Runoff Processes
Slides from Venkatesh Merwade
and Suzanne Anderson
Reading: Applied Hydrology
Sections 5.1 and 5.2
Surface water
• Watershed – area of
land draining into a
stream at a given
location
• Streamflow – gravity
movement of water in
channels
– Surface and
subsurface flow
– Affected by climate,
land cover, soil type,
etc.
Sources of streamflow
http://uregina.ca/~sauchyn/geog327/outline.html
Streamflow generation
•
Streamflow is generated by three
mechanisms
1. Hortonian overland flow
2. Subsurface flow
3. Saturation overland flow
•
Some texts mention groundwater ridging
as an additional mechanism contributing
to streamflow
Welcome to the Critical Zone
Denudation
Erosion and
weathering
control the extent
of critical zone
development
Weathering front
advance
Sediment
Water,
solutes and
nutrients
Critical zone
architecture
influences sediment
sources, hydrology,
water chemistry and
ecology
Fort
Collins
The Colorado Front Range
Boulder
Golden
Fort
Collins
The Boulder Creek watershed
•Precambrian crystalline bedrock
•Uniform climate history
•Three erosional states
Boulder
Limits of glacial
and fluvial rejuvenation
Golden
Boulder Creek Critical Zone Observatory
•How do weathering
and transport
processes control the
structure of the CZ?
Glacial limits
Slope map draped over shaded relief.
•What is the impact of
CZ structure on
hydrological,
geochemical and
biological functions?
Three experiments in one:
the geomorphic context of Boulder Creek
Upper: glaciated
Middle: steady, old
Lower: rejuvenated
Front Range ‘Rocky mountain surface’
Lower: Boulder Canyon-Betasso
Oregon Coast Range- Coos Bay
soil
Channel head
weathered
rock
water flow path
fracture
zone
bedding
5m
5m
Anderson et al., 1997, WRR.
Montgomery et al., 1997, WRR
Torres et al., 1998, WRR
Hortonian Flow
• Sheet flow described by
Horton in 1930s
• When i<f, all i is
absorbed
• When i > f, (i-f) results in
rainfall excess
• Applicable in
Rainfall, i
i>q
Infiltration, f
– impervious surfaces
(urban areas)
– Steep slopes with thin soil
– hydrophobic or
compacted soil with low
Later studiesinfiltration
showed that Hortonian flow rarely occurs on vegetated surfaces in humid regions.
Subsurface flow
• Lateral movement of water occurring through the
soil above the water table
• primary mechanism for stream flow generation
when f>i
– Matrix/translatory flow
• Lateral flow of old water displaced by precipitation inputs
• Near surface lateral conductivity is greater than overall
vertical conductivity
• Porosity and permeability higher near the ground
– Macropore flow
• Movement of water through large conduits in the soil
Soil macropores
Saturation overland flow
• Soil is saturated from below by
subsurface flow
• Any precipitation occurring over a
saturated surface becomes overland flow
• Occurs mainly at the bottom of hill slopes
and near stream banks
Streamflow
hydrograph
• Graph of stream
discharge as a
function of time at a
given location on the
stream
Ephemeral river
Direct runoff
Baseflow
Perennial river
Snow-fed River