion, animals, fertilizer/pesticide
application, leaking sanitary sewers,
direct connections of sanitary sewers
to storm sewers, poorly operating
septic systems, illegal disposal of oils,
paints, etc. to the storm sewer system,
accidental spills, leaking
underground storage tanks, leachate
from landfills, and leakage from
hazardous waste sites
www.ec.gc.ca/inre-nwri/default.asp?lang=En...
70
The top 50 percent of
pollutants found in
Nationwide Urban Runoff
Program samples are, lead,
zinc, copper, chromium, and
arsenic. Besides these
pollutants, other water
quality characteristics affect
the behavior and fate of
materials in water.
The characteristics include
temperature, pH, dissolved
oxygen, alkalinity, hardness,
and conductivity .
www.icpi.org/techspecs/index.cfm?id=17&tech=08
71
Estimates of the quantity of runoff
are determined first by evaluating
several key drainage area
characteristics. The first
characteristic is the drainage area
size, which is determined using
topographic maps.
Other characteristics are the shape
of the drainage basin and its various
slopes. The steeper the surfaces and
channels of the basin the faster
runoff can drain to its outlet. An
elongated drainage area with more
gradual slopes will result in slower
runoff and drainage. An elongated
drainage area with a longer distance
from its upper reaches to its outlet
may have a longer response time
than a rounded one of equal size and
therefore, a lower peak runoff rate
www.caveconservancyofvirginia.org/livingonkar...
72
Soil characteristics in a
watershed have a direct
effect on the rainfall
runoff process and they
are included in most
runoff estimating
techniques.
These characteristics
include soil layer
thickness, permeability or
infiltration rate, and the
degree of moisture in the
soil before the rain event.
The greater the soil
permeability, the ability to
infiltrate rainfall to its
lower strata, the less
remains to become runoff
73
The quantity of runoff can be estimated
using one of several different methods.
redac.eng.usm.my/html/USWM/SUDS.htm
The most common methods include
steady state, unsteady state, and
computer models. Steady-state methods
use uniform rainfall intensities, soil
infiltration rates, and representative
watershed response times. Steady-state
conditions are a reasonably accurate
way to estimate peak runoff rates from
high to moderate frequency storms in
small watersheds with relatively short
response times. Unsteady state methods
allow rainfall intensity, soil infiltration,
and watershed response time to vary
with time. These methods can more
accurately compute runoff
characteristics from rainfalls of varying
intensities in single or even multiple
storm events.
74
The quantity of runoff can be estimated
using one of several different methods.
Sustainable Urban Drainage System
(SUDS) is a concept that includes long
term environmental and social factors
in decisions about drainage. It takes
account of the quantity and quality of
runoff, and the amenity value of
surface water in the urban
environment .
Many existing urban drainage systems
can cause problems of flooding,
pollution or damage to the environment
and are not proving to be sustainable.
Computer models can analyze rainfallrunoff process for a series of storm events
over an extended time period from several
years to several decades.
These models account for changes in
watershed factors and parameters during
the time between storms as well as during
storms.
Factors that must be considered are
temperature, relative humidity, surface
evaporation and evapotranspiration, and
groundwater levels and movement. All of
these factors may significantly affect runoff
response to the next rainfall in the analysis
75
Runoff control represents
various practices designed to
keep water from contacting
bare soil and/or controlling its
velocity. Runoff control
includes drains for surface and
subsurface water, dikes, and
channels placed along slopes to
interrupt and divert runoff and
roughness created on the
surface to reduce velocity.
A temporary pipe slope drain is
effective in preventing runoff
erosion on a slope from a
higher elevation.
Up slope runoff needs to be
collected and directed into the
drain and then discharged in a
controlled way to prevent
erosion at the slope bottom
76
rpitt.eng.ua.edu/.../WinSlamm/Ch1/M1.html
An urban watershed
not only has
modified stream
channels created by
the increased peaks
and volumes of
runoff, but also has
a series of total
artificial closed
channels, which are
created when
sewerage is installed
under a city.
geoscape.nrcan.gc.ca/.../images/outdoors_e_.jpg
Most urban areas
collect runoff in
gutters and sewers
and discharge it at
various points to
receiving waters
77
Porous Pavements – open
concrete blocks, paving
stones, and turf-stones.
