poster - International Conference of Agricultural Engineering

As evidenced by Rana et al. (2005), for irrigated crops r c is not a constant, but it varied
depending on the available energy and the vapour pressure deficit. Katerji and Perrier (1983)
proposed to calculate r c as:
*
rc
r
= a + b
(5)
* ∆ + γ ρCpD
r = ⋅
(6)
ra
ra
∆γ A
where a and b are empirical calibration coefficients which require experimental
determination; r* (s m -1 ) is given as (Monteith, 1965).
In our study, the canopy resistance was calculated from eq. 4, by introducing the λE corr
values calculated by eq. 2, together with the measured values of D and A, and the estimated
values of r a :
ln(z − d) /(hc
− d)
ra
= (7)
*
ku
where z is the reference point above the canopy (8 meter), d (m), the zero plane
displacement is estimated as a portion of the canopy height where an intermediate scaling is
d=0.75 h c , h c is the mean height of the orchard (3.75 m), k=0.4 is the von Karman constant
and u* is the friction velocity (m s -1 ) measured by the EC method. The obtained values of r c
were combined with eq. 5 to estimate the parameter a and b.
The model was calibrated using 3 months of data (June-August) during the irrigation season
2010. A linear curve fit resulted in a=0.364 and b=0.0422 (coefficient of determination
R 2 =0.6287).
The final expression of the model at hourly time scale is:
∆A
+ ( ρCp
D ra
)
λ Emod
=
(8)
∆ + γ( 1.0422 + 0.364(r * / ra
))
The calculation of the orange orchard ET c,mod at daily time scale was obtained by summation
of the hourly values of λE mod (from eq. 8), after dividing by λ.
2.5. Determination of the crop coefficient Kc
Crop coefficients are determined by calculating the ratio K c = ET c,mod /ET o , where ET c,mod is the
evapotranspiration of a well-watered crop and ET 0 is the reference evapotranspiration
calculated by the Penman-Montetih method (Allen et al., 1998). The variables used for ET 0
determination were measured in an agrometeorlogical station of the Sicilian
Agrometerological Service (SIAS) located 3.0 km away from the experimental field. The
station was equipped with instruments for measuring the standard meteorological variables
(solar radiation, wind speed and direction, air temperature, relative humidity).
3. Results and discussion
3.1. BR-correction of measured sensible and latent heat fluxes
The energy balance ratio, i.e. the ratio of turbulent energy fluxes to available energy was
0.89 in 2010 and 0.86 in 2011. The RMSE (root mean square error) for 1 h values turbulent
fluxes was 1.27 and 1.29 MJ m -2 h -1 , during 2010 and 2011 respectively, evidencing the good
quality of the dataset. The latent heat flux (λE) was always in excess of the sensible heat flux
(H) during daylight hours. The H was higher than the soil heat flux (G). At the night the
results from Eddy Covariance showed H and LE approaching to zero. Unstable atmospheric
conditions predominated above the orchard, with the sensible heat flux (H) accounting for
about 30% of R n during both the monitoring periods. The significant leaf area index (LAI) of
orange crop within the study site (LAI of about 4-4.7 m 2 m -2 ) caused solar radiation to hardly
penetrate through the canopy. As a consequence, the soil heat flux (G) at daily scale was
small and negative, with daily average value less than 1% of R n . The largest part of R n was
used as latent heat flux (λE), that represents on average 67% of R n during the year 2010 and
57% of R n during 2011. The corresponding evaporative fractions (E F =λE/(R n -G)) were 0.70
and 0.60. Monthly data show that, after forcing (through the BR approach) the measured
energy balance data to close, the discrepancy between the measured available energy (R n -
G) and the turbulent fluxes (H corr +λE corr ) tend to be neglected. In particular, H corr increased of