SWRM : SUSTAINABLE WATER RESOURCES MANAGEMENT

SWRM:SUSTAINABLE

WATER RESOURCES MANAGEMENT

Smno.psdl.pdkl.ppsub2013

MK. PENGELOLAAN SDALH

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 sungai-sungai mengalir di bawah mereka, kemudian Kami binasakan

mereka karena dosa mereka sendiri, dan kami ciptakan sesudah mereka generasi yang lain”

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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 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.

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DAUR HIDROLOGI

CURAH HUJAN

VEGETASI

PERMUKAAN TANAH

CADANGAN SALURAN AIR TANAH

AIR BAWAH TANAH

EVAPOTRANSPIRASI KEBOCORAN LIMPASAN

transpirasi

evaporasi

Aliran permukaan

banjir

Aliran bawah permukaan

Aliran air bawah tanah

intersepsi

infiltrasi

perkolasi

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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

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.

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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 terpisah-pisah atau bergabung menjadi gumpalan yg

bentuknya tak teratur (hail)

www.fao.org/docrep/r4082e/r4082e05.htm

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.

Effective rainfall (8) = (1) - (4) - (5) - (7)

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Shift in Thinking is Neededfrom Blue Water to Green Water

Under the pressures of population growth, developmentaspirations 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.

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Green water is a significant water resource, much larger volume-wise than the water replenishing streams, lakes and aquifers (blue water).

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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

ecosystems.

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GREEN = BLUE BALANCE

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The reality of the big picture is that in a drainage basinperspective, 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.

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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 feed the world of tomorrow.

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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.

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.

Green water is the soil moisture,

exhaled during plant growth as vapour flow from land to the atmosphere.

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 rainfall-based sources of

income.

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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.

Climate change is a strong driving force for lessening society’s large-scale dependence on fossil fuels

through increased use of renewable energy, though such a move will

increase consumptive water use for biomass-based fuel alternatives.

The future conflicts of interest will be over land

use-water use, water quantity-quality,

upstream-downstream availability, and humans-

ecosystems.

A sustainable water future needs to incorporate the waterfrom 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.

Water is of central importance in other

sectors: industryproduction, forestry

and fibre production,

fisheries, etc.

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

consumesfreshwater) a determinant of

blue water availabilitydownstream.

A sustainable water future needs to incorporate the waterfrom 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 tosustain 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 anti-

desertifi cation.

Water supply for various sectors of society is gettingincreasingly complicated as water contaminationescalates, and awareness grows among water users

of the links between upstream polluters of

water withdownstream water users.

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Key Recommendations1. 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.

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.

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DAS adalah kesatuan terkecil dari pengelolaan

air, aspek pengelolaannya

meliputi:

Daerah tangkapan hujan

Kuantitas air Kualitas air

Pengendalian banjir

Jasa Lingkungan DAS

Prasarana pengairan

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CALCULATING A MONTHLY WATER BUDGET

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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)

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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

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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 ke dalam tanah.

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Sprinkler IrrigationMany types micro-

sprinklers, 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

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KANDUNGAN AIR DAN

TEGANGAN

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 samaTanah yang Strukturnya baik mempunyai total pori lebih banyak, shg mampu menahan air lebih banyakPori medium dan mikro lebih kuat menahan air dp pori makro

Tegangan air tanah, Bar 10.000

Liat

Lempung

Pasir

0.0110 Kadar air tanah, % 70

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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

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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.

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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

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Gerakan Jenuh (Perkolasi)

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

Lempung berpasir Lempung berliat

cm 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

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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

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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

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PERKOLASI

Jumlah air perkolasiFaktor yg berpengaruh:

1. Jumlah air yang ditambahkan2. Kemampuan infiltrasi permukaan tanah3. Daya hantar air horison tanah4. 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 & medium2. Liat tipe 2:1 yang mengembang-mengkerut sangat berperan

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LAJU GERAKAN

AIR TANAH

Kecepatan gerakan air dlm tanah dipengaruhi oleh dua faktor:1. Daya dari air yang bergerak2. Hantaran hidraulik = Hantaran kapiler = daya hantar

i = k.fdimana 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 bawah2. 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 tanah2. Besarnya tegangan untuk menahan air

Pada gerakan jenuh, tegangan airnya rendah, shg hantaran hidraulik berbanding lurus dengan ukuran poriPd tanah pasir, penurunan daya hantar lebih jelas kalau terjadi penurunan kandungan air tanahLapisan pasir dlm profil tanah akan menjadi penghalang gerakan air tidak jenuh

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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 HIGROSKOPISMolekul 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.

