AOM 4932 - Surface Water
Surface runoff - Precipitation or snowmelt which moves across the land surface
ultimately channelizing into streams or rivers or discharging into lakes
Watershed - Land area which contributes surface runoff to a specified point of interest 
typically stream outlet
Note: entire area within watershed may not contribute runoff due to depressions,
detention ponds, etc.
Watershed divide - Imaginary lines delineating adjacent watersheds. Normally follow
ridges and can be delineated with topographical maps.
Relationship between precipitation and runoff is influenced by various storm and basin
characteristics:
Storm
Basin (or Watershed)
intensity
area, shape, slope
duration
soil, vegetation, geology
areal extent
stream patterns (length, degree of branching)
uniformity
Initially large portion of precipitation goes into surface storage (initial abstraction), then
soil moisture storage (governed by infiltration equations).
Both of these types of storage can be classified as either:
i) retention storage - long term, depleted by evaporation
ii) detention storage - short term, depleted by outflow
After detention storage volume begins to fill, outflow begins to occur. Can be
1) groundwater flow
all can ultimately
2) unsaturated flow
end up as streamflow
3) overland flow
4) channel flow
Streamflow Hydrograph
Plot of volumetric flow rate vs. time at a particular point in stream or river. Typically at
outlet of watershed. Gives spatially and temporally integrated measure of runoff
production at a point in stream.
Annual streamflow hydrograph shows long term balance of precipitation, evaporation and
streamflow.
Individual storm hydrograph - most widely used method of evaluating surface runoff.
Shows relationship between peak streamflow’s and individual storms.
Hydrographs are used to predict peak flow rates so that hydraulic structures can be
designed to accommodate flow safely and to evaluate water quality effects associated
with surface runoff. Also integrating hydrograph over time gives volumes needed to
design reservoirs, detention ponds etc.
Typical Storm Hydrograph
if crest segment
stabilizes and
plateaus max.
storage attained
inflow - outflow
crest segment
reflects mostly
storm
characteristics
Q
m3/s
entire drainage
area contributes
to runoff but still
storage remains
to be filled
rising limb
first arrival of runoff from
nearby regions  may be
delay from onset of storm due
to rainfall absorbed by soil
and transmission time
falling limb - reflects mostly
basin geometry
(stored water drains)
next storm
begins
time
furthest point
begins to drain
no direct
runoff reflects
base flow
Components of hydrograph:
channel
precipitation
Q
rainfall
begins
rainfall
ends
1)
2)
3)
4)
direct
surface
runoff
direct surface runoff (Horton and Dunne)
interflow (lateral flow at shallow depths)
groundwater (base) flow
channel precipitation
interflow
groundwater flow (baseflow)
time
Direct surface runoff can be generated by two mechanisms:
1) Hortonian Overland Flow - Occurs when rainfall exceeds infiltration capacity of the
soil. Saturation of soil occurs from land surface down - applicable for impervious
surfaces in urban areas. Low permeability soils.
2) Dunne Overland Flow (also called Saturation Overland Flow) - All rainfall infiltrates
and results in a raising of the watertable.
saturation
overland flow






increased
outflow to
stream
If rains long enough saturation of soil occurs from below. Get no more infiltration 
Overland flow. Occurs in shallow water table flatwoods regions of Florida. First in lowlying areas near streams and wetlands.
How do we determine when Dunne runoff will occur?
Total available soil moisture storage (in inches of rain) = (total porosity - water content at
beginning of storm)*depth to water table at beginning of storm.
Therefore time to onset of Dunne runoff= total available soil moisture storage/rainfall rate
Before onset of Dunne runoff: runoff rate=0 (all rainfall infiltrates to fill storage)
After onset of Dunne runoff: runoff rate=rainfall rate