All allow vegetative
growth within the paving
matrix. Allows partial
degradation of oil and
grease.
Does require routine
maintenance.
78
Greenbelts – simple biological
process
Runoff. is channeled away
from parking lot to the
greenbelt, which is composed
of porous topsoil supporting
vegetation.
Gravel layer beneath allows
percolation into surrounding
soils.
Greenbelts can be curbed to
handle large flows when
infiltration and storage
capacity are exceeded.
www.dep.state.pa.us/.../poster_annotati
ons.htm
79
Adsorbents in sewer
inlets – much of grease
and oil exist in a
dissolved or colloidal
state, not captured by
catch basins and/or
storage tanks.
Runoff would flow
through a bed of
adsorbent material
before discharging to the
storm sewer.
Periodically the
adsorbent is replaced or
regenerated by a
desorption and recovery
process.
80
Surface cleaning using
wet scrubbing – spray
of water and
biodegradable
detergent would be
applied to solubilize oil
and grease on paved
surfaces.
Combination of
sweeping and
vacuuming would
recover the pollutants.
On-board filtration
system would then
remove the pollutants
and allow the reuse of
the detergent mix.
denversewerscam.hypocrisy.com/.../hello-world/
81
Runoff Control
Essentially only three methods for control – storage tanks, settling
tanks, and retention ponds, reservoirs, etc.
Storage tanks are inline for average runoff – offline when heavy rain
event and are best suited for high gradient
Settling tanks are similar to primary settling tanks in a WWTP and
best suited for no gradient urban areas
82
HOUSEHOLD
Properly dispose of car
oil, paints, and
pesticides. Never dump
motor oil, paints,
solvents, pesticides, or
other pollutants into
the street, gutter, lake,
or on your lawn.
Use phosphorus-free
soaps when washing
your car. This will help
to reduce the amount of
phosphorus entering
the storm sewer system
from your driveway
and street.
www.okobojiprotectiveassociation.org/stewards...
83
Rainfall-runoff
analysis
Runoff is generated
by rainstorms and its
occurrence and
quantity are
dependent on the
characteristics of the
rainfall event, i.e.
intensity, duration
and distribution.
84
Rainfall characteristics
Precipitation in arid and semi-arid zones results largely
from convective cloud mechanisms producing storms
typically of short duration, relatively high intensity and
limited areal extent. However, low intensity frontal-type
rains are also experienced, usually in the winter season.
When most precipitation occurs during winter, relatively
low-intensity rainfall may represent the greater part of
annual rainfall.
Rainfall intensity is defined as the ratio of the total amount
of rain (rainfall depth) falling during a given period to the
duration of the period It is expressed in depth units per unit
time, usually as mm per hour (mm/h).
85
Variability of annual rainfall
Water harvesting planning and management in arid and semi-arid zones present
difficulties which are due less to the limited amount of rainfall than to the inherent
degree of variability associated with it.
In temperate climates, the standard deviation of annual rainfall is about 10-20
percent and in 13 years out of 20, annual amounts are between 75 and 125 percent
of the mean.
In arid and semi-arid climates the ratio of maximum to minimum annual amounts
is much greater and the annual rainfall distribution becomes increasingly skewed
with increasing aridity. With mean annual rainfalls of 200-300 mm the rainfall in
19 years out of 20 typically ranges from 40 to 200 percent of the mean and for 100
mm/year, 30 to 350 percent of the mean. At more arid locations it is not uncommon
to experience several consecutive years with no rainfall.
For a water harvesting planner, the most difficult task is therefore to select the
appropriate "design" rainfall according to which the ratio of catchment to
cultivated area will be determined
86
The surface runoff
process
When rain falls, the
first drops of water are
intercepted by the
leaves and stems of the
vegetation. This is
usually referred to as
interception storage.
Schematic diagram
illustrating relationship
between rainfall,
infiltration and runoff
(Source: Linsley et al.
1958)
87
As the rain continues, water reaching the ground surface infiltrates into the soil
until it reaches a stage where the rate of rainfall (intensity) exceeds the infiltration
capacity of the soil. Thereafter, surface puddles, ditches, and other depressions are
filled (depression storage), after which runoff is generated.
