stream flow estimation. why try to estimate it? gauges don’t exist on every stream important for...
Post on 14-Jan-2016
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Stream flow estimationStream flow estimationStream flow estimationStream flow estimation
Why try to estimate it?Why try to estimate it?
• Gauges don’t exist on every stream
• Important for water quality modeling
• Permitting discharge sites
• Biological importance
• Recreation
• Flood forecasting
• Influenced by human factors
• More USGS gauges are now off-line
Drainage Area WeightingDrainage Area Weighting
STREAMSTATSSTREAMSTATS
STREAMSTATSSTREAMSTATS
7
Using drainage area to estimate stream flowUsing drainage area to estimate stream flow• More of a “back of the envelope” approach• Uses ratio between drainage area and gauged flow to
estimate flow• Assumes stream flow is the same per unit area as a
nearby gauge• Best for sites near gauged locations (ratio .3 to 1.5) and
in the same watershed
Gauge Drainage area = 50
Upstream Sites Drainage area = 15
Downstream Site
Drainage area = 75
Area good to go for estimating flow with the ratio method!
Steps for ratio methodSteps for ratio method
1.Find USGS gauge location downstream and in the same watershed
2.Summarize the recorded flow from gauge (USGS web site)
3.Find the FAC values at gauge and location of interest
4.Apply ratio of flow to drainage area at gauge to all upstream drainage areas in the FAC grid
Application to the Big SandyApplication to the Big Sandy
http://waterdata.usgs.gov/nwishttp://waterdata.usgs.gov/nwis
MapperMapper
MapperMapper
Stream flow infoStream flow info
ArcMapArcMap
ArcMapArcMap
ArcMapArcMap
Drainage area calculationDrainage area calculation
5,169,555 FAC units
FAC * cell size squared = Drainage area in meters squared (10 * 10)
= 516,955,500 meters squared
or
= 127,781 acres
or
= 199.65 square miles ~ 200 square miles
Stream flow infoStream flow info
Stream flow infoStream flow info
ExampleExample• Rockville gauge on the Big Sandy has a drainage area of 5,169,555
FAC units and an instantaneous flow of 226 cfs
Drainage area value --------------------------------------------
Measured or recorded flow cfs
Drainage area value at a site
-----------------------------------
Flow at a site in cfs
=
5,169,555 -------------------
226 Flow
FAC = -------------------
Cross multiply…
Flow * 5,169,555=FAC * 226
Flow=
USGS Gauge Info: Ungauged location of interest:
-------------------5,169,555
FAC * 226
ArcMapArcMap
Are we done?Are we done?
Revised legend for ratioflowRevised legend for ratioflow
Converting grid to linesConverting grid to lines
Possible extent that can use ratio methodPossible extent that can use ratio method
Hydrological limitationsHydrological limitations
• Lack of interception, evapotranspiration, baseflow from groundwater
• Distribution of a rainfall event
Application limitationsApplication limitations
• Best when applied to the same stream as the gauge within .5 to 1.5 of gauge DA.
• Can go down to .3 and into side tributaries but estimate is less reliable.
• Limited to natural systems (no dams or impoundments)
Questions / comments?Questions / comments?
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