Agricultural Water Management 98 (2011) 1091–1096
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Agricultural Water Management
journal homepage: www.elsevier.com/locate/agwat
Irrigation requirements and transpiration coupling to the atmosphere of a citrus
orchard in Southern Brazil
Fabio R. Marin a , Luiz Roberto Angelocci b,∗
a
b
Embrapa Agriculture Informatics, Av. André Tosello, 209 – Barão Geraldo, CP 6041, 13083-886 Campinas, SP, Brazil
University of Sao Paulo, College of Agriculture “Luiz de Queiroz”, Av.: Pádua Dias, 11 CP 9, 13418-900 Piracicaba, SP, Brazil
a r t i c l e
i n f o
Article history:
Received 31 July 2010
Accepted 4 February 2011
Available online 3 March 2011
Keywords:
Micrometeorology
Plant-atmosphere coupling
Leaf conductance
Micro-sprinkler
a b s t r a c t
Crop evapotranspiration (ETc) was measured as evaporative heat flux from an irrigated acid lime orchard
(Citrus latifolia Tanaka) using the aerodynamic method. Crop transpiration (T) was determined by a stem
heat balance method. The irrigation requirements were determined by comparing the orchard evapotranspiration (ETc) and T with the reference evapotranspiration (ETo) derived from the Penman–Monteith
equation, and the irrigation requirements were expressed as ETc/ETo (Kc) and T/ETo (Kcb) ratios. The
influence of inter-row vegetation on the ETc was analyzed because the measurements were taken during
the summer and winter, which are periods with different regional soil water content. In this study, the
average Kc values obtained were 0.65 and 0.24 for the summer and winter, respectively. The strong coupling of citrus trees to the atmosphere and the sensitivity of citrus plants to large vapor pressure deficits
and air/leaf temperatures caused variations in the Kcb in relation to the ETo ranges. During the summer,
the Kcb value ranged from 0.34 when the ETo exceeded 5 mm d−1 to 0.46 when the ETo was less than
3 mm d−1 .
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Brazil is the greatest citrus producer in the world. The state of
São Paulo in southern Brazil produces more than 80% of Brazilian citrus. Approximately 15% of the citrus-cultivated area is under
irrigation (Alves et al., 2007), whereas the remaining areas are
subject to weather variability and its effects on fruit yield and
quality.
Despite the advancement of technologies for water supply and
the economic importance of citrus, irrigation management remains
inadequate in most areas. The lack of basic information on crop
water needs is one of the causes of inadequate irrigation management.
Good irrigation practices lead to higher yields and incomes for
producers, but also increase the demand for water. Although water
resources are reasonably available in southeast Brazil, water use for
irrigation has become a major issue in this region during the winter
and spring seasons when rainfall is scarce.
The crop coefficient (Kc) approach (Allen et al., 1998) for
determining evapotranspiration makes it possible to consider the
independent contributions of the soil evaporation and crop transpi-
∗ Corresponding author. Tel.: +55 19 3211 5876; fax: +55 19 3211 5754.
E-mail addresses: [email protected], [email protected]
(F.R. Marin), [email protected] (L.R. Angelocci).
0378-3774/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2011.02.002
ration by splitting Kc into two separate coefficients as follows: Ke,
a soil evaporation coefficient; and Kcb, a crop transpiration coefficient (referred to as the basal crop transpiration coefficient). This
approach can be adopted as the procedure for scheduling and quantifying the water amount to be applied. However, few experiments
have been carried out to determine the evapotranspiration for citrus orchards (Van et al., 1967; Kalma and Stanhill, 1969; Alves et al.,
2007). Orchards have discontinuous soil cover and, when the plants
are fully developed, they are normally tall, allowing intense air mixing in the environment. These features should be taken into account
in the determination of Kc values.
In this study, we determined the values of Kc and Kcb for an acid
lime orchard. We also discussed the paradigm involving the crop
water requirements approach for orchard crops considering the
orchard-atmosphere coupling effect on crop water requirements.
