Transcript Some Ecohydrology Examples to Animate

Primer on Ecosystem Water
Balances
Lecture 2
Ecohydrology
Water Balance
• Inputs (cross-boundary flows)
– Rainfall
• Stochastic in interval, intensity and
duration
– Runin/Groundwater?
• Outputs
– Evapo-transpiration
– Surface runoff
– Infiltration
• Key internal stores/processes
–
–
–
–
–
–
Soil moisture
Interception
Stomatal regulation
Sap-flow rates
Boundary layer conductance
Capillary wicking
Water Balance
• P = ET + R + D + ΔS
– P – precipitation
– ET – evapotranspiration
• Contains interception (I), surface evaporation (E) and plant
transpiration (T)
– R – runoff
– D – recharge to groundwater
– ΔS – change in internal storage (soil water)
• Quantities on the RHS are functions of each other
– ET, R and D are a function of ΔS, and vice versa
– Plants mediate all of the relationships
Soil-Plant-Atmosphere Continuum
• ET through a chain of resistances in series
–
–
–
–
–
Boundary layer (canopy architecture)
Leaf resistance (stomatal dynamics)
Xylem resistances (sapwood area, conductivity)
Root resistances (water entry and transport)
Soil (matrix resistance)
• Individual plasticity and changes in composition (i.e.,
species level variability) affect each process at
different time scales. Creates important feedbacks
between the ecosystem and it’s resistance properties
Figuratively
• Driven by a vapor pressure
deficit between the soil and
atmosphere and net radiation
• Soil evaporation is a minor (~5%)
portion of total ecosystem water
use
Atmospheric
Demand
Boundary layer
Leaf control
– Most water passes through plant
stomata even in wet areas with
low canopy cover
Stem control
• Evolutionary control on
resistances and response to
stresses
Root control
– For example, cavitation of
the SPAC in the xylen
Soil resistance
Soil Moisture
The SPAC (soil-plant-atmosphere continuum)
Yw (atmosphere)
 -95 MPa
Yw (small
branch)
 -0.8 MPa
Yw (stem)
 -0.6 MPa
Yw(soil)  -0.1 MPa
Yw (root)
 -0.5 MPa
How Does Water Get to the Leaf?
Water is PULLED, not pumped.
Water within the whole plant
forms a continuous network of
liquid columns from the film of
water around soil particles to
absorbing surfaces of roots to
the evaporating surfaces of
leaves.
It is hydraulically connected.
Radiation, Wind
-
Vapor
Pressure
Deficit
Rainfall
+
-
+
Boundary,
Leaf, Stem, Soil
Conductance
+
+
Intercepted
Water
-
+
Primary
Production
-
+
Infiltration
+
Soil Moisture
-
Runoff
Vapor Deficit (Dm = es – ea)
• Distance between
actual conditions and
saturation line
– Greater distance =
larger evaporative
potential
• Slope of this line (s) is
a term in ET
prediction equations
– Usually measured in
mbar/°C
Key Regulatory
Processes
• Interception
– I = S + a*t
– Interception (I) is canopy
storage plus rain event
evaporation rate * time
• Mean I ~ 20% of P
• Annual I in forests > crops
and grasses because of
seasonal effects
Zhang et al. (1999)
Key Regulatory Process - ET
ENERGY
AERODYNAMIC
• Penman-Monteith Equation
• Ω is a decoupling coefficient (energy vs. aerodynamic terms; 0-1)
– Vegetation controls this; higher in forests, lower in grasslands
• s is the slope of saturation vapor pressure curve, γ is the psychrometric
constant, ε is s/γ, Rn is net radiation, G is ground heat flux, ρ is the density
of air, Cp is the specific heat capacity of air, Dm is the vapor pressure
deficit, rs is the surface resistance and ra is the aerodynamic resistance
ET and Surface Resistance
• ra is the resistance of the
air to ET, controlled by
wind speed and surface
roughness
• rs is resistance for vapor
flow through the plant
or from the bare soil
surface
• Vegetation effects
ET (indexed to PET) from a dry canopy as
a function of surface resistance (rs) at
constant aerodynamic resistance (ra)
– Leaf area index (LAI)
– Stomatal conductance
– Water status (wilting)
Albedo Effects
• Species type affects ecosystem energy budget
Net-radiative forcing of
boreal forests following fire
is dominated by albedo
effects (Randerson et al
2006)
Stomata – “Ecohydrologic Engineers”
• Air openings, mostly
on leaf under-side
– 1% of leaf area, but ~
60,000 cm-2
– Function to acquire
CO2 from the air
– Open and close
diurnally, and in
response to soil H2O
tension
• React to wilting (loss
of leaf water)
Guard cells (shape
