The SPAC soilplantatmosphere continuum: 1D, vertical fluxes

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The SPAC (soil-plant-atmosphere continuum
1-D, vertical fluxes
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Veg. ecologists (and atmospheric scientists?)
have a pretty good handle on soil-plant-atmosphere
interactions within homogeneous voxels
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I think that some of this knowledge can be
applied very usefully to current thinking in
ecohydrology
Rodriguez-Iturbes basic equation WRR 363-6
nZ ds/dt I (s,t) E (s,t) L (s,t)
nsoil porosity, Zsoil depth, ssoil moisture
content, I(s,t)infiltration, E(s,t)
evapotranspiration, L(s,t) leakage
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Hydrology offers veg. ecology huge opportunities
from a better understanding of landscape-level
(inter-voxel?) processes
Hydrology gravity-driven flows of liquid water
can we call this a landscape perspective?
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For example, to study areas with mixed
vegetation or soil properties, veg. ecologists
typically a) Subsample to try to estimate a
representative mean for the whole, or b)
Divide the area into a grid, apply a model or
other approach to each pixel or voxel, and sum up
the pieces.
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Georgiannes study used the mean value
approach. Sapwood basal area was estimated from
vegetation sampling in transects
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She measured sapflow in various species,
selecting individuals of various sizes and
various positions relative to the stream. In an
ideal situation the sampling would be randomized
through the watershed
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Transpiration on an land area basis was estimated
from the product of average sapflow per unit
sapwood area x total sapwood basal area
Average difference in Daily sap flow 1.23 mm
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• Our goals at the HJ Andrews
• Identify and quantify the ways that vegetation
  affects stream flow in harvested and control
  watersheds in the H.J. Andrews Experimental
  Forest (eco -gt hydro)
• 2. Use streamflow records to test hypotheses
  about age-related change in vegetation structure
  and function (hydro -gt eco)

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But the voxel approach has limitations for
questions like these, For example ..
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We want to know whether streamflow records
confirm the hypothesis that less water is used
for transpiration in watersheds with old growth
vegetation than in watersheds with vigorous,
maturing re-growth.
Average difference in Daily sap flow 1.23 mm
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Average difference in daily stream flow 0.26 mm
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Average difference in daily stream flow 0.02 mm
Average difference in daily stream flow -0.37 mm
Average difference in daily stream flow 0.18 mm
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Are the transpiration estimates completely
wrong? If not, why dont differences in summer
streamflow account for the differences in
transpiration?
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Perhaps the soil in the harvested watershed is
more depleted in moisture, but the impact does
not affect streamflow until this capacitor is
refilled by fall rains.
We need to know more about soil water depletion,
re-wetting, and transport to streams.
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Another limitation of the voxel approach what
if voxels interact
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On what scales of time and space, and what
conditions, is the watershed buffered i.e.
there will no change in a response variable
(summer stream flow?) in response to changes in
vegetation?
Does low water use in patches with poor
regeneration result in greater water use by
downslope vegetation later in the season?
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To begin answering this question we need to
understand water flow paths and transport times.
Complete removal of vegetation here may have
little or no effect on stream flow if the water
not used in transpiration is later used by
downslope vegetation.
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Finally Lets determine future directions for
ecohydrology that cover new territory rather
than reinvent wheels.
For example, Rodriguez-Iturbe suggests hydrologi
sts need to explain the effects of low water
availability .. on productivity of natural
ecosystems
My take ecologists have a pretty good
understanding of relationships between soil water
availability and productivity. We need to
identify questions and goals where the
interdisciplinary thinking opens up whole new
opportunities.
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Correspondence between 24-hour patterns of peak
transpiration and streamflow
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