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