The infiltration capacity of the soil depends on its texture and structure, as well as
on the antecedent soil moisture content (previous rainfall or dry season). The initial
capacity (of a dry soil) is high but, as the storm continues, it decreases until it
reaches a steady value termed as final infiltration rate
The process of runoff generation continues as long as the rainfall intensity exceeds
the actual infiltration capacity of the soil but it stops as soon as the rate of rainfall
drops below the actual rate of infiltration.
The rainfall runoff process is well described in the literature. Numerous papers on
the subject have been published and many computer simulation models have been
developed. All these models, however, require detailed knowledge of a number of
factors and initial boundary conditions in a catchment area which in most cases are
not readily available.
88
Factors affecting runoff
Apart from rainfall characteristics such as intensity, duration and
distribution, there are a number of site (or catchment) specific
factors which have a direct bearing on the occurrence and volume
of runoff.
Soil type
The infiltration capacity is among others dependent on the porosity of a soil which
determines the water storage capacity and affects the resistance of water to
flow into deeper layers.
Porosity differs from one soil type to the other. The highest infiltration capacities
are observed in loose, sandy soils while heavy clay or loamy soils have
considerable smaller infiltration capacities.
The difference in infiltration capacities measured in different soil types.
The infiltration capacity depends furthermore on the moisture content prevailing in
a soil at the onset of a rainstorm.
The initial high capacity decreases with time (provided the rain does not stop) until
it reaches a constant value as the soil profile becomes saturated
89
Infiltration capacity curves for
different soil types
90
Vegetation
The amount of rain lost to interception storage on the foliage
depends on the kind of vegetation and its growth stage.
Values of interception are between 1 and 4 mm. A cereal crop, for
example, has a smaller storage capacity than a dense grass cover.
More significant is the effect the vegetation has on the infiltration
capacity of the soil. A dense vegetation cover shields the soil from
the raindrop impact and reduces the crusting effect as described
earlier.
In addition, the root system as well as organic matter in the soil
increase the soil porosity thus allowing more water to infiltrate.
Vegetation also retards the surface flow particularly on gentle
slopes, giving the water more time to infiltrate and to evaporate.
In conclusion, an area densely covered with vegetation, yields less
runoff than bare ground.
91
Slope and catchment size
Investigations on experimental runoff plots have shown that steep
slope plots yield more runoff than those with gentle slopes.
In addition, it was observed that the quantity of runoff decreased
with increasing slope length.
This is mainly due to lower flow velocities and subsequently a longer
time of concentration (defined as the time needed for a drop of water
to reach the outlet of a catchment from the most remote location in
the catchment). This means that the water is exposed for a longer
duration to infiltration and evaporation before it reaches the
measuring point. The same applies when catchment areas of different
sizes are compared.
The runoff efficiency (volume of runoff per unit of area) increases
with the decreasing size of the catchment i.e. the larger the size of the
catchment the larger the time of concentration and the smaller the
runoff efficiency.
92
Runoff efficiency as a function of catchment size
93
Infiltrasi
Infiltrasi adalah proses meresapnya air atau proses meresapnya air dari permukaan
tanah melalui pori-pori tanah.
Dari siklus hidrologi, jelas bahwa air hujan yang jatuh di permukaan tanah sebagian
akan meresap ke dalam tanah, sabagian akan mengisi cekungan permukaan dan
sisanya merupakan overland flow.
Sedangkan yang dimaksud dengan daya infiltrasi (Fp) adalah laju infiltrasi
maksimum yang dimungkinkan, ditentukan oleh kondisi permukaan termasuk
lapisan atas dari tanah.
Besarnya daya infiltrasi dinyatakan dalam mm/jam atau mm/hari. Laju infiltrasi
(Fa) adalah laju infiltrasi yang sesungguhnya terjadi yang dipengaruhi oleh
intensitas hujan dan kapasitas infiltrasi.
94
Infiltrasi
Infiltrasi mempunyai arti penting terhadap :
a. Proses Limpasan
Daya infiltrasi menentukan besarnya air hujan yang dapat diserap ke dalam tanah.
