Stream flow estimation Why try to estimate it

Why try to estimate it? • • Gauges don’t exist on every stream Important

Using drainage area to estimate stream flow • More of a “back of the

Steps for ratio method 1. Find USGS gauge location downstream and in the same

Drainage area calculation 5, 169, 555 FAC units FAC * cell size squared (10

Example • Rockville gauge on the Big Sandy has a drainage area of 5,

Possible extent that can use ratio method

Hydrological limitations • Lack of interception, evapotranspiration, baseflow from groundwater • Distribution of a

Application limitations • Best when applied to the same stream as the gauge within.

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Stream flow estimation

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 Weighting

STREAMSTATS

Using 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 Upstream Sites Drainage area = 15 Downstream Site Drainage area = 75 7 Gauge Drainage area = 50 Area good to go for estimating flow with the ratio method!

Steps 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 Sandy

http: //waterdata. usgs. gov/nwis

Mapper

Stream flow info

Arc. Map

Drainage area calculation 5, 169, 555 FAC units FAC * cell size squared (10 * 10) = Drainage area in meters squared = 516, 955, 500 meters squared or = 127, 781 acres or = 199. 65 square miles ~ 200 square miles

Stream flow info

Example • Rockville gauge on the Big Sandy has a drainage area of 5, 169, 555 FAC units and an instantaneous flow of 226 cfs Ungauged location of interest: USGS Gauge Info: Drainage area value ---------------------- Drainage area value at a site ------------------ = Flow at a site in cfs Measured or recorded flow cfs 5, 169, 555 ---------226 = Cross multiply… FAC ---------Flow FAC * 226 = Flow * 5, 169, 555 FAC * 226 ---------5, 169, 555 = Flow

Arc. Map

Are we done?

Revised legend for ratioflow

Converting grid to lines

Possible extent that can use ratio method

Hydrological limitations • Lack of interception, evapotranspiration, baseflow from groundwater • Distribution of a rainfall event

Application 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)

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