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.
precipitation/runoff generation:
•  Snowmelt
•  Rain-on-snow
•  Rainfall
-  low intensity (cold fronts)
-  high intensity (thunderstorms)
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
broken
power law
dN =
dQ ⎛
⎜⎝
a0: intercept
a0 a1
⎞
1⎟⎠
Qa
α
+Qa
⎛
⎜⎝
⎞
1⎟⎠
β
a1: inflection pt
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