reference grass ET ET
o
by the crop coefficient K
c
= crop ETgrass ET, which depends on ground
cover and crop characteristics FAO method, Doorenbos and Pruitt, 1977; Allen et al., 1998. In
the case of olive the information on K
c
is scarce and obtained mainly from ET measurements us-
ing the soil water balance e.g. Mickelakis et al., 1994. Orgaz and Fereres 1997 reported crop
coefficients from 0.45 to 0.75 in different locations which are far below the values of annual crops,
typically from 1.0 to 1.2 Doorenbos and Pruitt, 1977. The variability of K
c
measured at different locations makes it difficult to apply the FAO
method to locations where no experimental infor- mation exists. An alternative approach to deter-
mine olive ET is to calculate its two components, transpiration E
p
and evaporation from the soil surface E
s
, independently, with an E
p
model based on the equation of Penman – Monteith
Monteith, 1965 and an E
s
model like the one proposed by Ritchie 1972. The E
p
model re- quires a parameterization of canopy conductance
G
c
as a function of environmental variables e.g. Stewart, 1988 which has to be based on accurate
measurements of evaporation at short time steps e.g. Dolman et al., 1988.
The objectives of this work were a to develop a joint E
s
– E
p
model to determine evapotranspira- tion of intensive irrigated olive orchards, and b
to analyze temporal variations in the crop coeffi- cient.
2. Materials and methods
The experiments were performed in a 6 ha drip-irrigated olive Olea europaea L. cv. ‘Picual’
orchard located at the Agricultural Research Cen- ter of Cordoba, Spain 38°N, 4°W, altitude 110
m. Plant spacing was 6 × 6 m. The trees had an average leaf area index LAI of 1.5 in May 1996
and 1.2 in May 1997, as determined with a Plant Canopy Analyzer model LAI-2000, Li-Cor Inc.,
Lincoln, NE following the procedure of Villalo- bos et al. 1995. Mean tree height was 4 m and
ground cover was 40.
Hourly weather data were determined over an irrigated grass Festuca arundinacea L. plot of 1.5
ha, located 400 m northwest of the olive orchard.
2
.
1
. Experiment
1
Sensible H and latent heat flux LE were measured using the eddy covariance method
above and below the trees from day of year DOY 162 11 June to DOY 181 30 June,
1997. Transpiration LE
p
was computed as the difference between LE above ET and LE below
E
s
. The fluxes were measured with a single-axis sonic anemometer model CA27, Campbell Scien-
tific, Logan, UT and a krypton hygrometer model KH20, Campbell Scientific. The sensors
above were set at a height of 5 m with a horizon- tal separation of 0.3 m, while the sensors below
were separated 0.13 m at a height of 0.4 m. Sampling frequency was 10.67 Hz inverse of 664
s. Fetch was 170 m in the west direction, which might be considered adequate as it contributed
90 of the measured fluxes according to the footprint analysis of Schuepp et al. 1990. Cor-
rection to the LE fluxes due to sensible and latent flux Webb et al., 1980, oxygen absorption Tan-
ner et al., 1993 and sensor separation Villalobos, 1997 were applied. Fluxes were calculated and
stored using a datalogger model CR10X, Camp- bell Scientific for 10-min periods and then aver-
aged for hourly periods.
2
.
2
. Experiment
2
Energy balance and evaporation measurements were performed from March 1996 to June 1996.
LE and H were determined above the canopy as in Section 2.1. Net radiation R
n
was measured above and below the canopy using four net ra-
diometers model Q-7, Radiation and Energy Bal- ance Systems, Seattle, WA in locations of
maximum above tree and minimum ground cover middle point among four trees. Measure-
ment heights were 5 and 0.3 m for sensors above and below the canopy, respectively. Soil heat flux
was determined at the same two locations of maximum and minimum ground cover. The com-
bination method was applied with soil tempera- ture
measured using
a copper-constantan
thermocouple at 0.025 m and heat flux measured with a soil heat flux plate model HFT3.1, Radia-
tion and Energy Balance Systems at 0.05 m. Soil
heat flux plates and thermocouples were located beside the net radiometers below the canopy.
Measurements of R
n
and G were performed at 60 s intervals while 10 min averages were stored
using a datalogger model CR10X and then averaged for hourly periods. Net radiation above
the orchard was regressed on solar radiation measured using a pyranometer see below. Soil
heat flux was regressed on net radiation below the canopy. These relationships were later used
to run the evaporation model using weather data.