2. Materials and methods
The study was carried out in an orchard in the experimental
area of the College of Agriculture “Luiz de Queiroz” at the University of São Paulo, Piracicaba, São Paulo State, Brazil (latitude
22◦ 42 S; longitude 47◦ 30 W; 546 m a.m.s.l.) from January 1998 to
August 2000. The micrometeorological measurements were carried
out during two seasons of 2000: the wet, hot summer (January)
and the dry, cold winter (July). The experimental field had 0.63 ha
(63 m × 100 m) of 7-year-old plants of Citrus latifolia Tanaka grafted
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F.R. Marin, L.R. Angelocci / Agricultural Water Management 98 (2011) 1091–1096
on Citrus limonia Osbeck rootstock growing in an orchard with the
largest dimension oriented predominantly northwest and southeast. The spacing at planting was 7 m between plants and 8 m
between rows. The average crown dimensions were 4.5 m (height)
by 4 m (diameter).
The soil was classified as Typic Rhodustults. On the northwest
side at 50 m distance from of the citrus orchard, there was a Pinus
sp. wind break. The predominant wind direction at Piracicaba was
southeastern. At the south and east sides of the field, there were
irrigated passion fruit and mango crops (2 m in height), nearly 5 m
from the orchard edge.
The diurnal course of leaf diffusive resistance (rs ) was determined at least once a month throughout 1998 using a steady state
pre-calibrated porometer (LI 1600; Li-Cor, Inc.). The rs was measured on exposed and shaded leaves in the upper, middle and
bottom canopy layers by sampling 20 leaves 7 times each day from
0900 h to 1600 h (local time) (Angelocci et al., 2004).
The mean values were used to compute the decoupling factor
(˝) for a hypostomatous leaf, which was defined by the following
equation, as described by McNaughton and Jarvis (1983) and Jarvis
(1985b):
˝=
1
1 + (2rs )/((S/ + 2)ra )
(1)
The daily ETc was calculated, recorded at 10-s intervals, averaged over 15 min and stored by a datalogger (CR7; Campbell
Scientific, Inc.). The following equation was used to calculate the
ETc:
ETc = −k2
ue z 2
fe
(2)
where is the air density (1.26 kg m−3 ); is the water latent
heat (2.45 × 106 J kg−1 ); k is the von Karman constant (0.4); P is
the local atmospheric pressure (kPa); z̄ and z are the average
between two measurement heights and the distance between two
heights, respectively (m); d is the zero-plane displacement height
(m), which is assumed to be 2/3 of crop height (approximately
4.5 m) based on the reports by Stanhill and Kalma (1972) and Kalma
and Fuchs (1976); u is the wind speed difference between the two
heights (m s−1 ); e is the difference of water vapor pressure at the
same two heights (kPa); and fe is an empirical correction function
to take into account the atmospheric stability described by Thom
et al. (1975). The following equations describe the fe function:
fe = (1 − 16Ri)
0.75
fe = (1 + 16Ri)
−2
fe = 1
Ri = g
where rs is the stomatal resistance to vapor diffusion, and ra is
the bulk aerodynamic resistance of acid lime orchards following
Landsberg and Jones (1981), with shape factor values ranging from
6.3 to 7.9.
Conceptually, the extreme values for the decoupling factor are as
follows: (a) ˝ → 1 as rs /ra → 0, implying that the net radiation is the
only contributor to the evapotranspiration process and that vegetation is completely decoupled from the atmospheric conditions;
(b) ˝ → 0 as rs /ra → ∞ indicating complete coupling of vegetation
with atmospheric vapor pressure deficit and wind speed.
In 1999, micrometeorological and sap flow measurements were
taken. For these measurements, a 10-m-tall tower was positioned
in the middle of the field (50 m from the crop edge in the predominant wind direction). Net radiation (Rn) was measured with a net
radiometer (NR Lite; Kipp and Zonen, Inc.) mounted 6.5 m above
the soil surface. The soil heat flux (G) at the surface was measured
using three heat flux plates (HTF3, REBS) that were placed 30 mm
below the soil surface, with two plates beneath the plant canopies
and another in the inter-row space.