Sekali air hujan tersebut masuk ke dalam tanah ia akan diuapkan kembali atau
mengalir sebagai air tanah. Aliran air tanah sangat lambat. Makin besar daya
infiltrasi, maka perbedaan antara intensitas curah dengan daya infiltrasi menjadi
makin kecil. Akibatnya limpasan permukaannya makin kecil sehingga debit
puncaknya juga akan lebih kecil.
b. Pengisian Lengas Tanah (Soil Moisture) dan Air Tanah
Pengisian lengas tanah dan air tanah adalah penting untuk tujuan pertanian. Akar
tanaman menembus daerah tidak jenuh dan menyerap air yang diperlukan untuk
evapotranspirasi dari daerah tak jenuh tadi. Pengisian kembali lengas tanah sama
dengan selisih antar infiltrasi dan perkolasi (jika ada). Pada permukaan air tanah
yang dangkal dalam lapisan tanah yang berbutir tidak begitu kasar, pengisian
kembali lengas tanah ini dapat pula diperoleh dari kenaikan kapiler air tanah.
95
Infiltrasi
Faktor-faktor yang mempengaruhi infiltrasi adalah:
1. Karakteristik –karakteristik hujan
2. Kondisi-kondisi permukaan tanah
• Tetesan hujan, hewan maupun mesin mungkin memadatkan permukaan tanah dan
mengurangi infiltrasi.
• Pencucian partikel yang halus dapat menyumbat pori-pori pada permukaan tanah
dan mengurangi laju inflasi.
• Laju infiltrasi awal dapat ditingkatkan dengan jeluk detensi permukaan.
• Kepastian infiltrasi ditingkatkan dengan celah matahari.
• Kemiringan tanah secara tidak langsung mempengaruhi laju infiltrasi selama
tahapan awal hujan berikutnya.
• Penggolongan tanah (dengan terasering, pembajakan kontur dll) dapat
meningkatkan kapasitas infiltrasi karena kenaikan atau penurunan cadangan
permukaan.
96
Infiltrasi
Faktor-faktor yang mempengaruhi
infiltrasi adalah:
3. Kondisi-kondisi penutup
permukaan
• Dengan melindungi tanah dari
dampak tetesan hujan dan dengan
melindungi pori-pori tanah dari
penyumbatan, seresah mendorong laju
infiltrasi yang tinggi
• Salju mempengaruhi infiltrasi
dengan cara yang sama seperti yang
dilakukan seresah.
• Urbanisasi (bangunan, jalan, sistem
drainase bawah permukaan)
mengurangi infiltrasi.
www.futurewater.nl/uk/projects/landcover
97
Infiltrasi
Faktor-faktor yang mempengaruhi infiltrasi adalah:
4. Transmibilitas tanah
• Banyaknya pori yang besar, yang menentukan sebagian dari
setruktur tanah, merupakan salah satu faktor penting yang mengatur
laju transmisi air yang turun melalui tanah.
• Infiltrasi beragam secara terbalik dengan lengas tanah.
5. Karakteristik-karakteristik air yang berinfiltrasi
• Suhu air mempunyai banyak pengaruh, tetapi penyebabnya dan
sifatnya belum pasti.
• Kualitas air merupakan faktor lain yang mempengaruhi infiltrasi.
98
Infiltrasi
Faktor-faktor yang mempengaruhi daya infiltrasi antara lain :
a.
Dalamnya genangan di atas permukaan tanah (surface detention) dan tebal
lapisan jenuh
b. Kadar air dalam tanah
c. Pemampatan oleh curah hujan
d. Tumbuh-tumbuhan
e. Karakteristik hujan
f Kondisi-kondisi permukaan tanah
Sedangkan faktor-faktor yang mempengaruhi laju infiltrasi antara lain :
a. Jenis permukaan tanah
b Cara pengolahan lahan
c. Kepadatan tanah
d. Sifat dan jenis tanaman.
99
Pengertian
Infiltrasi adalah proses
masuknya air dari
permukaan ke dalam tanah.
Perkolasi adalah gerakan
aliran air di dalam tanah
(dari zone of aeration ke
zone of saturation).
Infiltrasi berpengaruh
terhadap saat mulai
terjadinya aliran permukaan
dan juga berpengaruh
terhadap laju aliran
permukaan (run off).
100
One of the first attempts to
describe the process of infiltration
was made by Horton in 1933.
He observed that the infiltration
capacity reduced in an exponential
fashion from an initial, maximum
rate f0 to a final constant rate fc.
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Faktor yang Berpengaruh Terhadap Laju Infiltrasi
Dalamnya genangan di atas permukaan tanah dan tebal lapisan yang
jenuh.
