over the canopy height, w is the shrub mean leaf width ( 0.02 m), a w and a 0 are two constant 
coefficients respectively equal to 2.5 (dimensionless) and 0.005 ms"2. The resistance of the 
substrate, which represents the aerodynamic resistance between the source height of the 
substrate and the source height of the entire canopy (canopy+substrate) within the first 
compartment, was defined from the standard relation between the turbulent transfer 
coefficient, faction velocity and height as : 
ras- hex p(g w ){exp(—— ——-exp( + -°)^ /( cuK(h)) (11) 
h h 
where d s and z G s, d and zq are the displacement height and roughness length for the substrate 
and for the shrubs respectively and are defined as a function of the height of the shrubs and 
of the substrate respectively. K(h) is the value of eddy diffusivity (m^s’l) at the canopy 
height h, and was obtained from its value at reference height by assuming an exponential 
extinction with respect to the height (Brutsaert, 1982). 
2.2 Aerodynamic resistances 
For both compartments, the equations given by Mahrt and Ek (1984) were 
adapted and used to formulate the aerodynamic resistance. r a i (i=l, 2 ) and was expressed 
under stable conditions as: 
r ai = [log (& + )/*][(log(^4 + =oi ))/£]( 1 —15 Ri,) / V1 + 5 Ru / u (12) 
Zoi Zoi 
and for the unstable conditions (i.e. T e i - T a > 0) as: 
rai= [log( : ~ d ‘ + r °') / k ][log( .T ~. d- + - «) / k ](1 - 1 5Ri.) / ( 1 + Ca-JZru ) / u (13) 
Rq and Cj are the bulk Richardson number and a stability correction factor, and are defined 
as: 
G = 75 k 2 
\z-di + Za 
'[log( 
- — di + ^o 
■)(log( 
z-di + Zc 
■))3 
R, = g 
Ta-Ta 
Tall 2 
(14) 
(15) 
where u is wind speed (m/s) measured at reference height z, k is von karman’s constant (0.4), 
and T e i is the equivalent temperature of the compartment i (T e j=T e and T e 2 =T s ), and dj and 
z 0 i are, respectively, the displacement height and roughness length of the compartment i. 
3- DATA USED 
Bowen ratio-energy budget data were collected in Smoke Creek Desert, 
Nevada, from June through August, 1991 and in Railroad Valley, Nevada, from June through 
August, 1992 to characterize the energy budget for sparse greasewood (Sarcobatus 
vermiculatis). Smoke creek Desert is located in northwestern Nevada. The field study site 
780