Energy partitioning

2. Energy partitioning Rnet = H + LE + G + S Rnet = net radiation H = sensible heat flux LE = evapotranspiration G = ground heat flux S = change in storage

5. Energy available for turbulent exchange (momentum) • Available energy = Rnet – (G + S) = H + LE • Bowen ratio: H/LE • inversely related to proportion of Rnet that drives water loss • Driven by water availability

6. Characterizing functional differences among ecosystems… • Albdeo—controls amount of Rnet • Bowen ratio—controls fate of Rnet

10. The water cycle

11. Basic principle of water balance in ecosystems • Inputs • Mainly precipitation, some groundwater, fog, periglacial ice melt… • Outputs • Evaporation • Transpiration • Drainage

12. 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 *

13. Ecosystems differ in canopy storage Depends mainly on LAI Differs among species

14. Infiltration • Water movement into soil • Depends largely on hydraulic conductivity • Texture • Aggregate structure • Macropores made by animals and roots

15. Water potential () 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) *

16. Water movement to root • Moves along water potential gradient* • Rate depends on hydraulic conductivity and path length • Depends on root volume

17. Soil depth/volume exploited differs among ecosystems

18. Transpiration is major driving force of water movement • Water moves in continuous column from film on soil particles to leaf cells • Moves upward because of strong cohesive forces among water molecules

19. 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)

20. Water loss from leaves • Driving force is vapor pressure gradient • Depends on temperature and water vapor in bulk air • Stomata are major resistance • Stomatal conductance depends on • Soil moisture • Vapor pressure of air (in some species)

21. Rate of water loss from leaves depends on water supply (and leaf area) • Influences water potential gradient • Plants adjust stomatal conductance to match water supply (short time scale) • Plants adjust leaf area to match water supply (long time scales)

22. Water inputs to ecosystem determines water outputs • P + ∆S = ET + R • P = Precipitation • S = Storage • ET = Evapotranspiration (LE) • R = Runoff

23. Controls over ET in moist soils • Leaf surface conductance • Depends on stomatal conductance • Becomes increasingly important as soils dry • Ecosystem boundary layer conductance • Physical control over water loss • Depends on vegetation structure, temperature and wind • THE major vegetation control when soils are moist

24. Boundary layer conductance • With wind, moisture content of air is the major control • Without wind, net radiation is the major control • These environmental controls are the same in wet and dry canopies

25. Streamflow is the “leftovers” after soil storage and ET are met • Over long term, runoff depends on ppt and ET

26. In moist ecosystems, ET is relatively insensitive to ppt. Precipitation directly regulates streamflow

28. Jackson et al. 2005