Boundary-Layer Meteorology

, Volume 74, Issue 4, pp 397–418 | Cite as

An intercomparison of single and dual-source vegetation-atmosphere transfer models applied to transpiration from sahelian savannah

  • C. Huntingford
  • S. J. Allen
  • R. J. Harding
Article
Revised:

Abstract

Three models of the partitioning of net radiation into latent and sensible heat fluxes over Sahelian savannah are described. Each model has a different configuration of stomatal and aerodynamic resistances. Their performance was assessed by comparison against field measurements of latent heat flux over savannah vegetation consisting of bushes interspersed with a herbaceous understorey. The modelled results indicate that in dry conditions, a Penman-Monteith based single source model performs adequately when predicting the latent heat flux. However, the models with two sources demonstrate that the bushes and herbs have very different responses to local climate. In all the models, evaporation is highly sensitive to stomatal resistance, suggesting that a better understanding of stomatal response would improve the accuracy of the models.

Keywords

Radiation Evaporation Heat Flux Latent Heat Field Measurement 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Nomenclature

a(z)

Projected area of vegetation per unit volume (m−1)

a2

Constant parameterizing stomatal dependence on vapour pressure deficit (mb)

a3

Constant parameterizing stomatal dependence on temperature (°C)

a4

Constant parameterizing stomatal dependence on solar radiation (W m−2)

a5,a6

Constants parameterizing stomatal dependence on soil moisture deficit (mm)

A

Available energy (W m−2)

B

Stanton number

cp

Specific heat of air at constant pressure (J kg−1 K−1)

cd

Bulk drag coefficient of vegetation

d

Zero plane displacement (m)

D

Vapour pressure deficit athaws(mb)

DB

Vapour pressure deficit within canopy (mb)

e

Vapour pressure athaws(mb)

eS(T)

Saturated vapour pressure athaws(mb)

fi

Empirical functions describing stomatal response to local climate and soil conditions

G

Soil heat flux (W m−2)

H

Sensible heat flux (W m−2)

haws

Height of climate measurements at automatic weather station (m)

hc

Canopy or vegetation component heights (m)

k

Von Kármán constant

K

In-canopy eddy diffusivity (m2 s−1)

Kv,h,m

Eddy diffusivity for vapour, heat and momentum (m2 s−1)

Lc

Characteristic length (m)

\(L_{c_{B, H} } \)

Characteristic leaf dimension for bushes and herbs (m)

LAI

Leaf area index

n

In-canopy extinction coefficient

Pr

Prandtl number

rau

In-canopy aerodynamic resistance (s m−1)

\(r_{a_{v,h,m} } \)

Surface-layer aerodynamic resistance to vapour, heat and momentum (s m−1)

rb

Boundary-layer resistance (s m−1)

rS

Bulk stomatal resistance (s m−1)

rST

Stomatal resistance (s m−1)

\(r_{ST_{\min } } \)

Minimum stomatal resistance (s m−1)

Rn

Net radiation (W m−2)

Rsolar

Total incoming solar radiation (W m−2)

Re

Local Reynolds number at leaf

T

Temperature athaws(°C)

TL,TU

Fixed constants parameterizing stomatal response to temperature (°C)

Twet

Wet bulb temperature athaws(°C)

u(z)

Mean horizontal windspeed (m s−1)

uc

Characteristic fluid velocity (m s−1)

u*

Friction velocity (m s−1)

V

Correlation matrix

z

Height coordinate (m)

z0v,h,m

Roughness length for vapour, heat and momentum (m)

α

Areal fraction of herb coverage

γ

Psychrometric ‘constant’ (mb °C−1)

Δ

Rate of change ofes(T) w.r.t.T (mb °C−1)

Θ

Soil moisture deficit (mm)

κ

Molecular heat diffusion coefficient (m2 s−1)

λE

Latent heat flux (W m−2)

λEmeas

Eddy correlation measurement of latent heat flux (W m−2)

λEsap

Sap flow gauge measurement of latent heat flux (W m−2)

μ

Empirical factor in calculation of Stanton number

ν

Kinematic viscosity of air (m2 s−1)

ρ

Density of air (kg m−3)

σ1, σ2

Statistics used in model comparison

τ

Shearing stress (kg m−1 s−2)

φv,h,m(z)

Stability corrections to eddy diffusivities for vapour, heat and momentum

ψv,h,m(z)

Integrated stability corrections to eddy diffusivities for vapour, heat and momentum

Subscripts

B

Bush

H

Herb

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Copyright information

© Kluwer Academic Publishers 1995

Authors and Affiliations

  • C. Huntingford
    • 1
  • S. J. Allen
    • 1
  • R. J. Harding
    • 1
  1. 1.Institute of HydrologyWallingfordU.K.

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