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Status Air Tanah

Perubahan status air dalam tanah, mulai dari kondisi jenuh hingga titik layu

Jenuh Kap. Lapang Titik layu

100g 8g udara

Padatan Pori

100g 20g udara

100g 10 g udara

100g air 40g tanah jenuh air

kapasitas lapang

koefisien layu

koefisien higroskopis

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Agihan air dalam tanah

Berdasarkan tegangan air tanah dapat dibedakan menjadi tiga bagian: Air bebas, kapiler dan higroskopis

Koef. Higroskopis Kap. Lapang Jml ruang pori kurang lebih 31 atm kurang lebih 1/3 atm

Lapisan olah

Air higros- Air Kapiler Ruang diisi udara kopik Peka thd gerakan Biasanya jenuh uap air Hampir tdk kapiler, laju pe- Setelah hujan lebat menunjukkan nyesuaian me- sebagian diisi air, sifat cairan ningkat dg me- tetapi air cepat hi- ningkatnya ke- lang krn gravitasi

lembaban tanah bumi Lapisan bawah tanah Karena pemadatan ruang

pori berkurang

Strata bawah (jenuh air)

Kolom tanah Jumlah ruang pori

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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

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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 penyerap2. Pertumbuhan akar ke arah zone tanah yang mengandung air

LAJU GERAKAN KAPILER

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%)

Bulu akar menyerap

air

Jumlah air tanah

berkurang

Tegangan air tanah

meningkat Terjadi perbedaan Tegangan

dg air tanah di sekitarnya

Terjadi gerakan kapiler

air menuju bulu akar

Laju gerakan tgt perbedaan

tegangan dan daya hantar pori tanah

Gerakan kapiler 2.5 cm

sagt penting

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KEHILANGAN UAP AIR

DARI TANAH

HADANGAN HUJAN OLEH TUMBUHANTajuk 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.

Awan hujan Pembentukan Awan

Tanah permukaan

Groundwater Batuan

Sungai - laut

presipitasi

infiltrasi

perkolasi

Run off

transpirasi

evaporasi

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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 70.3 65.0Melalui batang 22.8 20.4

Jumlah di tanah 93.1 85.4Yang tinggal di atmosfer 6.9 14.6

Sumber: J.L.Haynes, 1940.

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Lingkaran Tanah-Air-Tanaman

LTAT mrpk sistem dinamik dan terpadu dimana air mengalir dari tempat dengan tegangan rendah menuju tempat dengan

tegangan air tinggi.

Serapan bulu akarPenguapan

Hilang melalui stomata daun (transpirasi)

Air kembali ke atmosfer

(evapo-transpirasi)

Air dikembalikan ke tanah melalui hujan

dan irigasi

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EVAPO-TRANSPIRASI

Kehilangan uap air dari tanah:1. EVAPORASI: penguapan air dari permukaan tanah2. TRANSPIRASI: Penguapan air dari permukaan tanaman3. EVAPOTRANSPIRASI = Evaporasi + TranspirasiLaju 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 Penyinaran2. Tekanan uap air di atmosfer3. Suhu4. Angin5. Persediaan air tanah

Air tanah Evapotranspirasi (cm:Jagung Medicago sativa

Tinggi 17.7 24.4Sedang 12.7 20.5

Sumber: Kelly, 1957.

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WATER BALANCE

Gains: precipitation

Soil moisture storage                                                                           Losses: utilization and evapotranspirationactual 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

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Ketersediaan Air Tanah vs

Evapo-transpirasi

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 24.25 23.45 23.2717.5 - 180.0 20.75 21.17 22.25

Sumber: Dreibelbis dan Amerman, 1965.

55

Soil moisture balance diagrams

(Thornthwaite and Mather - 1955) - soil

depth 50 cm.

56

Soil moisture balance diagrams

(Thornthwaite and Mather - 1955) - soil

depth 300 cm.