change with turgor
pressure)
Stomatal Conductance
• Rate of CO2 (H2O)
exchange with air
(mmol m-2 s-1)
Specific Variation
• Conductance properties vary
by species
– Feedbacks between water use
and succession
– Comparative climate change
vulnerability
Rooting Depth
Rooting Depth Effects
Surface
2 months later
Hydraulic Redistribution
• Roots equilibrate soil moisture (even
when stomata are closed)
– Cohesion-tension theory, where
tension is exerted by potential
gradients, and water forms a
continuous “ribbon” because of
cohesion forces
• Water transport from well watered
locations to dry locations
– Local spatial variation in irrigation
– Deep water access via tap-roots
(“hydraulic lifting”)
• Facilitation effects (deep-rooted
plants supplying shallow moisture)
Richards and Caldwell (1987)
A Simple Catchment Water Balance
• Consider the net effects of
the various water balance
components (esp. ET)
– At long time scales (e.g., > 1
year) and large spatial scales
(so G is ~ 0): P = R + ET
• The Budyko Curve
– Divides the world into “water
limited” and “energy limited”
systems
– Dry conditions: when Eo:P → ∞,
ET:P → 1 and R:P → 0
– Wet conditions: when Eo:P → 0 ET
→ Eo
Budyko Curve
Evidence for One Feedback – Forest Cover
Affects Stream Flow
CO2
Jackson et al. (2005)
H2O
1 : 300
Moreover – Species Matter
Evidence for Another Feedback – Composition
Effects on Water Balances
• Halophytic salt cedar • Pataki et al. (2005) studied stomatal
invades SW riparian
conductance for both species in response
areas
to increased salinity
• Displaces cottonwoods, de-waters
riparian areas
Pataki et al. (2005)
Adding Processes (and Feedbacks)
• Organic matter affects soil moisture dynamics
• Vegetation affects soil depth (erosion rates)
• Soil moisture affects nutrient mineralization
(esp. N)
• Inter- and intra-specific interactions
(facilitation, inhibition)
Coupled Equations to Describe PlantWater Relations in a Forest
• Peter Eagleson
(1978a-g)
– 14 parameter model
links rain to
production via soil
moisture
– Posits three
“optimality criteria” at
different scales
In Equation Form (yikes)
Eagleson’s Optimality Hypothesis #1
• Vegetation canopy density will equilibrate with
climate and soil parameters to minimize water stress
(= maximize soil moisture)
– Idea of an equilibrium is reasonable
• “Growth-stress” trade-off
• Stress not explicitly included in the model
– Evidence is contrary to maximizing soil moisture
• Communities self-organize to maximize productivity subject to
risks of overusing water between storms
– Tillman’s resource limitation hypothesis predicts excess
capacity in a limiting resource will be USED
Optimality Criteria #2
• Over successional time, plant interactions with repeated
drought will yield a community with an optimal transpiration
efficiency (again maximizing soil moisture, because that is
how a plant community buffers drought stress)
– Actually impossible (or nonsense at least)
• A community that uses less water will replace a community that uses
more (contradicts all of successional dynamics)
• The equilibrium occurs at “zero photosynthesis” because that is the state
at which transpiration loss is minimized.
– While the central prediction is probably in error, the basic idea of
some non-obvious equilibrium emerging from the negotiation
between climate, plants and soils is an idea that others have built on
Optimality Criteria #3
• Plant-soil co-evolution occurs in response to
slow moving optimality
– Changes in soil permeability and percolation
attributes
– Assumes no change in species transpiration
efficiencies
– First inkling that, embedded in the collective control
of plant communities on abiotic state variables has
evolutionary implications
• Selection based on group criteria
• Constraints of efficiency
•
Unlikely to hold in Eagleson’s formulation (presumes
stasis in environmental drivers over deep time, which is
inconsistent with climate dynamics), but as a prompt to
think more deeply about plant-water relations, it is a
huge milestone
permeability
Pore “disconnectedness”
Simplifying Complex Dynamics
• Emergent behavior from reciprocal
adjustments between soil moisture and
ecosystem “resistances” (water use, biomass
growth) in response to climate (rainfall)
• Read Porporato et al. (2004)