Chapter 4 Terrestrial Water and Energy Balance

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Chapter 4 Terrestrial Water and Energy Balance

• Part II Mechanisms
• Chapin, Matson, Mooney
• Principles of Terrestrial Ecosystem Ecology

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We can look at energy budget of an ecosystem just
as we did for Earth
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Net radiation

• Energy input to ecosystem
• Balance between
• Input (long- and short-wave radiation)
• Output (long- and short-wave radiation)

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Albedo

• Reflectance of shortwave radiation
• Depends on absorptivity
• Depends on ecosystem structure

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Examples of albedoes

• Oceans and lakes 0.03-0.10
• Sea ice 0.30-0.45
• Fresh snow 0.75-0.95
• Tundra 0.15-0.20
• Conifer forest 0.09-0.15
• Broadleaf forest 0.15-0.20
• Desert 0.20-0.45

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Abedo differs depending on the type of vegetation
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Absorptivity changes from summer to
winter Changes following fire
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Energy Partitioning

• Rnet H LE G
• Rnet net radiation
• H sensible heat flux (dry heat)
• LE evapotranspiration (wet heat)- water
  transpired by plants
• G ground heat flux heat from soils

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Energy partitioning

• Ecosystems differ substantially in
• Albedo a fancy name for reflectivity
• Bowen ratio (H/LE) When the magnitude of B is
  less than one, a greater proportion of the
  available energy at the surface is passed to the
  atmosphere as latent heat than as sensible heat,
  and the converse is true for values of B greater
  than one

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Comparison of diurnal trends at three pine
sites Rn was generally highest at the Douglas
Fir 40 yr forest. Values of H were similar at
the DF 20 and 40 yr forests, and G was greatest
at the Jack pine 12 yr site, but, G is generally
negligible.
DF 40 yr
DF 20 yr
JP 12 yr
Energy flux density (W m-2)
Hour
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The Bowen ratio was generally highest at the
20-year Douglas-fir stand, with the 12-year Jack
pine and 40-year Douglas-fir having generally
similar Bowen ratios.
Precipitation (mm)
Bars with the same letter for a given month are
not significantly different (a 0.05).
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Variations in seasonal snow cover and the
associated changes in albedo and atmospheric
heating are key components of the climate system.
Recent studies have found reductions in
high-latitude snow cover. (Dye, 2002, Stone et
al., 2002, Chapin et al., 2005, Euskirchen et
al., 2006)
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Energy budget feedbacks to regional summer climate

• Feedbacks from vegetation change
• Tussock to shrub transition 3.4 W/m2
• Tundra to forest transition 4.7 W/m2
• 2 change in solar input 4.6 W/m2
• (glacial to interglacial change)
• Doubling atmospheric CO2 4.4 W/m2

Chapin et al.
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Total Annual Area Burned in Alaska 1950-2006
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Fire alters energy exchange Negative feedback to
climate warming Importance to society depends on
total area burned
Baldochi
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Changes in surface albedo at the Delta
Chronosequence
Grey line Recent burn Black line Control
Liu et al., 2005
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Does boreal forest fire warm or cool climate?
Positive feedback loop via greenhouse gases
leads to warming
Negative feedback loop via albedo leads to
cooling
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Sum of forcing agents yields net warming over the
first year but cooling over 80 years
Randerson et al. 2006
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Climate warming feedbacks

• Trace-gas feedbacks
• CO2 exchange
• Modest C sink in the Arctic
• Modest C source in boreal forest
• Permafrost thaw potential large future source
• CH4 substantial source
• Energy-budget feedbacks predominate
• Earlier snow melt in the Arctic and Boreal
• positive feedback to warming
• Vegetation change
• Increased shrubs in arctic (positive feedback to
  warming)
• Switch to deciduous in boreal (negative feedback
  to warming)

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Drought in Sahel
Foley et al. 2003
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Response to drought
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Drought feedbacks

• Trace-gas feedbacks small
• Very small NEP
• Energy-budget feedbacks predominate
• Reinforce climate drying or wetting
• Ecosystem feedbacks create non-linear response
• Creates memory that sustains climate regime
• Effects are regional
• Important role for restoration ecology

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Foley et al. 2003
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Boreal deforestation causes net cooling

