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A Practicum in Applied GIS AnalysisA Practicum in Applied GIS Analysis A real-world problem...
Transcript of A Practicum in Applied GIS AnalysisA Practicum in Applied GIS Analysis A real-world problem...
APracticuminAppliedGISAnalysisAreal-worldproblem(reduced)
CreateaperfectdatainaGeodatabase(digitizesome,downloadsome),enforcingTopologicalRuleswithinandbetweenlayers
Conditiondata-createneededattributes,assignvalues;turnitinandthenneveruseitagain
Identifyaworkflow(flowchart,analysisroadmap).Receivelargerareadata,andconditionthis
Calculatebasecaseconditions
Modifythelandscapetoachievezerorunoff
WhattheHeckWeDoing?
Practicebuilding/testingaGeodatabases
Calculatingrainfallrunoffatkeylocations
Proposingmodificationsthatreducerunofftozero
Calculatingcosts
WhattheHeckWeDoing?
• ReviewedtheProblemStatement(lastweek)• We’lluseanextremelysimplifiedrainfall/runoff
massbalanceapproach• Firstpartofthesemester,focusonbuilding,
evaluatingthedata• Secondpart,calculatingandproviding
recommendationstoreducerunoff
GrossRainfall
“Throughfall”rainfallminuscanopyinterception
“Hard”surface,concrete,asphalt,building-allrainfallisrunoff
“soft”surface,lawn,uncompactedsoil,flowerbeds-someinfiltration,somerunoff
Accumulatednettoasewergrate(watershed)
CalculatingRunoff
GrossRainfall
“Throughfall”israinfallminuscanopyinterception
CanopyInterception
photosource:ChrisMetcalf
MeasurementstoEstimates
Runoff depth is Rainfall minus Canopy interception minus Absorption, orRunoff = R - CInt - ABS
Watershed water volume is watershed area times runoff, orWVol = WA * Runoff
Whyisthisadifficultproblem?
First,Interception,AbsorptionAren’tUniform
Second,DuringOverlandFlow,ExcessCanBeAbsorbed
SimplifyingAssumptions:
1) Wewillassumeabsorption/infiltrationduetooverlandflowiszero(firstapproximation,allowscellbycellorpolygonbypolygonanalysis)
2) Weassumecategoricalmeansforcanopyinterceptionandsoilabsorption/infiltrationarevalid
GrossRainfall
“Throughfall”rainfallminuscanopyinterception
“Hard”surface,concrete,asphalt,building-allrainfallisrunoff
“soft”surface,lawn,uncompactedsoil,flowerbeds-someinfiltration,somerunoff
Accumulatednettoasewergrate(watershed)
CalculatingRunoff
XX
Throughfall is Rainfall minus Canopy interceptionThroughfall = R - CInt
Here, we give simple calculation, from model, so you can determine the fixed amount, depending on tree type (conifer or hardwood) and size (simple height category)
e.g. “Conifers taller than 9 meters absorb the first 0.85 cm of any storm”
We need a data layer of canopy extent, by height and type
SoilInfiltration/Absorption
Runoff is Throughfall minus Infiltration/absorptionRunoff = Throughfall - ABS
Here, we give simple calculation, from model, so you can determine the fixed amount, depending on soil type
e.g. “Well drained can absorb 75% the first 2.5 cm (1”) of rainfall, all after that is runoff ”
We need a data layer of soil type
To solve this problem then, I need to apply Runoff = R - CInt - ABS
So I need to build my rainfall, canopy interception, and surface absorption layers for current conditions
And sum them over appropriate watersheds (defined by sewer grates) to calculate runoff volumes
I then modify my canopy and surface absorption layers until the sum of runoff over the watershed equals zero