The overall crop evapotranspiration (ETc) was determined by
the aerodynamic method (Thom et al., 1975) during the summer
and winter of 1999 (Eqs. (2–6)). To measure the vapor concentration, we used four aspirated copper–constantan thermocouple
psychrometer (Marin et al., 2001) mounted at 2.5 m, 3.5 m, 4.5 m
and 6.5 m above the ground in a row between two trees. The wind
speed was measured with Met-One anemometers (model OA14;
0.45 m s−1 starting speed) at the same heights with an extra sensor
at 8.5 m above the ground. Combinations of measurement heights
for the vertical gradients were previously tested, and 2.5 m and
6.5 m were used as the most appropriate levels of measurement
(Pereira et al., 2002).
The dry period was preceded by 110 d without rain, and the
orchard was irrigated with micro-sprinklers wetting the area under
the crowns; the mini-sprinklers were scheduled to ensure 80% as
minimal available soil water content in the 1 m soil profile. In the
same orchard, Machado and Coelho (2000) showed that the roots
reached 1.5 m depth and that the bulk of the roots were in the
top 0.4 m of the soil. The frequency and amount of irrigation were
scheduled to avoid a shallow root system that would lead to a water
supply insufficient to match the atmospheric demand.
0.622
2
(z̄ − d)
P
Ri < −0.01 (unstable)
(3)
Ri > 0.01 (stable)
(4)
− 0.01 ≤ Ri ≤ 0.01 (neutral)
(5)
(/z)
T (u/z)
2
(6)
where Ri is the gradient Richardson number; g is the gravitational
acceleration (9.8 m s−2 ); and is the vertical difference of potential temperature (K) set equal to T as suggested by Rosemberg
et al. (1983) due to the small z used.
In 1999, the measurements started during the summer season
(wet period) with high regional soil moisture and full inter-row
ground cover by small grass vegetation. The winter period was
characterized by the decrease of regional soil moisture and by interrow grass drying, when citrus leaves and wet soil bulbs were the
main water vapor sources in the area.
The meteorological data were collected from an automatic standard weather station (CR10X; Campbell Scientific, Inc.), which was
located over grass 2 km from the experimental field. They were
used to compute daily values of reference evapotranspiration (ETo)
based on the Penman–Monteith method, as parameterized by the
FAO-56 Bulletin (Allen et al., 1998). Another identical weather station was installed inside the orchard measuring the same variables
at the same time during the entire experimental period.
In parallel with the micrometeorological measurements, sap
flow was measured in two trees with different crown sizes. These
measurements were used to observe the effect of the size of the leaf
area on tree transpiration using the stem heat balance technique
(Sakuratani, 1981; Baker and Van Bavel, 1987). Due to the large
size of the trunk (diameter greater than 0.2 m) and irregularity of
its shape (resulting in poor contact with the sensor), it was necessary to install one sensor in each of the three main branches. The
respective values were summed to determine the whole tree sap
flow. We built each sensor, and each sensor was fed by a DC power
supply, which dissipated between 1 W and 3 W, depending on the
branch diameter. The change in heat storage of the branch segment
was also measured (Marin, 2000). All signals were monitored every
10 s by a CR7 datalogger (Campbell Inc.), which gave mean values
every 15 min using the procedures described by Valancogne and
Nasr (1989).
The daily sap flow value for each branch was computed from
the summation of the values at every time interval of measurement
starting at sunrise when it was assumed that the tree had its maximal internal water capacitance. The 24 h integrated values of sap
flow were considered as representative of the daily transpiration
F.R. Marin, L.R. Angelocci / Agricultural Water Management 98 (2011) 1091–1096
1093
Fig. 1. Daily variation of acid lime plant transpiration (T), orchard evapotranspiration (ETc) and reference evapotranspiration (ETo) throughout the experimental period of
1999.
