Lecture 2
Ecohydrology

• 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

• 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

• 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

• 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
– Most water passes through plant stomata even in wet areas with low canopy cover
• Evolutionary control on resistances and response to stresses
– For example, cavitation of the SPAC in the xylen
Atmospheric
Demand
Boundary layer
Leaf control
Stem control
Root control
Soil resistance
Soil Moisture

The SPAC (soil-plant-atmosphere continuum)
Y w
(atmosphere)

-95 MPa
Y w
(small branch)

-0.8 MPa
Y w
(stem)

-0.6 MPa
Y w
( soil)

-0.1 MPa
Y w
(root)

-0.5 MPa

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
+
Boundary,
Leaf, Stem, Soil
Conductance
+
+
Primary
Production
-
-
Soil Moisture
+
+
Intercepted
Water
-
Infiltration
-
-
+
Runoff

m
s
a
• 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)

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, R of air, C deficit, r p s n is net radiation, G is ground heat flux, ρ is the density is the specific heat capacity of air, D is the surface resistance and r a m is the vapor pressure is the aerodynamic resistance

ET (indexed to PET) from a dry canopy as a function of surface resistance (r s
) at constant aerodynamic resistance (r a
)
• r a is the resistance of the air to ET, controlled by wind speed and surface roughness
• r s is resistance for vapor flow through the plant or from the bare soil surface
• Vegetation effects
– Leaf area index (LAI)
– Stomatal conductance
– Water status (wilting)

• Species type affects ecosystem energy budget
Net-radiative forcing of boreal forests following fire is dominated by albedo effects (Randerson et al
2006)

• Air openings, mostly on leaf under-side
– 1% of leaf area, but ~
60,000 cm -2
– Function to acquire
CO from the air
2
– Open and close diurnally, and in response to soil H tension
2
O
• React to wilting (loss of leaf water)
Guard cells (shape change with turgor pressure)

• Rate of CO
2
(H
2
O) exchange with air
(mmol m -2 s -1 )

• Conductance properties vary by species
– Feedbacks between water use and succession
– Comparative climate change vulnerability

Surface
2 months later

• 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)

• 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 E o
ET:P → 1 and R:P → 0
:P → ∞,
– Wet conditions: when E o
:P → 0 ET
→ E o

Evidence for One Feedback – Forest Cover
Affects Stream Flow
Jackson et al. (2005)
CO
2
H
300
2
O

Evidence for Another Feedback – Composition
Effects on Water Balances
• Halophytic salt cedar invades SW riparian areas
• Displaces cottonwoods, de-waters riparian areas
• Pataki et al. (2005) studied stomatal conductance for both species in response to increased salinity
Pataki et al. (2005)

• 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)

• Peter Eagleson
(1978a-g)
– 14 parameter model links rain to production via soil moisture
– Posits three
“optimality criteria” at different scales

• 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

• 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

• 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”

• 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)