Precipitation

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Precipitation
E
Q
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Precipitation
Canopy
Throughfall
Runoff
Infiltration
River
Soil Water
Discharge
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Canopy

• Canopy

evap(t)
p(t)
ScMax
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• Soilwater

trans(t)
R(t)
Qsubsurface(t)
SsMax
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Relationship between Actual and Potential
Evapotranspiration in soils
Data courtesy of John Albertson, California
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Throughfall at rate p falls on the segment of
area A and thickness D. A portion, R, of this
recharges the subsurface. Subsurface flow from
the segment occurs at rate qsubsurface. Surface
flow, qoverland, occurs from saturated areas
(saturation-excess overland flow). The local
slope at the outflow point, b, is considered to
be equal to the slope of the water table.
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Sequence of events

• Update canopy storage
• Calculate throughfall (p)
• Calculate recharge and direct runoff from the
  saturated area.
• Recharge (1-Asat)p
• Direct runoff Asatp
• Update spatially averaged soilwater storage
• Remove water by transpiration
• Distribute the storage within the catchment
  according to topographic index
• Calculate saturation excess runoff and saturated
  area
• Go back to 1.

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TOPMODEL Assumptions

• Runoff occurs through saturation excess
• Hydraulic conductivity decreases exponentially
  with depth
• The water table moves through a sequence of
  steady states
• The slope of the water table is approximately the
  same as the slope of the ground surface

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Application to Forsinard

• Topmodel on machines in the lab. You can also
  download a copy for your own machines
• For young pine the amount of water stored at the
  end of a storm on the leaves is minimal, so we
  will ignore it. Assume that all the precipitation
  ends up as throughfall.