Converse Boreal afforestation causes net
warming (Efforts to address C sequestration
alone are counter-productive)
Bala et al. 2007
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Tropical deforestation causes net warming
Bala et al. 2007
Converse Tropical afforestation causes net
cooling
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Tropical deforestation feedbacks (net warming)

• Trace-gas feedbacks (net warming)
• Carbon loss
• globally dispersed
• Energy-budget feedbacks (net warming)
• Increased ecosystem albedo (cooling)
• But cancelled out by reduced cloudiness
• Reduced transpiration (warming)
• Reduced precipitation
• Half of rainfall comes from recycled water
• Reduces forest growth
• Could trigger switch to savanna/dry conditions
• Climate feedbacks stabilize the vegetation change
• Socioeconomic implications

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Evapotranspiration accounts for 75 of turbulent
energy transfer to the atmosphere
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Water is THE major greenhouse gas

• Water-vapor feedback to warming
• Warm air holds more water
• This causes more water to evaporate
• This increases the water content of atmosphere
• This absorbs more infrared radiation
• This warms the atmosphere
• Etc..a VICIOUS CYCLE!

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Why do we care about ecosystem water budgets?
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Water budget Inputs and outputs (vertical and
lateral) Internal transfers
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Why do plants use so much water?
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Basic principle of water balance in ecosystems

• Soil is like a bucket
• Inputs
• Mainly precipitation
• Outputs
• Evaporation
• Transpiration
• Drainage
• Water storage depends on
• Size of bucket (soil depth and texture)
• Balance between inputs and outputs

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Water inputs to ecosystems

• Precipitation
• THE major water input to ecosystems
• Groundwater
• Fog deposition

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Ecosystems differ in canopy storage Depends
mainly on LAI Differs among species
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Basic principle of water movement in ecosystems

• Water moves along energy gradient
• From high energy to low energy
• What forces cause water to move?
• Pressures
• Gravity
• Forces created by organisms
• Osmotic gradients
• Matric forces (adsorption)

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Water movement in soil
? ? t
Js Ls
l
Js Water flux rate Ls Hydraulic
conductivity ? ? t Water potential gradient l
Path length
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Infiltration

• Water movement into soil
• Depends largely on hydraulic conductivity (Ls)
• Texture
• Aggregate structure
• Macropores made by animals and roots
• Impermeable layers
• Calcic layer in desert
• Permafrost in cold climates
• -Tundra both a wetland and a desert!

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Infiltration

• Surface runoff occurs when infiltration is slower
  than precipitation

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Plant-available water Difference between field
capacity (the quantity of water obtained by a
saturated soil after the gravitational water has
been drained and permanent wilting point (where
plants wilt b/c they cannot obtain enough water
from soils)

Coarse Fine Particle size
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Water movement in plantsEquation is identical to
that in soils
? ? t
Jp Lp
l
Jp Water flux rate Lp Hydraulic
conductivity ? ? t Water potential gradient L
Path length
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Water moves along a pressure gradient, with
differences between day and night
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Transpiration is major driving force of water
movement in plants

• Water moves in continuous column from film on
  soil particles to leaf cells
• Moves upward because of strong cohesive forces
  among water molecules
• Plant spends no energy in transporting water
• Passive transport driven be transpiration
• This is different than the considerable metabolic
  energy used by plants in photosynthesis and N
  acquisition!

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Soil moisture directly limits evapotranspiration
rates in dry soils.
Water potential potential energy of water
relative to pure water at the soil surface
BUT, IN MOIST SOILS -Transpiration and plant
water potential are insensitive to soil
moisture -Transpiration rate depends mainly on
transport properties of plants
Greater of capacity drier soil decline in
plant water potential
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Water loss from leaves

• Driving force is vapor pressure gradient
• Depends on temperature and water vapor in bulk
  air
• Plants control water loss by creating resistance
  between leaf and air
• Plants can adjust this resistance
• Stomatal conductance depends on
• Soil moisture
• Vapor pressure of air (in some species)

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Water movement through stem

• Driving force is difference in water potential
  between leaf and root
• So water moves through the stems to replace
  water lost in the transpiring leaves
• Resistance depends on path length and stem
  structure
• Like sucking water up through a straw

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Water movement to root

• Moves along water potential gradient
• Root has lower water potential than soil
• Rate depends on hydraulic conductivity and path
  length

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Under dry conditions, rate of water loss from
leaves depends on water supply

• Influences water potential gradient
• Plants adjust stomatal conductance to match water
  supply
• Remember
• Photosynthesis depends in part on the stomates
  response to water! Stomates open photosynthesis
  
• Stomates closed no photosynthesis, but also no
  water loss
• It is a tricky balance for the plant.