Kelembaban tanah
Pemampatan tanah oleh curah hujan
Penyumbatan oleh bahan yang halus (bahan endapan)
Pemampatan oleh orang dan hewan
Struktur tanah
Tumbuh-tumbuhan
Udara yang terdapat dalam tanah
Topografi
Intensitas hujan
Kekasaran permukaan
Mutu air
Suhu udara
Adanya kerak di permukaan.
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Infiltration devices
What are they?
Infiltration devices
Infiltration devices drain
water directly into the
ground. They may be used at
source or the runoff can be
conveyed in a pipe or swale
to the infiltration area.
They include soakaways,
infiltration trenches and
infiltration basins as well as
swales, filter drains and
ponds. Infiltration devices
can be integrated into and
form part of the landscaped
areas.
Soakaways and infiltration
trenches are completely
below ground, and water
should not appear on the
surface. Infiltration basins
and swales for infiltration
store water on the ground
surface, but are dry except in
periods of heavy rainfall.
103
Infiltration devices : How they work
Infiltration devices work by enhancing the natural capacity of the ground to store and drain water.
Rain falling onto permeable (eg sandy) soil soaks into it. Infiltration devices use this natural process to
dispose of surface water runoff.
Limitations occur where the soil is not very permeable, the water table is shallow or the groundwater
under the site may be put at risk.
104
Infiltration devices
Quantity
Infiltration techniques:
provide storage for runoff. In the case of soakaways and infiltration trenches, this storage is provided
in an underground chamber, lined with a porous membrane and filled with coarse crushed rock.
Infiltration basins store runoff by temporary and shallow ponding on the surface;
enhance the natural ability of the soil to drain the water. They do this by providing a large surface
area in contact with the surrounding soil, through which the water can pass.
The amount of water that can be disposed of by an infiltration device within a specified time depends
mainly on the infiltration potential of the surrounding soil. The size of the device and the bulk
density of any fill material will govern storage capacity.
Quality
Runoff is treated in different ways in an infiltration device. These include:
physical filtration to remove solids
adsorption onto the material in the soakaway, trench or surrounding soil
biochemical reactions involving micro-organisms growing on the fill or in the soil.
The level of treatment depends on the size of the media and the length of the flow path through the
system, which controls the time it takes the runoff to pass into the surrounding soil. Pre-treatment
may be required before polluted runoff is allowed into an infiltration device.
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The water table defines
the top of an unconfined
aquifer. Water in a well
penetrating an
unconfined aquifer will
remain at the elevation
of the water table. Some
streams and lakes
intercept the water table,
allowing direct
groundwater–surface
water exchange.
The potentiometric
surface reflects the
water pressure of a
confined aquifer, and is
the level to which water
in a well will naturally
rise (i.e., to an elevation
above the confined
aquifer it penetrates).
www.waterencyclopedia.com/Ge-Hy/Groundwater.html
106
SUMUR RESAPAN AIR HUJAN
Air hujan yang jatuh di permukaan tanah akan terdistribusi secara
evapotranspirasi, infiltrasi dan sebagian lagi mengalir sebagai air
permukaan.
Dengan makin luas penutupan permukaan tanah oleh bangunan maka
semakin besar air yang mengalir sebagai air permukaan dan berarti
semakin kecil air yang meresap ke dalam tanah.
Koefisien aliran permukaan untuk genting, beton dan aspal hampir
mendekati satu, dengan kata lain tidak ada air yang meresap kedalam
tanah.
Dari sini jelas, bahwa untuk atap yang pada umumnya genting, beton
atau perkerasan lainnya, diperlukan dimensi sistem drainasi yang
relatif besar dan sekaligus kehilangan air hujan karena langsung
masuk ke sungai.
107
SUMUR RESAPAN AIR HUJAN
Untuk penentuan dimensi suatu peresapan air hujan yang
mampu menampung air sebelum meresap kedalam tanah
perlu diperhitungkan terhadap beberapa hal antara lain:
1. lama hujan dominan,
2. intensitas hujan pada lama hujan dominan,
3. selang waktu dengan hujan dominan,
4. koefisien permeabilitas tanah,
5. tinggi muka air tanah, dan
6. luasan atap dan koefisien aliran permukaan.
