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?