SF E Studies of the Forest O Energy Budget by L. W. Gay H . R . Holbo . 11 4li Water Resources Research Institute STUDIES OF THE FOREST ENERGY BUDGET By ' L . W . Gay and H . R . Holbo Water Resources Research Institut e and Department of Forest Engineerin g Oregon State Universit y Corvallis, Oregon 97331 Summary Application of the mean-profile energy budget analysis to forests i s discussed, with regard to limitations imposed by the forest environment . These limitations are primarily associated with the diffuse structure an d aerodynamically rough nature . of, forest canopies, which contrast with th e dense and relatively smooth canopies of agricultural crops . The results of the few forest energy budgets that have been tomplete d are reviewed . The country of origin and number . of articles reviewed include : Australia, 2 ; Belgium, 2 ; Canada, 5 ; Federal Republic of Grmany, 8 ; German Democratic Republic, 2 ; Soviet Union, 2 ; United Kingdom, I ; and United States , 2 . Results from these studies are compared with a clear weather energ y budget for a second-growth Douglas-fir site on . the Cedar River east of Seattle , Washington . On August 10, 1972, the energy budget components of the 28- m forest were (cal/cm 2 24-hours) : net, radiation, 399 ; change in stored energy , 2 ; latent energy, -240 ; convection, -161 . Budgets are also tabulated for a clear and far an overcast day at this site during the summer of 1971 . The results are based on the Bowen ratio model . The Cedar River results for August 10, 1972 are the mean of estimate s made from two towers, 100 m apart . The individual latent energy fluxes, fo r example, were 18 percent above and below the mean value of -240 cal/cm 2 24 - hours . Despite obvious limitations upon interpretation of variability fro m just two estimates, this appears to be the first forest study to replicat e the energy budget measurements . The variability found here confirms th e necessity for replication in future experiments . The literature review confirmed that among mean-profile methods, th e Bowen ratio was the most widely used model for forest energy budget analysis . Graphical, and statistical techniques for estimating the Bowen ratio ar e discussed . A few studies have attempted to compare the Bowen ratio result s with those obtained by different methods . The aerodynamic methods have no t generally appeared satisfactory-over rough forest canopies ; this was also the case at the Cedar River site . Tests and comparison of methods must continu e in order to improve instrumentation and experimental design . Acknowledgements The work upon which this publication is based was supported in part by funds provided by the U .S . bepartthent of Interior, Office of Wate r Resources Research, as authorized u1der the Water Resources Research Ac t of 1964, and administered by the Water Feddardet Research Institute , Oregon State University ; and the %atib%al Stiehce Foundation Brant No . GB-36910X to the Coniferous Fbresf Blatt, V .S . Analysis of Ecosyttems s International Biological Program . This i-t contfibution no . 109 frof th e Coniferous Forest Biome . Portions of tht material in this report hav e been presented elSewhere 1 ' 2 . Professor Leo J . Fritschen, Collet Of Forest Resources, University of Washington, cooperated id fdtiy phll- of this Work . Professor Janusz Paszynski, Institute of Geography of tit POlish Academy of Sciences , Warsaw, and Dr . Nicola Tarsia, ENtC Cettral Forest Experiment Station ; Rome, provided valuable Ataidtance duriii+g the 1972 field seasi n . w 1 Gay, L . W . and J . B . Stewart . Energy balance studies in coniferou s forests . Unpublished paper presented to Swedish Coniferous Forest Biom e Seminar, JHdrags, Sweden, May 17, 1973 . 24 pp ., mimeo . 2Fritschen, L . J ., L . W . Gay and H . R . Hcflbo . Estimating evapotranspiratio n from forests by meteorological and lysimetric methods . Symposium Oh Evapotranspiration from Natural Terrain . American Geophysical Union, Fal l Annual Meeting, San Francisco, Calif ., Ddt . 12, 1973 . ii Table of Content s Summary Acknowledgement s List of Figure s List of Table s List of Symbol s 1. INTRODUCTION 1 2. ENERGY TRANSFER FUNDAMENTAL S 2 2 .1 Energy Budget Equation 2 .2 Radiation Exchange 2 .3 Latent and Sensible Energ y 2 .3 .1 Transfer Equation s 2 .3 .2 Bowen Ratio 2 .3 .3 Aerodynamic Method s 2 .3 .4 Eddy Correlation Methods 2 .4 Stored Energ y 2 .5 Photosynthesi s 2 .6 Remarks 3. RESTRICTIONS IN APPLICATION OF MICROMETEOROLOGICAL METHODS 6 3 .1 Steady-State Conditions , 6 3 .2 Surface Homogeneity 7 3 .3 Boundary Layer Integrity 7 4. THE SCOPE OF STUDIES CARRIED OUT ELSEWHERE 9 4 .1 Survey of Recent Forest Energy Budget Experiments 9 4 .1 .1 Australia 9 4 .1 .2 Belgium 9 4 .1 .3 Canada 10 4 .1 .4 Federal Republic of Germany 10 4 .1 .5 German Democratic Republic 11 4 .1 .6 Soviet Union 11 4 .1 .7 United Kingdom 12 4 .1 .8 United States 12 4 .2 General Comments 12 5. EXPERIMENTAL WORK IN DOUGLAS-FIR AT CEDAR RIVER 13 5 .1 Environmental Instrumentation 13 5 .2 Site Characteristics 14 5 .3 Field Measurements 14 iii 6 . RESULTS kid UISC J ON 6 . 