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8/3/2019 Efa3c_hydrologic Analysis With Gis
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Hydrologic AnalysisHydrologic Analysiswith GISwith GIS
Eric Anderson
3/18/03
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Outline
Outline
DTMs (Digital Terrain Models)
HydrologicModeling Procedures
Filling Sinks
Determining Flow Direction/Accumulation
Developing CN Numbers
Potential Runoff Map
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DTMsDTMs
Grid Overlay
Each cell assigned a
value (elevation, landcover, etc.)
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Color ClassifiedElevation Data
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Raster Tools in ArcMapRaster Tools in ArcMap
Spatial Analyst ± Raster processing
extension in ArcMap
± Very Powerful Tool
± Allows slope, aspect, azimuth, classifications,and other raster manipulations
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Deriving Attributes from RastersDeriving Attributes from Rasters
Using a DEM
± Slope±rate of elevation change expressed as
percentage or degrees
± Aspect²compass direction of the steepest slope
± END OF THE REVIEW
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Preprocessing DEMsPreprocessing DEMs
DEMs must be preprocessed to eliminate
inherent errors
Sinks
± Most common error is a sink
± Places where water might drain, but are
disconnected with the entire drainage system
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Sink ExampleSink Example
100 100 100 100 100
100 97 96 95 100
1001009998100
100 100 100 100 101
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Flow Direction and AccumulationFlow Direction and Accumulation
Basis of most all GIS hydrological analysis
Once flow direction is known it is possible
to determine how many cells flow into a
given cell
Basis of watershed delineation and stream
networking
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SLOPE
± Slope of each cell based on its eight neighbors
± Largest slope will be assigned to the center cell
Flow Direction and AccumulationFlow Direction and Accumulation
12 13 13
11 108 10 9
10
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FLOW DIRECTION GRID
± Determine flow direction with slope grid
± If one cell¶s slope is lower than its neighbor,
flow will be in the direction of that cell
Flow Direction and AccumulationFlow Direction and Accumulation
100 100 100 100 94
100 97 96 95 100
1001009998100
100 100 100 100 101
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FLOW ACCUMULATION
± Trace backwards up the flow direction grid and
determine the number of cells flowing into each cell ± Accumulated flow is the number of cells flowing into
each cell
± generate a grid to represent the total number of cells
flowing into each cell ± Cells that have high accumulation are designated as
stream channels
Flow Direction and AccumulationFlow Direction and Accumulation
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Flow Accumulation SurfaceFlow Accumulation Surface0 0 0 0 18
03 8 15 0
00220
0 0 0 0 0
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Predictive Eq
uations
Predictive Eq
uations
Runoff Availability Equation
± Can be used to predict the amount of water that will become
runoff given a specific rainfall event ± For example, how much runoff can be expected if there is an
inch of rain over a clay soil with dense vegetation
± Based on soil characteristics and land cover
Q = (sqrt(p ± 0.2 * (1000/CN ± 10))) / (p + 0.8 * (1000/CN ± 10))
Where: Q = total water available for runoff p = precipitationCN = curve number
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Runoff Availability EquationRunoff Availability Equation
Equation based on the CN number
± CN number based on soil and land cover
characteristics²both easily available in raster
or vector format for GIS
± CN values range from 0-100 (like a percentage)
Note: concrete has a CN value of nearly 100
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Runoff Availability Flow ChartRunoff Availability Flow Chart
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Combining Land Cover and HydrologicCombining Land Cover and Hydrologic
Soil Group LayersSoil Group Layers²²Raster Calculator Raster Calculator
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Raster Calculator Raster Calculator
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Combine Land Cover andCombine Land Cover and
Hydrologic Soil Grou
p LayersHydrologic Soil Grou
p Layers 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 thenassigned a value of Curve runoff based on a table you
can get from the NRCS
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Plug in CN Grid and PrecipitationPlug in CN Grid and Precipitation
Grid into Available Runoff EquationGrid into Available Runoff Equation Original Equation
± Q = (sqrt(p ± 0.2 * (1000/CN ± 10))) / (p + 0.8 *
(1000/CN ± 10)) Raster Calculator Operation
± ( ([Precipitation] - 0.2.AsGrid) * ((1000.AsGrid
/ [CN]) - 10.asgrid)).sqr / ( ([Precipitation] +
0.8.AsGrid) * ((1000.AsGrid /[CN]) -
10.AsGrid))
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Potential Runoff MapPotential Runoff Map
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QuestionsQuestions??
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