relations between precipitation and runoff: • forms of precipitation

Relations between precipitation and runoff:

• forms of precipitation / runoff

– effect on intensity and variability of floods

• associations between high flows and channel morphology

– bankfull discharge, dominant discharge, etc.

•  Snowmelt

•  Rain-on-snow

•  Rainfall -  low intensity (cold fronts)

-  high intensity (thunderstorms)

precipitation/runoff generation:

Snowmelt-dominated systems

- in the western US snowmelt produces ~80% of the total runoff (v. important for H2O supply) - energy to melt snow derived from:

• solar radiation

• sensible heat transfer (warm air)

maximum snowmelt rates = average rainfall rates (Cline, 1997)

Rainfall-dominated systems

- rainfall also provides runoff, sometimes too much

- intensity of rainfall is generally much higher than snowmelt

Rain-on-snow events

Additional factors: forest cover, soil types

Some definitions

mean annual discharge

discharge exceeded 10%

of the time

discharge exceeded 5% of the time

(about when sed. transp. begins)

bankfull discharge

(exceeded ~2% of the time)

Annual floods (instantaneous

peak discharges)

mean annual (2-yr) flood

Flood frequency distributions

probability density function (histogram)

cumulative density function (flood frequency curve)

log-transformation

slope of FF curve reflects variability of peak discharges

Rainfall Snowmelt

Relations between peak flows, precipitation, and drainage basin characteristics

Questions: • What factors determine peak flows in moderate-large drainage basins?

- seasonal patterns of precipitation and runoff?

- topography? slope, drainage density? • Are floods in small drainage basins more variable (flashier) than floods in

large drainage basins?

- what are the effects of drainage basin scale?

- effects of precipitation intensity and runoff?

Colorado Front Range

Conclusions

1.  Magnitude of the 100 yr flood varies significantly with precipitation and runoff-generating mechanisms

2.  Mean annual flood is most highly correlated with drainage area and mean annual precipitation

3.  No correlation between flood variability (standard deviation), basin characteristics or drainage area

4.  Most of the variability in floods reflects differences in the variability of precipitation

Variability in annual discharge and peak discharge is generally higher in areas where rainfall intensity is higher

Variability in annual discharge

Daily flow frequency distributions

probability density function (histogram)

cumulative density function

Goal: Model frequency (d/yr) of bed disturbance

Segura et al., 2010b

Segura and Pitlick, WRR, 2010a

32 sites in Colorado, 32 sites in Idaho

dNdQ

= a0 a1

Q a1⎛⎝⎜

⎞⎠⎟α+ Q a1

⎛⎝⎜

⎞⎠⎟β

broken power law a1: inflection pt

a0: intercept

as drainage area increases the inflection point (a1) shifts, meaning the frequency of flows < Qbf increases downstream

Implications.... Why is this important?

EPA: “Bankfull discharge is associated with a maximum flow that has an average recurrence interval of 1.5 years as determined using a flood frequency analysis (Dunne and Leopold 1978).”

We find:

• the frequency of bankfull discharge varies with scale

- intermediate Q occur more frequently as DA increases

• duration of flows that can transport sediment increases

• channel adjusts width / depth to accomplish this