Agricultural and Forest Meteorology 104 2000 289–301
Estimation of evapotranspiration from a field of linseed in central Italy
R. Casa
a,∗
, G. Russell
b
, B. Lo Cascio
a
a
Dipartimento di Produzione Vegetale, Università degli Studi della Tuscia, Via S. Camillo de Lellis, 01100 Viterbo, Italy
b
Institute of Ecology and Resource Management, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JG, Scotland, UK Received 6 December 1999; received in revised form 14 June 2000; accepted 14 June 2000
Abstract
A study was carried out in central Italy into the evapotranspiration from linseed Linum usitatissimum L.. In particular, the objectives were to a evaluate the accuracy of daily evapotranspiration estimates from partially incomplete Bowen ratio
data; b measure seasonal evapotranspiration from linseed; c test the FAO 56 method for estimating crop coefficients for linseed and d calculate the surface resistance of the crop as a function of soil water status. The study was carried out on a
crop of linseed growing in a 1.1 ha field near Viterbo using Bowen ratio apparatus. Soil water content and green area index were also monitored. It was found that 1 daily evapotranspiration could be estimated from partially incomplete Bowen
ratio data on a continuous basis throughout the growing season; 2 the linseed crop lost 240 mm over the 100-day growing season; 3 measured crop coefficients agreed with those obtained using the FAO 56 methodology; 4 surface resistance
could be expressed as a function of green area index and soil water deficit seemed to have an indirect effect by hastening leaf senescence. © 2000 Published by Elsevier Science B.V.
Keywords: Bowen ratio; Evapotranspiration; Linseed; Linum usitatissimum L.; Penman–Monteith equation; Water use
1. Introduction
In recent decades, many complex models have been proposed for the analysis and prediction of crop wa-
ter use. However, simplicity is essential for practical application at the farm or field level, so complicated
mechanisms and processes are often reduced to em- pirical coefficients. The procedure most widely used
by agronomists for the estimation of crop water re- quirements has probably been the FAO 24 approach
Doorenbos and Pruitt, 1977, where the evapotranspi- ration from a well-watered crop is estimated by multi-
∗
Corresponding author. Tel.: +390-761-357-554; fax: +390-761-357-558.
E-mail address: rcasaunitus.it R. Casa.
plying the evaporation from a reference crop by a crop coefficient K
c
that takes into account the differences in albedo, emissivity, soil heat flux, aerodynamic and
surface resistances between the crop considered and the reference crop. These depend to a large extent on
the leaf area index of the crop and its height and thus on the phenological stage. It has been shown that K
c
is site-specific Hanks, 1985 with the main sources of variation being the frequency of wetting of the soil
Jagtap and Jones, 1989 and the degree of coupling between the vapour pressure deficit at the canopy sur-
face and in the free airstream Jarvis and McNaughton, 1986. Variation also arises from the differences in
canopy development between locations. Less variable results are obtained when evaporation from the soil is
considered separately and K
c
is partitioned into a basal
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290 R. Casa et al. Agricultural and Forest Meteorology 104 2000 289–301
crop and a soil coefficient Wright, 1982. The coeffi- cients can then be used more safely in locations other
than those in which they were originally determined Ritchie and Johnson, 1990. The actual values of K
c
obviously depend on the method used to compute reference evapotranspiration and the lack of agree-
ment between the various models Choisnel et al., 1992 prevents the creation of a unique set of K
c
values Howell et al., 1995. The FAO 24 methodology has recently been re-
vised Allen et al., 1998. This methodology will be referred to, in the present paper, as FAO 56. The ref-
erence crop evapotranspiration E , now defined for
a ‘hypothetical crop’ resembling grass Allen et al., 1994, is to be computed using the Penman–Monteith
equation Monteith, 1965. In addition, an extended procedure for adjusting E
to actual crops has been developed Allen et al., 1996. This procedure takes
into account evaporation from the soil surface, and sub-optimal growth conditions such as water stress
and salinity, which result in a reduced leaf area index or an increased surface resistance. A water balance
model of the surface soil layer based on the two-stage drying model described by Ritchie 1972 is used to
estimate soil evaporation and a water balance model of the root zone is added when soil water availability
is sub-optimal.
Independent tests of the FAO 56 procedure are now needed in order to test its validity for a wide range of
crop species and environments. Lysimeters have of- ten been used to provide the ‘true’ evapotranspiration
Wright, 1982; Pruitt, 1991. However, some authors have claimed that measurement errors are more com-
mon than usually thought Allen et al., 1994. Increas- ingly, the Bowen ratio method is employed to estimate
crop evapotranspiration and calculate crop coefficients Hsiao et al., 1985; Grattan et al., 1998 because of its
portability and relative cheapness compared with the other micrometeorological techniques. Errors in the
Bowen ratio estimate of evapotranspiration are com- monly considered to be of the order of 10 Sinclair
et al., 1975; Angus and Watts, 1984. However, recent improvements in the instrumentation and careful data
analysis should enable this error to be reduced Malek and Bingham, 1993; Allen et al., 1994. The removal
of erroneous data Tattari et al., 1995 can result in substantial gaps in the record which can be filled us-
ing procedures that have been proposed for estimation of evapotranspiration by remote sensing Sugita and
Brutsaert, 1991; Zhang and Lemeur, 1995. In the present paper, evapotranspiration of lin-
seed Linum usitatissimum L. was estimated using the Bowen ratio technique during a growing season
in central Italy. Linseed, grown for the production of industrial oil, and its close relative, flax has a
world-wide cultivation area from Canada to India representing a wide range of environmental and man-
agement conditions. The K
c
values reported in FAO 56 and FAO 24 procedures come from a limited num-
ber of experiments carried out in Arizona and eastern Europe and require to be validated for Mediterranean
conditions. Data on the effect of soil water status on the actual water use of linseed in field conditions are
also lacking and this hinders the development and ap- plication of decision support systems for conditions
where shortage of water is important Casa et al., 1997. Most of the relevant informations come from
experiments carried out in India Gupta and Agrawal, 1977; Tiwari et al., 1988; Dutta et al., 1995 where
conditions are different from that in Italy.
The objectives of the present study were thus the following.
1. To evaluate the accuracy of daily evapotranspi- ration estimates from partially incomplete but
quality-checked Bowen ratio data. 2. To estimate linseed water use using the Bowen ratio
method. 3. To test the FAO 56 methodology for estimating K
c
. 4. To investigate the effect of water shortage on evap-
otranspiration by calculating the surface resistance of the crop.
2. Methods
