Primer on Ecosystem Water Balances

Lecture 2Ecohydrology

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– 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 xylenSoil Moisture

Atmospheric Demand

Boundary layer

Leaf control

Stem control

Root control

Soil resistance

Yw(soil) -0.1 MPa Yw (root) -0.5 MPa

Yw (stem) -0.6 MPa

Yw (smallbranch) -0.8 MPa

Yw (atmosphere) -95 MPa

The SPAC (soil-plant-atmosphere continuum)

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.

Boundary,Leaf, Stem, Soil Conductance

PrimaryProduction

Soil Moisture

Intercepted Water

Rainfall

Infiltration

Runoff

Vapor PressureDeficit

+

+

+

++

- --

-

--

Radiation, Wind

+

+

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

• 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

ENERGY AERODYNAMIC

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– Leaf area index (LAI)– Stomatal conductance – Water status (wilting)ET (indexed to PET) from a dry canopy as a

function of surface resistance (rs) at constant aerodynamic resistance (ra)

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

Fore

st So

il

Surface

2 months later

Rooting Depth Effects

• 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)

Hydraulic Redistribution

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

Jackson et al. (2005)

CO2 H2O

1 : 300

Moreover – Species Matter

Evidence for Another Feedback – Composition Effects on Water Balances

• Halophytic salt cedar invades SW riparian areas

• Displaces cotton-woods, de-waters riparian areas

• Pataki et al. (2005) studied stomatal conductance for both species in response to increased salinity

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 Plant-Water 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)