Table 1
The leaf area (LA) and leaf area index (LAI) for both trees in which sap flow was
measured and the mean orchard LA on the two dates in 1999.
Measurement date
Jan/14/2000
Jul./11/2000
Tree 1
Tree 2
Orchard average
Leaf area
LAI
Leaf area
LAI
Leaf area
48
64
4.5
5.6
99
87
5.5
5.5
5.1
4.8
of each plant. The transpiration rates were normalized by dividing
them by the leaf area (LA) of the plant to obtain the transpiration
rate on a LA unit basis (mm m−2 of the leaf).
The crop transpiration was scaled up to a ground area unit basis
by multiplying the average transpiration rate of the four plants by
the average leaf area index (LAI). The LA was measured on ten trees
in order to compute the mean orchard LAI. The LA was determined
with a LAI-2000 Canopy Analyzer (Li-Cor, Inc.) in the two seasons
(Table 1). The LAI of each plant was calculated by dividing the LA
by the canopy crown area projected on the ground.
3. Results and discussion
The ETo was consistently higher than the ETc in both seasons,
and the T was lower than the ETc in the summer. During the winter,
the ETc and T were similar for the season (Fig. 1). Gaps in Fig. 1 were
due to failures in the measurement system. The contribution of
the active inter-row vegetation for the overall orchard evapotranspiration may explain why Etc values were higher than T values
during the summer. Because the inter-row vegetation was dehydrated and apparently without transpiration activity during the
winter, the acid lime trees became the main source of water vapor,
leading to the similar values of ETc and T. In both seasons, values
for ETo were higher than those for ETc and T.As the evaporation of
the inter-row surface was computed by the difference between T
and ETc, the role of the inter-row vegetation was inferred by comparing these variables (Fig. 2). Except for a few days, Figs. 2a and 3a
show that T did not follow ETo and ETc for the whole range of values, but rather that T stabilized when ETo reached 4 mm d−1 . For
the winter (Figs. 2b and 3b), this relationship was not as clear as
it was in the summer, as ETo and ETc were lower than in summer.
Despite the poor linear relation between the T and ETc, the interrow evapotranspiration (ETg) represented nearly 35% of the overall
evapotranspiration during the summer, which become almost negligible during the winter. The difference of 8% between the T and
ETc (Fig. 2b) may be attributed to measurement errors rather than
any ecophysiological effect.The ratio of the overall orchard area
occupied by acid lime crowns to the overall orchard area ranged
between 0.35 and 0.40, which was a similar value to the T:ETc
ratio observed during the summer. Based on this observation, the
inter-row vegetation and trees showed similar daily transpiration
rates.
The mean daily Kc values (given by the ETc and ETo ratio) during
the summer was 0.65 ± 0.11, ranging from 0.51 to 0.94. In the winter, the mean Kc value ranged from 0.10 to 0.52 with a mean value
of 0.24 ± 0.12. The summer Kc value compared well with other Kc
values reported for humid climates (Rogers et al., 1983; Boman,
Fig. 2. Relation between the acid lime transpiration (T) and orchard evapotranspiration (ETc) during the summer and winter seasons.
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F.R. Marin, L.R. Angelocci / Agricultural Water Management 98 (2011) 1091–1096
Fig. 3. Relationship between acid lime transpiration (T) and orchard evapotranspiration (ETc) with the reference evapotranspiration (ETo) in two seasons.
1994; Doorenbos and Pruitt, 1977; Castel et al., 1987; Allen et al.,
1998), but the winter Kc values were nearly half of the Kc values
reported by Allen et al. (1998) for non-ground covered orchards.
The winter Kc values were also lower than those observed by Alves
et al. (2007) under the same climate and soil conditions.
