Photo: Don Buso

GAPS! Why they occur, and what we can do about it.

Craig See, Ruth Yanai, John Campbell and Amey Bailey

Causes of stream flow gaps

25%

50%

6%

13%

6%

Equipment failure

Maintenance

Operator error

Ice in v-notch

Debris in v-notch

Misc.68%

12%

8%

10%

1% 1%

Hubbard Brook Coweeta

Watershed 5 stream flow (ft3/s)

Wat

ersh

ed 6

stre

am fl

ow (ft

3 /s)

Using lots of fake gaps to simulate real ones

• For each real gap in the record, randomly select a point in the record to create a corresponding “fake” gap:o Same length of the real gap o Stratify sampling by flow rateo Calculate difference between

observed and expectedo 10,000 iterations to create an

error distribution• For multiple gaps in a year, sum the

errors of all simulated gaps for each iteration

Uncertainty in annual stream export due to gaps

Wet year~0.4%

Dry year~1.9%

Advantages:• Does not rely on statistical assumptions (normality,

homoscedasticity)• Can be modified to fit most gap-filling methods• Conceptually simple

Disadvantages:• Requires a long-term record• Hard to characterize extreme (infrequent) events

Code available soon! www.quantifyinguncertainty.org

Uncertainty in annual stream export due to gaps

-6.7% 6.7%

Annual stream flow uncertainty (mm)

Freq

uenc

y

14.5% 14.5%

Wet Year (1996) Dry Year (2001)

Causes of precipitation volume gaps at Hubbard

Brook

Gaps in the Streamflow Record(days)

Hubbard Brook Coweeta