Agricultural and Forest Meteorology 100 2000 183–197
Modelling and measurement of radiation interception by olive canopies
M.J. Mariscal
a,∗
, F. Orgaz
a
, F.J. Villalobos
a,b
a
Instituto de Agricultura Sostenible, Consejo Superior de Investigaciones Cient´ıf´ıcas, Apdo 4084, 14080 Córdoba, Spain
b
Departamento de Agronom´ıa, Universidad de Córdoba, Spain Received 2 February 1999; received in revised form 18 August 1999; accepted 20 September 1999
Abstract
We present the formulation, calibration and validation of a model to estimate photosynthetically active radiation PAR intercepted by olive Olea europaea L. canopies. The model calculates the PAR transmittance at any point located within the
four central trees of the orchard. The spatial and time integration of this process allows calculation of PAR transmitted to the ground and, thus, the PAR intercepted at any given time. Direct-beam radiation, diffuse radiation and scattering are separately
taken into account.
Model parameters are the G-function, leaf reflectance and transmittance. Leaf inclination distributions, measured in the field, were used to derive the G-function. A planophile distribution was found. Leaf reflectances measured in three olive
cultivars were, on average, 0.06 for adaxial and 0.12 for abaxial surfaces, while leaf transmittance was below 0.01. The model was barely sensitive to the parameters.
Predictions of diffuse and total transmittance were tested on clear and overcast days in winter, fall and summer at Córdoba, Spain, in seven olive orchards of quite different characteristics. The validation showed that when estimating average canopy
values, local errors compensate and the predictions of intercepted PAR seem correct for all canopy types, sun position and heights within the stand. The procedure seems applicable to any olive tree variety and does not present important systematic
errors. ©2000 Elsevier Science B.V. All rights reserved.
Keywords: PAR interception; Radiative transfer; Olea europaea L.; Crop modelling
1. Introduction
Among the various environmental factors determin- ing plant growth, photosynthetically active radiation
PAR intercepted by a crop canopy is the main factor that determines dry matter production, being the main
∗
Corresponding author. Present address: Pomology Depart- ment, University of California, One Shields Avenue, Wickson
Hall 0680, Davis, CA-95616, USA. Tel.: 1-530-752-1843; fax: 1-530-752-8502.
E-mail addresses: mariscalucdavis.edu M.J. Mariscal, ag2maarmuco.es F. Orgaz.
source of energy for the process of photosynthesis. Therefore, the supply of radiation sets a limit to po-
tential production. This amount is determined by the incident radiation conditions as well as by the optical
and architectural properties of the stand Ross, 1981.
Canopy photosynthesis, and thus biomass produc- tion, in a given radiation environment results from
the coupling between the photosynthetic response of leaves and the distribution of radiation on these
elements. Within a heterogeneous stand, the atten- uation of radiation resulting from absorption and
scattering by leaves creates a vertical gradient of mean irradiance. Further, horizontal variation of the
0168-192300 – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 9 9 0 0 1 3 7 - 9
184 M.J. Mariscal et al. Agricultural and Forest Meteorology 100 2000 183–197
transmitted radiation results from the highly direc- tional distribution of incoming direct solar radiation
creating sunflecks and shaded areas Baldocchi et al., 1986; Nilson, 1992. An olive orchard is a heteroge-
neous stand, since the leaves are located within an envelope that is distributed in the space according to
planting pattern and row orientation. For these types of canopy, interactions between stand structure and
radiation are complicated to deal with in field trials.
Models of radiative transfer in vegetation aim at de- riving both, the amount and distribution of PAR in-
tercepted by a crop. Since the work of Monsi and Saeki 1953, more than50 models of radiation atten-
uation in homogeneous canopies have been proposed and have been reviewed by Myneni et al. 1989.
Going from horizontally homogeneous canopies to row crops, orchards or forest canopies, those models
are no longer suitable because leaf area is not ran- domly distributed. Nonetheless, for hedgerow planta-
tions two-dimensional theories have been formulated e.g. Charles-Edwards and Thorpe, 1976; Cohen and
Fuchs, 1987; Ganis, 1997. In plantations composed of isolated trees, a three-dimensional characterization
of the grove is needed. Tree crown has been described using ellipsoids Norman and Welles, 1983, cones for
conifers Kuuluvainen and Pukkala, 1987 and cylin- dersparaboloidscones for fruit-trees Wagenmakers,
1991, or also using fractal geometry Myneni, 1991.
Other factors, such as variation in leaf inclination Wang and Baldocchi, 1989, in leaf area density
LAD, Wang and Jarvis, 1990 or penumbral effects Wang and Baldocchi, 1989 have been included on
a few occasions. Regarding the scattering due to leaf reflectance and transmittance, when taken into ac-
count, the approximations of Goudriaan 1977 and Norman and Jarvis 1975 have often been used.
Olive groves are the main component of numerous agricultural systems in the Mediterranean region, in-
cluding traditional extensive groves and new intensive orchards. Extensive cropping systems intercept around
20–30 of the seasonal solar radiation. New intensive olive orchards with fully developed canopies intercept
at most 70 of the solar radiation, but it takes many years to reach this level. This slowness in obtaining
a structure, that produces assimilates together with the low interception in extensive systems, presents an
opportunity to improve productivity Jackson, 1980. Moreover, the radiation interception by olive trees has
not received much attention, partly because of the complexity in describing the three-dimensional radia-
tion regime in these stands. The simulation of olive growth involves intercep-
tion of radiation, dry matter DM production and DM distribution. Regarding these last points, Mariscal
et al. 1998 have reported radiation use efficiency and dry matter partitioning for olive trees.
The objective of this work was to formulate, cal- ibrate and validate a model of PAR interception
by olive canopies. The procedure is based on the three-dimensional approach adopted by Norman and
Welles 1983, takes into account characteristics of olive cropping systems, and considers scattering,
diffuse and direct PAR radiation.
2. Material and methods