108
SUMUR RESAPAN AIR HUJAN
Lama hujan dominan:
Jumlah kejadian “lama hujan dominan” dirunut dan data Automatic
Rainfall Recorder (ARR) yang mana dari sini dapat ditentukan lama
hujan yang diperhitungkan untuk suatu daerah penelitian.
Sebagai kontrol dapat diambil pula untuk kejadian-kejadian lama
hujan yang lain yang mana pada akhirnya kapasitas isi resapan air
hujan yang menjadi sasaran pokok dan penelitian ini. intensitas hujan:
Setelah diketahui lama hujan dapat dihitung besar nilai intensitas
hujan.
Untuk beberapa kota telah tersedia grafik hubungan antara lama
hujan, intensitas serta frekuensi ke jadian sedangkan untuk kota yang
belum tersedia grafik tersebut dapat dihitung dari analisa frekuensi.
109
SUMUR RESAPAN AIR
HUJAN
Selang waktu hujan :
Setelah ditentukan lama hujan
sebagai dasar perhitungan
dapat ditelusur selang waktu
hujan yang berpengaruh
terhadap lama hujan yang
diperhitungkan.
Hal ini diperlukan untuk
mempertimbangkan
kemampuan resapan sebagai
reservoir sebelum seluruh air
dapat meresap kedalam tanah,
agar sebanyak mungkin air
meresap kedalam tanah dengan
dimensi resapan sekecil
mungkin.
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SUMUR RESAPAN AIR
HUJAN
Koefisien permeabilitas
tanah:
Dengan mempertimbangan
selang waktu hujan yang
berpengaruh terhadap lama
hujan yang diperhitungkan
dibanding dengan kemampuan
peresapan air dalam tanah yang
dalam hal ini sangat dipengaruhi
oleh koefisien permeabilitasnya
maka dapat diuji akan
kehandalan volume resapan yang
diperhitungkan.
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SUMUR RESAPAN AIR HUJAN
Tinggi Muka Air Tanah:
Hal ini perlu untuk mempertimbangkan dimensi maupun bentuk
resapan dan sekaligus akan mempengaruhi kecepatan peresapan. Di
samping itu perlu dikaji pula tinggi muka air tanah dalam kaitannya
dengan kemungkinan pengaruh negatifnya terhadap sistem perakaran
tanaman. Namun demikian dengan dasar pemikiran bahwa sistem
peresapan ini akan mengembalikan ke kondisi alami dalam pengisian
air tanah seperti sebelum ada bangunan, maka diperhitungkan bahwa
masalah tersebut bukan merupakan hal-hal yang sering dijumpai.
Luas permukaan penutupan dan koefisien aliran permukaan:
Luas permukaan yang dilayani oleh resapan ini merupakan faktor
utama dalam menentukan dimensi sumur resapan yaitu untuk atap
diperhitungkan luas permukaan horizontal.
SUMUR RESAPAN AIR HUJAN
METODE PERHITUNGAN
Metode perhitungan bertujuan agar daerah layanan bebas dari
genangan air dan Sekaligus meresapkan air hujan kedalam tanah.
Air yang jatuh pada bukan perkerasan dialirkan ke jaringan drainasi
yang diperhitung kan berdasar pada Formula Rational dengan it
didasarkan pada waktu konsentrasi (Tk), sedangkan air yang jatuh di
perkerasan dialirkan masuk kedalam konstruksi resapan dengan
volume resapan fungsi dan lama hujan dominan (Td), intensitas hujan
pada Td, koefisien permeabilitas tanah, selang waktu dengan hujan
dominan, tinggi muka air tanah, luasan atap layanan dan koefisien
aliran permukaan yang kemudian diperhitungkan berdasar formula
Darcy.
SUMUR RESAPAN AIR HUJAN
Kedalaman efektif sumur resapan dihitung dari tinggi muka air tanah
bila dasar sumur berada di bawah muka air tersebut dan diukur
dari dasar sumur bila muka air tanah berada di bawahnya.
Bila dari perhitungan dengan diameter sumuran 0,80 m (banyak di
pasaran) didapatkan H yang kurang menguntungkan :
(1) misalnya dasar sumuran berada di bawah muka air tanah atau
dasar akari terletak pada lapisan yang kurang porus, maka dapat
ditentukan H yang tepat kemudian diameter dapat dihitung
dengan
(2) Waktu yang diperlukan untuk mendapatkan H max. adalah T
dengan formula
(3) Hal ini diperlukan untuk memperbandingkan dengan waktu hujan
dominan (Td) dalam analisa hidrologi selanjutnya.
