primer on ecosystem water balances lecture 2 ecohydrology
TRANSCRIPT
![Page 1: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/1.jpg)
Primer on Ecosystem Water Balances
Lecture 2Ecohydrology
![Page 2: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/2.jpg)
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
![Page 3: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/3.jpg)
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
![Page 4: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/4.jpg)
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
![Page 5: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/5.jpg)
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
![Page 6: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/6.jpg)
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)
![Page 7: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/7.jpg)
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.
![Page 8: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/8.jpg)
Boundary,Leaf, Stem, Soil Conductance
PrimaryProduction
Soil Moisture
Intercepted Water
Rainfall
Infiltration
Runoff
Vapor PressureDeficit
+
+
+
++
- --
-
--
Radiation, Wind
+
+
![Page 9: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/9.jpg)
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
![Page 10: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/10.jpg)
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)
![Page 11: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/11.jpg)
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
![Page 12: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/12.jpg)
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)
![Page 13: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/13.jpg)
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)
![Page 14: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/14.jpg)
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)
![Page 15: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/15.jpg)
Stomatal Conductance
• Rate of CO2 (H2O) exchange with air (mmol m-2 s-1)
![Page 16: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/16.jpg)
Specific Variation• Conductance properties vary by
species– Feedbacks between water use
and succession– Comparative climate change
vulnerability
![Page 17: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/17.jpg)
Rooting Depth
Fore
st So
il
![Page 18: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/18.jpg)
Surface
2 months later
Rooting Depth Effects
![Page 19: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/19.jpg)
• 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
![Page 20: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/20.jpg)
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
![Page 21: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/21.jpg)
Budyko Curve
![Page 22: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/22.jpg)
Evidence for One Feedback – Forest Cover Affects Stream Flow
Jackson et al. (2005)
CO2 H2O
1 : 300
![Page 23: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/23.jpg)
Moreover – Species Matter
![Page 24: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/24.jpg)
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)
![Page 25: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/25.jpg)
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)
![Page 26: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/26.jpg)
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
![Page 27: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/27.jpg)
In Equation Form (yikes)
![Page 28: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/28.jpg)
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
![Page 29: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/29.jpg)
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
![Page 30: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/30.jpg)
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”
![Page 31: Primer on Ecosystem Water Balances Lecture 2 Ecohydrology](https://reader033.vdocument.in/reader033/viewer/2022061517/56649ebe5503460f94bc8982/html5/thumbnails/31.jpg)
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)