57

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 evapotranspirasiIni merupakan angka-praktis untuk keperluan pengairan

Dua faktor penting yg menentukan PK adalah:1. KEDALAMAN PERAKARAN TANAMAN2. 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 TRANSPIRASIFaktor yg berpengaruh adalah:

1. Perbandingan luas tutupan tanaman thd luas tanah2. Efisiensi pemakaian air berbagai tanaman3. Perbandingan waktu tanaman berada di lapangan4. Keadaan iklim

Di daerah basah : EVAPORASI TRANSPIRASIDi daerah kering:

1. EVAPORASI 70 - 75 % dari seluruh hujan yg jatuh2. TRANSPIRASI 20 - 25%3. RUN OFF 5%

58

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

dihasilkan2. Transpirasi (dalam kg) setiap kg jaringan tanaman yg dihasilkan

……… NISBAH TRANSPIRASI

Jumlah air yg diperlukan untuk menghasilkan 1 kgbahan kering tanaman

NISBAH TRANSPIRASIUntuk tanaman di daerah humid: 200 - 500, di daerah arid duakalinya

Tanaman Nisbah Transpirasi

Beans 209 - 282 - 736Jagung 233 - 271 - 368Peas 259 - 416 - 788Kentang 385 - 636

Sumber: Lyon, Buckman dan Brady, 1952.

59

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 & PLASTIKBahan mulsa dihamparkan di permukaan tanah, diikat spy tdk terbang, dan tanaman tumbuh melalui lubang-lubang yg telah disiapkanSelama 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 lainnyaBahan mulsa dipotong-potong dan disebarkan di permukaan tanahCara 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

60

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)

61

Olah Tanah vs Penguapan Air

Tanah

Alasan pengolahan tanah: 1. Mempertahankan kondisi fisika tanah yg memuaskan2. Membunuh gulma3. Mengawetkan air tanah.

Pengendalian Penguapan vs Pemberantasan Gulma

Perlakuan Hasil jagung (t/ha) Kadar air tanah (%) hingga kedalaman 1 m

Tanah dibajak dg persiapan yg baik 1. Dibebaskan dari gulma 2.9 22.3 2. Gulma dibiarkan tumbuh 0.4 21.8 3. Tiga kali pengolahan dangkal 2.5 21.9Persiapan Buruk 4. Dibebaskan dari gulma 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 tanamanPengolahan tanah yg berlebihan dapat merusak akar tanaman dan merangsang evaporasi, shg merugikan tanaman

62

Soil Moisture and Groundwater

63

Stream Flow 

64

Idealisasi Aquifer

65

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.

66

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 rainfall-runoff 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

67

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

68

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...

69

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

70

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...

71

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

72

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...

73

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

74

The quantity of runoff can be estimated using one of several different methods.

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.

redac.eng.usm.my/html/USWM/SUDS.htm

75

The quantity of runoff can be estimated using one of several different methods.

Computer models can analyze rainfall-runoff 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

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.

76

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

rpitt.eng.ua.edu/.../WinSlamm/Ch1/M1.html

77

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.

Most urban areas collect runoff in

gutters and sewers and discharge it at various points to receiving waters

geoscape.nrcan.gc.ca/.../images/outdoors_e_.jpg

78

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.

79

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_annotations.htm

80

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.

81

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/

82

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

83www.okobojiprotectiveassociation.org/stewards...

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.

84

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.

85

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).

86

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

87

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)

88

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.

89

Factors affecting runoffApart 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

90

Infiltration capacity curves for different soil types

91

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.

92

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.

93

Runoff efficiency as a function of catchment size

94

InfiltrasiInfiltrasi 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.

95

InfiltrasiInfiltrasi mempunyai arti penting terhadap :

a. Proses LimpasanDaya 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 TanahPengisian 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.

96

InfiltrasiFaktor-faktor yang mempengaruhi infiltrasi adalah:

1. Karakteristik –karakteristik hujan2. 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.

97

InfiltrasiFaktor-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

98

InfiltrasiFaktor-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.

99

InfiltrasiFaktor-faktor yang mempengaruhi daya infiltrasi antara lain :

a. Dalamnya genangan di atas permukaan tanah (surface detention) dan tebal lapisan jenuh

b. Kadar air dalam tanahc. Pemampatan oleh curah hujan

d. Tumbuh-tumbuhane. 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.

100

PengertianInfiltrasi 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).

101

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.

102

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.

103

Infiltration devicesWhat are they?

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.

Infiltration devices

104

Infiltration devices : How they workInfiltration 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.

105

QuantityInfiltration 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.

QualityRunoff 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.

Infiltration devices

106

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

107

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.

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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.

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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.

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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.

SUMUR RESAPAN AIR HUJANTinggi 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:

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.allencountygeology.indiana.edu/water-table-el...

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SUMUR RESAPAN AIR HUJANTinggi tekanan akan seimbang pada suatu titik tertentu untuk

permukaan air asin (S) maupun air tawar (F).

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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.