The performance of the eddy covariance system was assessed by regressing LE + H on R
n
− G
for 24-h periods. The intercept 5.9 W m
− 2
and the slope 0.88 were not different from 0 and 1,
respectively, at the 0.95 probability level. The regression forced through the origin yielded a
slope of 0.91 indicating an underprediction of the fluxes of less than 10 which is similar to that
reported by other authors e.g. Rochette et al., 1995 using the eddy covariance technique. Daily
evaporation data were corrected by dividing by the ratio LE + HR
n
− G.
2
.
3
. Calculations Hourly canopy conductance G
c
for daylight conditions was derived from the Penman – Mon-
teith equation:
G
c −
1
= DR
na
+ r C
p
VPD LE
p
− D−g
1 g
G
a
1 where D is the slope of saturation vapor pressure
on temperature, g the psycrometric constant, r air density, C
p
specific heat of air at constant pres- sure, VPD vapor pressure deficit and G
a
is aero- dynamic conductance see below. Absorbed net
radiation R
na
was estimated as the product of R
n
above the orchard and the fraction of intercepted photosynthetically-active radiation PAR, which
was in turn calculated using a model of PAR interception by olive orchards Mariscal, 1998.
The sensitivity of LE
p
to changes in G
c
and G
a
where calculated according to Raupach and Finnigan 1988.
2
.
4
. Experiment
3
The objective of this experiment was to deter- mine the relationship between aerodynamic con-
ductance over the olive orchard and wind speed over a grass reference plot.
A three-dimensional sonic anemometer model CSAT-3, Campbell Scientific was installed at a 5
m height over the olive orchard at the same position where the sensors were located during
measurements in 1996. The sensor was monitored at 10.7 Hz using a CR10X datalogger. The mea-
surements were performed from 280497 to 05 0597
DOY 118-DOY
125. Fluxes
were calculated and stored for 10-min periods and then
averaged for hourly periods. Simultaneous measurements of wind speed at 2
m height U
g
using a propeller anemometer Young 05103 were performed over the irrigated
grass plot. Aerodynamic conductance over the olive or-
chard was calculated as: G
a
= u
2
U
o
2a u =
−wu 2b
where u is friction velocity m s
− 1
, U
o
average horizontal wind speed m s
− 1
, u and w are instantaneous departures from the average hori-
zontal and vertical wind speed, respectively, and horizontal bar indicates an average. Empirical
relationships between G
a
and U
g
and between u and U
o
were derived.
2
.
5
. Transpiration model We fitted an empirical model e.g. Stewart,
1988 to estimate canopy conductance G
c
in the orchard described above as a function of irradi-
ance R
s
, vapor pressure deficit VPD and tem- perature T:
G
c
= G
cm
f
1
VPDf
2
R
s
f
3
T 3
f
1
VPD = 1 − K
D
D 0 B D B D
c
4a f
1
VPD = 1 − K
D
D
c
D \ D
c
4b
f
2
R
s
= R
s
1000 + K
R
1000R
s
+ K
R
5 f
3
T = T – T
L
T
H
– T
a
K
T
– T
L
T
H
– K
T a
6 where a = T
H
− K
T
K
T
− T
L
. In order to mini- mize the number of unknown parameters G
cm
, K
D
, D
c
, K
R
, K
T
, T
L
, T
H
we assumed that the lower and upper limiting temperatures were T
H
= 40°C and T
L
= 0°C, which are the values sug-
gested by Stewart 1988 for pine trees. The model was combined with Ritchie 1972
model as modified by Bonachela et al. 1999 to calculate E
s
. The combined model was tested against the data of Section 2.2 spring 1996. The
model was then used to calculate the daily crop coefficient K
c
, ratio of evaporation to grass ET of the orchard, case A, typical of intensive plan-
tations using weather data of Cordoba from 1964 to 1986, as K
c
is a commonly used parameter for calculating irrigation requirements. The model
was also applied to a traditional orchard case B with the same LAI and ground cover of 30,
assuming that maximum G
c
was 80 of that of the intensive orchard, due to the lower radiation
interception. The value of 80 corresponds to the ratio of intercepted radiation between the two
orchards. Although future work should be di- rected to the scaling-up of olive canopy conduc-
tance our assumption is supported by the general link between canopy conductance, and assimila-
tion e.g. Wang and Leuning, 1998 and the rela- tion
between assimilation
and radiation
interception of olive trees shown by Mariscal et al. 2000.
3. Results and discussion
