Efa3c_hydrologic Analysis With Gis

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Hydrologic AnalysisHydrologic Analysiswith GISwith GIS

Eric Anderson

3/18/03



Outline

Outline

DTMs (Digital Terrain Models)

HydrologicModeling Procedures

Filling Sinks

Determining Flow Direction/Accumulation

Developing CN Numbers

Potential Runoff Map



DTMsDTMs

Grid Overlay

Each cell assigned a

value (elevation, landcover, etc.)



Color ClassifiedElevation Data



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





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



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



Sink ExampleSink Example

100 100 100 100 100

100 97 96 95 100

1001009998100

100 100 100 100 101



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



SLOPE

± Slope of each cell based on its eight neighbors

± Largest slope will be assigned to the center cell


12 13 13

11 108 10 9

10



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


100 100 100 100 94

100 97 96 95 100

1001009998100

100 100 100 100 101



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 Accumulation SurfaceFlow Accumulation Surface0 0 0 0 18

03 8 15 0

00220

0 0 0 0 0



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



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



Runoff Availability Flow ChartRunoff Availability Flow Chart



Combining Land Cover and HydrologicCombining Land Cover and Hydrologic

Soil Group LayersSoil Group Layers²²Raster Calculator Raster Calculator



Raster Calculator Raster Calculator



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



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))



Potential Runoff MapPotential Runoff Map



QuestionsQuestions??

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Efa3c_hydrologic Analysis With Gis