Buchan, G D

Main, B

McKenzie, B A

English

University of Kentucky

Session 3 - Alternative Uses of Forages 3-7are of two types. The simpler ‘non-interacting’ model assumes theheat and vapour fluxes from the soil surface transfer directly to theatmosphere, with no interaction with the canopy. The ‘interacting’model (Fig. 1) admits mutual influences between the canopy andsoil surface via their heat and vapour fluxes. However both modelsare ‘radiatively interacting’, i.e. include full solar and longwaveexchanges between layers. A further subdivision of model typesassumes windspeed is either constant or varying throughout the day.Measurements were at two contrasting sites: Site 1, a classic,developed pasture on the Canterbury Plains; and Site 2, an inland,hill-country site characterised by a dense, clumpy sward.Field measurements included the following. Model input data: solarradiation, air temperature and vapour pressure, windspeed; canopyalbedo; canopy cover fraction. Other parameters included soil albedo,emissivities, and soil thermal conductivity and heat capacity. A sonicanemometer gave direct measurements of sensible heat flux, enablingcanopy resistance to be deduced via energy balance closure. Modeloutput data: soil temperatures to 1m depth; soil heat flux; canopytemperature Tc from infra-red thermometer; evapotranspiration frommini-lysimeters.For site 2, the more complex canopy was partiioned into two layers:an upper layer of protruding tops; and a lower, dense layer includingsenescing and dead material. For this lower, mulch-like layer, verticalheat transfer via vegetative conduction was also included.RESULTS AND DISCUSSIONFig. 2 shows illustrative results from the interacting model version,for the high-country site, for one 7-day period in late summer 1994.More generally, our results show that the above mechanistic, two-layer models of grassland energy balance can predict canopy andsoil surface temperatures in good agreement (typically within c.±1.5 K) with measurements. These climatic models can be used tocharacterize grassland microclimate, and have potential utility inassessing impacts of climate change.ACKNOWLEDGEMENTSThe authors are grateful to: the Miss E.L. Hellaby IndigenousGrasslands Research Trust (NZ) for generous financial support; theStapledon Memorial Trust (UK) for a Travelling Fellowship for G.Buchan; and to Prof. Mike Novak and Dr. Frank Kelliher for valuablediscussions.REFERENCESBuchan, G.D. 1982. Predicting bare soil temperature. Parts I to III.Journal of Soil Science 33: 185-197, 199-209 and 365-373.Main, B. 1996. Development of mechanistic energy balance modelsfor grassland. PhD thesis, Lincoln University, New Zealand.ABSTRACTKEYWORDSEnergy balance, soil temperature, canopy temperature, heat flux,evapotranspiration.INTRODUCTIONModelling grassland soil temperature is complicated by the sward’smediation of the energy flows between atmosphere and soil, and bythe wide diversity of grassland types, from developed to native. Thispaper describes: 1) models of the surface energy exchanges ofgrasslands, predicting from weather data both canopy temperatureand evapotranspiration, and the soil thermal regime; and 2) modeltesting at two strongly contrasting sites and grassland types inCanterbury, NZ.The models extend the bare soil models of Buchan (1982), whichare simple single-layer models (i.e. analyse energy exchanges onlyat the soil surface). The grassland models are two-layer, adding anequivalent canopy layer (Fig. 1). Key model features include:reductionism (minimum complexity); preferential application ofanalytical rather than numerical mathematics; and description of themulti-day average diurnal (24-hour) variation, i.e. of thermal‘climate’, as opposed to single-day ‘weather’.Previous studies have modelled two surface types. a) Vegetatedsurfaces. Commonly, models are numerical, and either single- ortwo-layer (i.e. neglect or include the soil surface, respectively); oreven multi-layer, for deeper canopies. However vegetation modelscommonly focus on the canopy microclimate. b) Mulched surfaces,with greater interest in predicting soil temperature and moistureregimes. Our models combine aspects of both surface types, byfocussing on the soil thermal regime, and admitting a mulch-likeheat conduction term for dense swards.MATERIALS AND METHODSModels. As shown in Fig. 1, the two-layer models partition thegrassland into: a soil surface layer; and an equivalent canopy layer,effectively a ‘porous sheet’ through which the soil exchangesradiation, heat and vapour with the atmosphere. This model convertsto a resistance network, with a ‘node’ for each energy source or sink.Key canopy characteristics include: the canopy cover fraction, c(1-c represents both the canopy’s radiation transmissivity, and thefractional contribution of bare soil to the total surface energy budget);aerodynamic characteristics, including roughness length; radiativecharacteristics, i.e. albedo and emissivity; and (especially for thedense sward at site 2), the thermal conductivity of stems and leaves,providing an additional conduction path for heat flow. We aimed todevelop simplified models characterizing ‘climate’ rather thatn day-to-day ‘weather’ (e.g. for climate change sutdies). Hence we modelledthe multi-day (typically c. 7-day) mean diurnal variation. The modelsID NO. 7MECHANISTIC MODELLING OF GRASSLAND ENERGY BALANCEG.D. Buchan1, B. Main1, and B.A. McKenzie21Department of Soil Science, PO Box 84, Lincoln University, New Zealand2Department of Plant Science, PO Box 84, Lincoln University, New Zealand

Mechanistic Modelling of Grassland Energy Balance

https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=6811&context=igc

Mechanistic Modelling of Grassland Energy Balance

Abstract

Similar works

Full text

Available Versions

University of Kentucky