poster - International Conference of Agricultural Engineering

• The effectiveness of the crop coefficient method to determine water requirements of
the orange orchard and to verify the suitability of the K c value proposed for a generic
citrus crop.
2. Materials and methods
2.1. Site information and field measurements
The trial was carried out during the monitoring period 2010-2011 within an orange orchard
located in Sicily, Southern Italy (Lentini, lat. 37°16’N, long. 14°53’E). This area has a
Mediterranean semi-arid climate (annual mean air temperature 17°C and rainfall less than
600 mm). The experimental field was planted with 15-25 year old orange trees, grown in an
orchard of about 120 ha. The mean canopy height was 3.75 m. Leaf area index (LAI, m 2 m -2 )
was found to be in the range 4.0-4.7. For the dominant wind direction (the main were W and
NW), the fetch was larger than 550 m. The crop was maintained in a well-watered conditions
by irrigation supplied every day during hot months (May-October). Water was supplied by
drip irrigation, with on-line labyrinth drippers, in a number of four per plant, spaced at 0.80 m,
with discharge rate of 4 l/h at a pressure of 100 kPa.
Continuous energy balance measurements were made from January 2010 until December
2011. Net radiation (R n , W m -2 ) was measured with two CNR 1 Kippen&Zonen net radiometer
at height of 8 meter. Soil heat flux density (G, W m -2 ) was measured with three soil heat flux
plates, which were placed horizontally 0.05 meter below soil surface. The air temperature
and the three wind speed components were measured at two heights, 4 and 8 meter, using
fine wire thermocouples (76 µm diameter) and sonic anemometers (CSAT, Campbell Sci.). A
gas analyzer (CSAT, Campbell Sci.) operating at 10 Hz was deployed at 8 meter. The raw
data were recorded at a frequency of 10 Hz using two synchronized data loggers.
Low frequency measurements were taken for air temperature and humidity, wind speed and
direction, and atmospheric pressure at 4 and 8 meter. Rainfall was measured nearby.
2.2. Energy closure at the orange orchard and correction of measured sensible and latent
heat fluxes
Neglecting the amount of available energy at the land surface that is used for photosynthesis
(typically less then 1% of net radiation), the surface energy balance can be expressed at
hourly or longer time scale as:
R n = H + λE
+ G
(1)
R n -G is commonly termed available energy and the surface heat storage (G) at daily or
longer time scales is negligible.
The EC method, applied at the site selected for this study, allows for estimates of H and λE
from direct measurements of fluctuations in the vertical wind velocity and the scalar
concentration. The lack of closure in the energy budget is commonly quantified by the
relative difference between (R n -G) and (H+λE), expressed as a percentage: 100×[((R n -
G)/(H+λE))-1]. Figure 1 shows the hourly data on (H+λE) plotted against (R n -G). Assuming
that R n and G measurements are rather accurate, Fig. 1 shows that during both the
monitored years (H+λE) is underestimated at the site. This may arise from an
underestimation of H or λE, or both. The mean annual energy imbalance is 29.3% during
2010 and 31.1% in 2011, showing peaks during winter and a variation of about 30%.
The BR approach assumes that the EC technique provides correct estimates of the Bowen
ratio (β=H/λE) even though it underestimates H and λE, as some studies tend to confirm (El
Maayar et al., 2008). Thus rearrangement of eq. 1 yields:
Rn − G
λE
=
(2)
1+ β