KONSTRUKSI SUMUR RESAPAN
Untuk keamanan konstruksi, resapan perlu dilengkapi
dengan pelindung dinding.
Karena bentuk umum resapan ini adalah sumuran maka
pelindung dinding ini dapat dilaksanakan dengan konstruksi
pasangan batu kosong, pasangan batu cadas atau buis beton
yang kesemuanya akan mempengaruhi perhitungan sesuai
dengan formulasinya.
Sedangkan air yang ditampung adalah air dari atap melalui
talang datar dan tegak kemudian masuk ke resapan, atau air
dari atap ditampung oleh selokan keliling tritisan (tanpa
talang) kemudian masuk ke resapan.
Sumur Resapan dilengkapi
dengan peluap untuk
melewatkan air hujan yang
tidak diperhitungkan hingga
kelebihan air dapat
disalurkan.
Untuk daerah di mana muka
air tanah cukup dangkal
bentuk sumuran seperti di
atas kurang tepat.
Dasar resapan diletakkan
serendah-rendahnya pada
muka air tanah tertinggi
untuk mendapatkan
efektifitas masuknya air ke
dalam tanah.
SUMUR RESAPAN AIR HUJAN
KEUNTUNGAN
Reduksi dimensi:
Dimensi jaringan drainasi akan dapat diperkedil karena sebagian
besar air meresap kedalam tanah sebelum masuk ke jaringan drainasi.
Debit yang diperhitungkan dalam perancangan ini adalah debit air
dari permukaan bukan perkerasan.
Walau akan terjadi biaya tambahan untuk pembuatan sumur resapan
namun dalam pembangunan suatu sistem drainasi dapat dipikul
bersama yaitu pemerintah untuk janingan drainasi dan masyarakat
dengan beban pembuatan resapan di halaman masing-masing jadi
disamping memperkecil dana pembangunan sekaligus meningkatkan
partisipasi swadaya masyarakat dalam pembangunan.
SUMUR RESAPAN AIR HUJAN
KEUNTUNGAN
Memperkecil probabilitas genangan:
Untuk bagian kota yang rendah, selain menanggung beban banjir dan
lokasi itu sendiri juga mendenita karena beban air dari daerah-daerah
tinggi di Sekitar. Dengan pemakaian resapan ini berarti memperbesar
“retarding basin” yang berarti memperkecil probabilitas genangan.
Memperkecil konsentrasi pencemaran:
Daerah pemukiman pada umumnya mempunyai potensi besar dalam
pencemaran air tanah maka dengan bertambah besarnya cadangan air
di daerah tersebut akan memperkedil konsentrasi pencemaran. Hal ini
cukup penting karena salah satu usaha untuk mencegah pencemaran
adalah memperkecil konsentrasi polucan.
SUMUR RESAPAN AIR
HUJAN
KEUNTUNGAN
Mempertahankan muka air
tanah:
allencountygeology.indiana.edu/water-table-el...
Tinggi muka air tanah sangat
berpengaruh terhadap iklim
mikro dan ini sangat penting
untuk tata tanam (tanaman
keras).
Di samping itu penurunan muka
air tanah akan berpengaruh pula
pada energi yang dipergunakan
untuk memompa air sumur, dan
untuk skala regional,
pengesampingan hal ini
merupakan sumberdaya
terbengkelai.
SUMUR RESAPAN AIR HUJAN
Tinggi tekanan akan seimbang pada suatu titik tertentu untuk
permukaan air asin (S) maupun air tawar (F).
120
SUMUR RESAPAN AIR HUJAN
Dari perhitungan di atas dapat dilihat bahwa penurunan permukaan
air tawar akan menyebabkan kenaikan permukaan air asin sebesar 40
kali penurunan tersebut, hingga pengaruh air hujan yang masuk
kembali ke dalam tanah adalah sangat besar guna menghindari intrusi
air laut.
Melihat perimbangan air secara sektoral maupun secara regional yang
memberikan arah akan kekurangan air serta keuntungan-keuntungan
yang didapatkan dari penggunaan resapan air hujan sebagai
komponen sistem drainasi maka sistem “Drainasi Air Hujan Ramah
Lingkungan” perlu segera dibakukan.
121