1 Cedar River Results 1 8 6 . 2 Comparison with Other Fot#sjnergy Budget s 20 6 .3 Compa I geen of Methods 2 1 6 .3 .1 CogpatisOhs Between I'ettbroio ±cal Met fides 2 1 6 .3 .2 Cdinparisons With Dire`ct Measureunl b 22 6 .3 .3 Tests by RepliQhtlOn 2 2 7 . PRACTICAL DIFFICIJITIES N ESTIMATING FORESt E\APOTOA SPIRA ON 2 3 7 .1 Problems of Dimension 1t 7 .1 .1 Canopy Dimension 2 7 .1 .2 Abrodyna iic Diit nsidiis 23 7 .2 R b1 ins of rnstrumetttatic ► 24 7 .3 DIrtIcultle$ in Analysis 24 8 . -CONCLUDING REMARKS 2 7 9 . LITERATURE CITED 29 APPENDIX I . Energy Flux Studies in a Cdtilferou Eoie"st t.ciigsteb . 3 5 iv List of Figure s Figure 1 . The Cedar River site in 1971 . Figure 2 . The Cedar River site in 1972 . Figure 3 . The instrument hoist . Figure 4 . The suspended instrument package . Figure 5 . Energy budget components under clear skies . Figure 6 . Energy budget components under overcast skies . Figure 7 . Similarity between gradients of potential temperature an d . vapor pressure . List of Table s Table 1 . Energy budget totals at four coniferous forest sites . Table 2 . Energy budget components at Cedar River, Washington . .ti L. List of Symbol s Symbol Definition C .carbon .dioxide concen.tirdtion, !ppm C . volumetric heat •capacU.ty df oir., fiioma s., ,ar .►soil, cal/cm 3OC C -epecifis heat of air., cal/tret DP effective displacement height, '•D =d - zo , cm d actual displacement 'height where -u =%, , .and d = D + z o , cm E water vapour :flux, igm/oni 2 •mfn .e vapor pressure, mb es saturation vapor pressure, :mb G storage heat flux in air, biomass and cal/cm2 ►Vitt g acceleration :due to gravity., 980 cm%sec t H sensible -heat flux, ,cal/cm2 min ih height of trees, m xKC eddy diffusivity for carbon dioxide, cm 2 /sec IKE eddy diffusivity for water vapor, cm 2 /se c =Kg eddy diffusivity for sensible heat, cm2 /sec eddy diffusivity for momentum, cm 2 /sec K+ global radiation flux, cal/cm2 min KT reflected global radiation flux, cal/cm2 min K Viet global radiation flux, cal/cm2 min Ton Karmans constant, 0 .4 1 L4 longwave radiation flux, incoming, cal/cm 2 min Lt‘ 1.ongwave radiation flux, outgoing, cal/cm 2 min L laet longwave radiation flux, cal/cm2 min P net photosynthetic heat flux, cal/cm2 min p atmospheric pressure, mb Q net allwave radiation flux, cal/:cm2 min q specific humidity, (g water/g air) •fluctuations in specific humi ity RI Richardson number - `" T temperature, °C or ° K t time, sec ra aerodynamic resistance, sec/d m rs surface resistance, sec/cm u oindspeed, cm/se c w fluctuation in vertical wind, cm/sec z height above a reference plane, cm r z o roughness length, cm coefficient in the stability correctdon Tfunict46 Bowen ratio exponent in the stability correction function an operator denoting a finite differenc e ratio of the mole weight cif water :to . Lr, 0 .l62 2 potential temperature, ° K latent heat of fusion, cal/-,g latent heat flux, cal/cm2 min heat of assimilation of carbon, cal/ g density of air, g/cm3 stability correction vi 1, INTRODUCTION The key to a comprehensive theory of environment lies in a descriptio n of the transformation and transfer of energy at or near the surface of the earth . Most of the energy transformations take place at the interface . between the solid (or liquid) surface and the gaseous atmosphere, Sola r radiation readily reaches the surface through the relatively transparen t atmosphere . If the quantity of solar radiation is measured, the major - problem remaining is to evaluate the rate at which it is transformed into r If ' other forms of energy . The majority of these transformations involve forms of radiant , latent or sensible energy . The major effect of these transformations i s the creation and maintenance of the general circulation of the earth . The relatively minor fraction that is photochemically fixed by plants, thoug h small in quantity, does have a significant effect upon mans activities . The cycle of energy transformations at the earth-air interface is also A associated with a corresponding cycle of mass . For example, the mass o f water cycled at this interface can readily be expressed as energy required I I to effect its transformation from one phase to another, Likewise, th e mass of carbon cycled between the biomass of vegetation and the atmospher e can be expressed as the equivalent energy required to effect a net chang e in phase from gas to solid during the photosynthesis process .