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

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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.

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Abbreviations

a(z) :

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

a 2 :

Constant parameterizing stomatal dependence on vapour pressure deficit (mb)

a 3 :

Constant parameterizing stomatal dependence on temperature (°C)

a 4 :

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

a 5,a 6 :

Constants parameterizing stomatal dependence on soil moisture deficit (mm)

A :

Available energy (W m−2)

B:

Stanton number

c p :

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

c d :

Bulk drag coefficient of vegetation

d :

Zero plane displacement (m)

D :

Vapour pressure deficit ath aws(mb)

D B :

Vapour pressure deficit within canopy (mb)

e :

Vapour pressure ath aws(mb)

e S (T) :

Saturated vapour pressure ath aws(mb)

f i :

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)

h aws :

Height of climate measurements at automatic weather station (m)

h c :

Canopy or vegetation component heights (m)

k :

Von Kármán constant

K :

In-canopy eddy diffusivity (m2 s−1)

K v,h,m :

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

L c :

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

r au :

In-canopy aerodynamic resistance (s m−1)

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

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

r b :

Boundary-layer resistance (s m−1)

r S :

Bulk stomatal resistance (s m−1)

r ST :

Stomatal resistance (s m−1)

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

Minimum stomatal resistance (s m−1)

R n :

Net radiation (W m−2)

R solar :

Total incoming solar radiation (W m−2)

Re:

Local Reynolds number at leaf

T :

Temperature ath aws(°C)

T L,T U :

Fixed constants parameterizing stomatal response to temperature (°C)

T wet :

Wet bulb temperature ath aws(°C)

u(z) :

Mean horizontal windspeed (m s−1)

u c :

Characteristic fluid velocity (m s−1)

u * :

Friction velocity (m s−1)

V :

Correlation matrix

z :

Height coordinate (m)

z0 v,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 ofe s (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)

λE meas :

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

λE sap :

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

B :

Bush

H :

Herb

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Huntingford, C., Allen, S.J. & Harding, R.J. An intercomparison of single and dual-source vegetation-atmosphere transfer models applied to transpiration from sahelian savannah. Boundary-Layer Meteorol 74, 397–418 (1995). https://doi.org/10.1007/BF00712380

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