Fraction of maximum leaf conductance as a function of environmental variables

1. Fraction of maximum leaf conductance as a function of environmental variables Cleaf= C*leaf • fK(Kin) • f(v) •fT(Ta) • f(v) If all functions =1, then leaf conductance and ET are maximized for the environmental conditions If f(v) = 1 then ET= PET

2. Penman-Monteith Equation for estimating evapotranspiration • ET = •(K + L) + caa Cat {e*a – ea} • vw {+ (1+Cat/Ccan )} • ET = •(K + L) + caa Cat (1-Wa) e*a • vw {+ (1+Cat/Ccan )} • Where: • = slope of the saturation vapor pressure vs. temperature relationship at Ta • K = short wave radiation net input ca = heat capacity of the atmosphere • L = longwave radiation net input a= density of air • = psychrometric constant v = latent heat of vaporization w= density of water Cat = atmospheric conductance e*a= saturation vapor pressure of atm. Ccan= canopy conductance ea= water vapor pressure in atm. Wa = relative humidity (fraction)

3. Wind velocity appears to have its greatest influence at low velocities, which are not very common. This may explain why simple models of ET or PET work reasonably well without including a wind variable.

4. Limitations of the Penman-Montieth Model Does not explicitly separate out evaporation from soil or leaf surfaces (Shuttleworth & Wallace developed a model that adds explicit conductances for evaporation from soil and leaf surfaces) Requires some data about vegetation that are not generally available for many species: maximum leaf conductance, variation in leaf conductance with environmental stresses, shading factors, and albedo. There is considerable uncertainty about estimates of long wave radiation (L).

5. AmeriFlux Program Objectives Generally: establish an infrastructure for guiding, collecting, synthesizing, and disseminating long-term measurements of CO2 , water, and energy exchange from a variety of ecosystems collect critical new information to help define the current global CO2 budget enable improved predictions of future concentrations of atmospheric CO2 enhance understanding of carbon fluxes, Net Ecosystem Production (NEP), and carbon sequestration in the terrestrial biosphere http://public.ornl.gov/ameriflux/ Data collection includes net radiation, leaf area index and evapotranspiration (using eddy covariance) hourly. Bondville, IL Morgan Monroe State Forest, IN

6. Net all wave radiometer, measuring the net of incoming and outgoing radiation

7. Net Radiometer measuring incoming and outgoing shortwave and long wave radiation Pyranometers (short wave) Pyrgeometers (long wave)

8. Comparison between measured and simulated ET for corn at Bondville

9. PM= Penman Monteith SW= Shuttleworth Walace (Vorosmarty et al. 1998)

10. Hammon Potential Evaporation Equation PETH = 2.98  D  ea*(Ta) Ta + 273.2 Where: PETH = Potential ET (mm/day) D = Day length (hrs) Ta = mean daily temperature (oC) ea*(Ta) = saturation vapor pressure evaluated at Ta

11. Gross Rainfall (R) Canopy Interception (Ec) Throughfall (Rt) Stemflow (Rs) Net Rainfall (Rn) Litter Interception (El) Rn = Rt + Rs Rn = R – Ec - El

12. Measuring throughfall

13. Measuring Stem Flow