- channel precipitation
- direct interception of precipitation
by streams
- much less than 5% and usually less
than 1% of runoff reflecting the area of surface water as a
proportion of basin area, i.e., only important where there
is a large area of wetlands or lakes or during floods
- but it always occurs
- overland flow
- unconfined runoff, expressed as
discharge per unit width of slope
Hortonian
- saturation from above
- described by Horton in a series of
papers in the '30s and '40s, where he suggested it was the
principal, if not only, mechanism of overland flow and occurred on
virtually all slopes
- sheet flow or anastomosing rivulets
move as a kinematic wave (independent of the force, e.g.,
like traffic jams moving at a different rate than the traffic)
- it occurs when precipitation
intensity exceeds infiltration capacity, i.e. only with
- soil that is saturated with
antecedent water or after a long duration rain
- frozen ground
- steep slopes and thin soil
- hydrophobic or compacted soil
- bare soil, because on vegetated
surfaces, organic matter and biological activity contribute to
higher hydraulic conductivity and thus infiltration capacity
- in the 1960s, the partial-area
concept was introduced, whereby Hortonian overland flow occurs in
a limited contributing area that varies from basin to basin but is
fairly constant within a basin, except for extreme events
- then field studies demonstrated that
Hortonian overland flow was far from ubiquitous and that more
often saturation occurs from below
saturation overland flow
- saturation from below
- water is returned to the surface by
exfiltration
- first identified for recharge of
shallow groundwater, i.e., adjacent to humid region streams
- it also occurs:
- in hillslope hollows where
groundwater flow lines converge
- at slope concavities (i.e.,
base of many slopes); with decreasing hydraulic gradient, flow
decelerates and therefore the depth of subsurface flow
increases
- in thin soils over an
impermeable surface, e.g., rock, frozen ground, heavy
soil horizon
- where hydraulic conductivity
decreases with depth and thus soil water becomes perched
- variable source area concept
- the area of saturation from below
varies greatly with time, reflecting the overall wetness of the
watershed
-
- interflow
- lateral movement of soil water
between the ground surface and a regional or perched water table
- can account for a significant
proportion of the runoff in humid regions
- in sloping landscapes, interflow may
exfiltrate at the surface producing saturation overland flow
matrix flow
- "water-content-dependent
conductivity anisotropy:" (Dingman), i.e., depends
on antecedent soil water conditions and flow is preferentially
lateral given greater lateral hydraulic conductivity than
vertical
- a common condition since the
porosity and permeability of soil tend to decrease with depth
given the weight of overlying soil and the translocation of
material in percolating water
macropore flow (rapid interflow)
- soil water passing quickly to a
stream through root canals, animal tunnels or pipes produced
by subsurface erosion
- groundwater flow
- in the zone of saturation below a
perched or regional water table
- days, weeks or even years may pass
before water that seeps to the water table eventually reaches a
stream
- in dry soil, infiltrating water
largely replenishes soil water storage and thus does not
contribute to groundwater recharge
- however, some groundwater can reach
a stream during or shortly after an input event via
- translatory flow, when a belt of
antecedent water is forced by new seeping water
- a rapid local rise in the water
table producing a groundwater mound or ridge near streams; the
higher local hydraulic gradient produces a rapid flux of water
from the streamward side of the mound; it may also cause
saturation overland flow on the floodplains
- a perched groundwater below a
slope; these sloping slabs are not connected to the regional
groundwater circulation; they thicken downslope (again
producing saturation overland flow) and can contribute
baseflow within days or less with intense rain and/or
permeable sediments
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