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Sekolah Ilmu dan Teknologi Hayati
Analisis Hidrologi
Materi Perkuliahan n Tujuan Perkuliahan: § Digital Terrain Model § Prosedure Pemodelan Hidrologi • • • • •
Mengisi Sink Membuat Flow Dirrection Membuat Flow Accumulation Membangun Cell Numbers Membuat Batas DAS dan Sub DAS
§ Aplikasi • Model Potensi Runoff
n Pustaka : § Pemodelan
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Daerah Aliran Sungai (Watershed)
Outlet Point A watershed boundary can be sketched by starting at the outlet point and following the height of land defining the drainage divides using the contours on a map.
Minnesota GIS/LIS Consortium Spring Workshops 2000’
Page 3
Daerah Aliran Sungai (Watershed) n Drainage system - The area upon which water falls and the network through which it travels to an outlet. n Drainage Basin - an area that drains water and other substances to a common outlet as concentrated flow (watersheds, basins, catchments, contributing area) n Subbasin - That upstream area flowing to an outlet as overland flow n Pour Point - A location at which the contributing area can be determined. n Drainage Divide - The boundary between two basins. This is an area of divergent flow.
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Daerah Aliran Sungai (Watershed) Stream Network A stream network is composed of those areas where concentrated flow is occurring. Stream Ordering Shreve - When two links intersect, their magnitudes are added and assigned to the downslope link. Strahler - Stream order only increases when streams of the same order intersect
DTM
Y
ELEVATION VALUES
X
n Grid / Raster n Setiap cell assigned a value (elevation, land cover, etc.)
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DTM
Data Elevasi dengan pewarnaan
DTM The elevation values of the DEM can be grouped into intervals. Each interval is then represented by a different gradient color.
Minnesota GIS/LIS Consortium Spring Workshops 2000’
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PROSES DEM n DEM harus di proses terlebih dahulu sebelum dimasukan dalam model hidrologi agar bersih dari kesalahan n Sinks § Kesalahan yang umum adalah sink (sumur/tenggelam) § Sink adalah lokasi dimana air dapat dialirkan, tetapi tidak terhubungkan dengan sistem drainase Sink Example 100
Stream
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100-meter Elevation contour Sink Area
100
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Cell Elevation
Flow Direction & Flow Accumulation n Dasar dari hampir semua analisis hidrologi dgn SIG n Sekali flow direction diketahui, maka memungkinkan untuk membuat berapa banyak cell yang masuk pada satu cell. n Merupakan dasar untuk mendeliniasi daerah aliran sungai (watershed) dan jaringan aliran sungai (stream)
100
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94
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97
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100
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0
0
0
0
18
3
8
15
0
0
2
2
0
0
0
0
0
0
0
0
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Flow Direction n Slope § Slope(kelerangan) pada satu cell bergantung pada 8 cell sekitarnya. § Slope terbesar akan “menyerahkan” ke cell yang tengah.
n Flow Direction Grid § Membentuk flow direction dengan grid kelerangan (slope). § Jika satu cell slope lebih rendah dari cell sekitarnya, aliran akan mengealir ke cell tersebut.
12 13 13 11 10 10 8 10 9
CN elevation CN flowdirection
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32 64 128 16 1 8 4 2
Target Cell
Flow Direction Original Surface 100
Flow Direction Surface
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100
97
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94
2
2
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1
1
1
128
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64
32
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80
1 100
128
64
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Flow Direction
Flow Accumulation n Menelusuri aliran air berdasarkan grid flow direction dan menentukan jumlahh sell yang mengalir pada cell tersebut. n Akumulasi aliran adalah jumla cell yang masuk ke setiap cell nya. Flow accumulation membentuk sebuah grid untuk merepresentasikan total jumlah cell yang mengalir pada setiap cell n Cell yang memiliki jumlah akumulasi yang tinggi dapat dijadikan sebuah kanal/sungai (stream channels) 100
100
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94
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97
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0
0
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0
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Flow Accumulation
0
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Stream
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Flow Length Surface
Flow Direction Surface 2
2
2
1
1
1
128
128
128
128
64
128
64
100
32
1
Flow Length Surface 84.6
30
102.3
72.3
42.3
32
144.6 114.6
84.6
72.3
80
156.9 126.9 114.6 126.9
128
144.6 114.6 132.3
64
0
30 84.6
0
Assuming 30 meter sq. cells
Flow Length Surface Flow Length Surface
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DAS dan Sub-DAS
Pour Point
Contributing Area
DAS dan Sub-DAS Minimum Catchment Area = 5000 cells
Minimum Catchment Area = 10000 cells
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Summary
Membuat Cell Number Pertanian
+
Jalan Sumur
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Run Off
Combining Land Cover and Hydrologic Soil Group Layers—Raster Calculator
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Raster Calculator
Combine Land Cover and Hydrologic Soil Group Layers n Enter in Raster Calculator
§ [Hydrologic Soil Group].Combine( { [Land Use and Cover] } ) § Raster calculator will create a new grid § Values in the new grid represent zones of unique combinations of values in the input grids § Each of these soil/landcover combinations is then assigned a value of Curve runoff based on a table you can get from the NRCS
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Plug in CN Grid and Precipitation Grid into Available Runoff Equation n Original Equation § Q = (sqrt(p – 0.2 * (1000/CN – 10))) / (p + 0.8 * (1000/ CN – 10))
n Raster Calculator Operation § ( ([Precipitation] - 0.2.AsGrid) * ((1000.AsGrid / [CN]) 10.asgrid)).sqr / ( ([Precipitation] + 0.8.AsGrid) * ((1000.AsGrid / [CN]) - 10.AsGrid))
Run Off
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