Assuming a measurement error of about 8%, as discussed previously (Fig. 2b), we increased the ETc and found average Kc values
of 0.70 and 0.26 for the summer and winter, respectively. These
corrected Kc values were still slightly lower than those observed
by Alves et al. (2007), but were within a similar range reported by
others (Rogers et al., 1983; Boman, 1994; Doorenbos and Pruitt,
1977; Castel et al., 1987; Allen et al., 1998).
The Kcb value was 0.41 ± 0.08 for the wet period and 0.28 ± 0.07
for the dry period, and these Kcb values were comparable to previously reported values (Castel, 1994; Boman, 1994; Allen et al.,
1998; Alves et al., 2007). Although the Ke was originally defined
for bare soil, it represents the bare soil below the citrus canopies
plus inter-row water loss, which includes weed transpiration. The
average Ke value was 0.23 during the summer, but it was as low
as 0.02 during the winter due to the severe drought observed
during the season.The variety, root-stock, plant age and management practices are responsible for differences in the Kc and
Kcb values, but the differences in micrometeorological conditions have an important role, especially regarding the atmospheric
water demand (Marin et al., 2005). As mentioned before, there
is evidence that Kcb values may decrease under high ETo values (Fig. 3a), even under high soil water content. Furthermore,
papers concerning the relationship between leaf diffusive conductance and environmental variables have demonstrated that
citrus leaves restrict water loss under high atmospheric water
demand. Several papers have noticed dependence of stomatal conductance on air temperature and VPD with stomatal closure in
response to an increase of weather variables (Hall et al., 1975;
Kahiri and Hall, 1976; Cohen and Cohen, 1983; Angelocci et al.,
2004).
The non-linear relationship between T and ETo (Fig. 3a) may be
due to an increase of inner resistances to water transport of acid
lime trees when subjected to conditions of high atmospheric water
demand, as verified for other horticultural species (Syverstsen and
Lloyd, 1994; Tardieu and Simonneau, 1998) due to an opposite
tendency of transpiration and stomatal movement in relation to
increased air vapor pressure deficit (McNaughton and Jarvis, 1983).
Fig. 4 shows the response of stomatal vapor diffusion conductance (gs) to weather variables, with gs decreasing when the
atmosphere demands high transpiration rates. Based on Fig. 4, it is
possible to visually detect 650 W m−2 , 27 ◦ C and 1 kPa as gs maximal
points for solar radiation, air temperature and VPD, respectively.
Using these values to estimate the ETo (Allen et al., 1998), a daily
rate of 4.9 mm d−1 was found. This estimation was done with the
assumption that the net radiation was 50% of the solar radiation
and that the photoperiod was 12 h. Moreover, the estimation was
carried out using the average values of wind speed and relative
humidity data collected throughout the experiment (1.5 m s−1 and
73%, respectively).
Despite the correlation between the air temperature and VPD
and the poor correlations in Fig. 4, the decreases in gs under high
atmospheric water demand leads to the assumption that trees control the transpiration as the ETo increases, which could support the
use of different Kcb values for discrete ETo ranges. Table 2 shows
the Kc and Kcb values for different ETo ranges. For both seasons, the
Kcb values decreased as the ETo increased, which may represent a
way to improve water management in orchards under localized
irrigation.
The decoupling factor, ˝, (Jarvis, 1985a) was calculated using
aerodynamic and canopy resistance data collected during 1998
(Fig. 5). The low values of ˝ demonstrated the influence of wind
Fig. 4. Relationship between daily mean leaf conductance and mean values of (a) solar radiation, (b) air temperature and vapor pressure deficit (VPD).
F.R. Marin, L.R. Angelocci / Agricultural Water Management 98 (2011) 1091–1096
1095
Table 2
The values of Kc and Kcb for different ETo ranges; the standard deviation is found in the brackets.
Range
Wet summer
Dry winter
Kc
1.3 < ETo =< 3
3 < ETo =< 5
5 < ETo =< 6.6
0.72
0.69
0.67
Kcb
[0.13]
[0.12]
[0.11]
0.46
0.44
0.34
speed and VPD on ETc and T, i.e., the crop transpiration was conditioned by aerodynamic conditions rather than radiation conditions,
which imposed a tendency of larger crop evapotranspiration rates.