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Soil water profiles in adjacent shrub and
grassland communities at the end of the summer
drought period. Predawn water potentials are a
good index of soil moisture and the degree of
drought stress experienced by the plant.
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Hydraulic Lift vertical movement of water
through roots from moist soils to dry soils along
a gradient in water potential - Occurs in most
arid ecosystems and many moist forests
-During the day, water moves from soils to the
atmosphere. -At night, water moves from wet soils
at depth to dry surface soils
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Transport system can collapse under very dry
conditions

• Water column within stems break
• No longer possible for transpiration to suck
  water up from roots

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Plants in dry environments have a larger safety
factor They operate at much higher water
potential than is likely to cause
cavitation. -The 11 line is the line expected if
the plants lost all conductivity at the lowest
water potential observed in nature, that is, if
there is no safety factor. -Large departure from
the 11 line, such as in the diagram, is
characteristic of a plant in dry environment
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Water-transport capacity of stems is balanced
with LAI Species from dry environments have more
transport cells
Moist sites more vessels, more LAI per unit
sapwood
Dry sites smaller vessels, less LAI per unit
sapwood
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Plants have some water storage capacity Quite
limited in most plants (2 hours in this
graph) Most of water must come from soil (not
plant storage)
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Water inputs to ecosystem determines water outputs

• P ?S E R
• P Precipitation
• S Storage
• E Evapotranspiration
• R Runoff

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EVAPOTRANSPIRATION The major factors controlling
evapotranspiration from a plant canopy.
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Wet canopy evaporation

• Depends on driving force
• Temperature of canopy water
• Governs water vapor concentration at leaf surface
• Humidity of bulk air
• Governs water vapor concentration of bulk air

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Wet canopy evaporation

• Large where canopies are wet
• Large in ecosystems with high surface roughness
• Well coupled to atmosphere
• Depends on conditions that drive ET (dryness of
  atmosphere)
• Less evaporation from smooth canopies
• Net radiation drives ET

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Dry canopy evapotranspiration

• Soil moisture is major control in dry soils
• Regulated by stomatal conductance
• Same controls as described for individual leaves
• Vegetation structure and atmospheric conditions
  determine ET in moist soils

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Controls over ET in moist soils

• Boundary layer conductance
• Physical control over water loss
• Depends on vegetation structure and wind
• THE major vegetation control when soils are moist
• Surface conductance
• Physiological control over water loss
• Depends on stomatal conductance
• Insensitive to vegetation structure
• Becomes increasingly important as soils dry

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Surface conductance (contd)

• Variable among ecosystems
• No clear trend with LAI
• Tends to be higher in crops
• The ecological controls over this are not well
  defined in natural ecosystems
• Higher in fertile soils?
• Higher in deciduous than evergreens?

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Climatic effects on ET

• Moisture content of air is major control in
  rough, well-mixed canopies
• Net radiation is major control in smooth, poorly
  coupled canopies
• These environmental controls are the same in wet
  and dry canopies

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Water outputs

• Streamflow is the leftovers after soil storage
  and ET are met
• Over long term, runoff depends on ppt and ET
• Groundwater flow is usually a net output

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In moist ecosystems, ET is relatively insensitive
to ppt.- Precipitation directly regulates
streamflow In this example, based on 19 years of
data collected at Hubbard Brook,
evapotranspiration varies little over the years,
but streamflow is directly linked with
precipitation.
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Runoff from the clearwater (non-glacial) river
peaks at snowmelt, whereas the glacial river has
peak discharge when temperatures are warmest in
mid-summer.
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Stream flow increases linearly with the
proportion of the watershed that is
deforested. Many graphs like this can be plotted
for the Western U.S., although this one is from
the Southern U.S.
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Water budget Inputs and outputs (vertical and
lateral) Internal transfers
Interception is substantial (10 50) in closed
canopy forests.