As Jarvis (1985b) postulated, ˝ tends to gradually lessen in tall
horticultural crops with discontinuous ground cover, due to a
reduction of aerodynamic resistances of the canopy caused by a vigorous air mixing and a high crop roughness. Therefore, in conditions
of high available energy, wind speed and VPD, which are normally
found when ETo surpasses 4.0 mm d−1 , it may be expected that tall
horticultural species with high inner resistances to water flow do
not respond directly to the atmospheric water demand.
Fig. 3A shows a linear relationship between the ETc and ETo,
indicating that the decrease in the transpiration rate under high
atmospheric demands was compensated by a direct response
inter-row evapotranspiration rate (i.e., soil evaporation plus grass
transpiration) during the summer. This compensation was based on
the fact that the rate of transpiration by short vegetation and soil
evaporation are normally decoupled from the atmospheric conditions because Rn is the major contributor to the evapotranspiration
process (McNaughton and Jarvis, 1983). This compensation, in turn,
was an effect of acid lime trees on inter-row vegetation and soil
reducing the wind speed near the ground.
Allen et al. (1998) claimed that Kc values must be used under
standard climatic conditions (sub-humid climate, minimum relative humidity of 45% and wind speeds averaging 2 m s−1 ) and that
variations in wind speed may alter aerodynamic resistance and,
hence, crop coefficients mainly for tall crops. They also asserted
that Kc tends to increase under conditions of high wind speeds and
low relative humidity.
However, some aspects we observed for the lime acid orchard
were different from the aspects postulated by Allen et al. (1998).
First, we observed high wind speed and low air relative humidity
affecting acid lime transpiration and changing Kcb as the ETo varied. Second, we observed that T did not linearly follow ETo, which
resulted in a decrease in Kcb values as ETo increased. Third, the Kc
values did not show the same pattern during the summer due the
inter-row role, compensating the citrus transpiration when there
was enough soil water (Table 2).
Fig. 5. Time variation of porometry measured stomatal conductance to vapor diffusion (gs) and decoupling factor (˝) measured in the acid lime orchard throughout
1998.
Kc
[0.09]
[0.02]
[0.06]
0.32
0.28
0.19
Kcb
[0.15]
[0.15]
[0.05]
0.38
0.29
0.22
[0.08]
[0.05]
[0.03]
Therefore, for tall and sparse citrus orchards using localized irrigation, such as drip and micro-sprinkler systems, it is inadequate
to adopt unique values of Kcb and Kc. Finally, it should be noted
that Kc has a small variation as the ETo ranges from 1.3 mm d−1 to
6.5 mm d−1 , which is mainly due to the role of inter-row vegetation.
In spite of the sparse field data available in this study, Marin et al.
(2005) demonstrated the same trend in coffee in southern Brazil,
and this trend may be generalized for other tall sparse horticultural
crops with low ˝ values after further experiments.
4. Conclusions
The mean Kc value during the summer was 0.65 and during
the winter 0.24. The average Kcb values ranged from 0.41 to 0.28
for the summer and winter, respectively. The transpiration represented 35% of overall orchard evapotranspiration, a value close
to the orchard area occupied by acid lime trees. The acid lime
leaves reduced the stomatal conductance under high temperature,
VPD and solar radiation as a consequence of the strong transpiration coupling to the atmosphere. The transpiration coupling to the
atmosphere was cited as the main cause for the reduced Kcb as ETo
increased. During the summer, Kc values had small variation as the
ETo ranged from 1.3 mm d−1 to 6.5 mm d−1 , which was mainly due
to the role of inter-row vegetation. It was proposed that the Kcb recommendation for practical purposes may be variable in function of
ETo. In this study, the Kcb values decreased nearly 40% when the
ETo increased from 3 mm d−1 to 5 mm d−1 .
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