2. 56 APPLIED ENGINEERING IN AGRICULTURE
While Jordan has witnessed the advent of modern,
pressurized irrigation systems, these networks are not
realizing their full potential and it has become ever-more
necessary to exploit this potential through proper
management techniques and improved irrigation
scheduling. Regulated deficit irrigation (RDI) is one of the
promising management techniques that can be applied to
Jordan’s agricultural sector. RDI, in essence, maximizes
water use efficiency by purposely reducing water
application during certain stages of growth that are less
sensitive to water stress. In these stages, water is supplied
at a rate lower than the evapotranspiration rate but still at a
rate high enough to avoid any negative impacts on crop
yield or quality (Fereres et al., 2003; Kijne, 2003; Velez
et al., 2007). As a result, water is saved and crops are
maintained.
Several studies have already been carried out to
investigate the impact of RDI on crop yields in countries
with arid environments such as Australia, Spain, Turkey,
Morocco, the United States, and China [Food and
Agriculture Organization (FAO), 2002], with some also
focusing on olive trees. Dabbou et al. (2010), in their
experiment on olive trees in Tunisia, found that the best oil
and fresh fruit production occurred with a 75% RDI
scheme. Ramos and Santos (2010) likewise found the
highest oil and fruit yields occurring with a 60% sustained-
deficit irrigation regime within their olive tree test case in
Portugal. And the olive trees tested by García et al. (2013)
in Spain with RDI treatments saw higher oil yields and no
difference in quality as compared to control cases. On the
other hand, Gómez-del-Campo (2013) reports that while
the control treatment with no RDI on olive trees in Spain
still had the highest oil content, one of the RDI treatments
had an oil content very close to the control and just as
importantly reaped significant water savings, unlike the
control group.
With regard to the particular timing of RDI application,
it has been found that the second phase of fruit develop-
ment in the olive tree, when pit hardening occurs, is the
most resistant to water deficit (Goldhammer, 1999), while
the blooming growth stage is extremely sensitive to water
stress (Moriana et al., 2003). The third phase of olive fruit
development, oil accumulation, while difficult to pinpoint,
is also said to be sensitive to water stress (Lavee and
Wodner, 1991). Tognetti et al. (2005) have recommended
that RDI be applied to olive trees after the pit hardening
stage to cover 66% of the crop evapotranspiration. It has
been seen elsewhere that olive trees show only a slight
reduction in fruit production and oil yield under severe
water stress during this stage of growth (Goldhammer
et al., 1994; Moriana et al., 2003).
While these studies reveal useful information with
regard to olive trees and deficit irrigation, the present study
seeks to add to this body of knowledge in two ways. First,
the specific suitability of RDI for olive trees within
Jordan’s unique and local climatic conditions is newly
examined. In particular, the study is conducted in the
Highlands region, which uses limited groundwater
resources as the main source for irrigation and also suffers
from high soil salinity. Farmers have shifted to olive trees
and away from other crops in past years due to their more
drought-resistant and saline-tolerant nature (Chebaane
et al., 2004). But the sustainability of olive tree cultivation
will still depend on farmers’ abilities to use less water and
this study’s support of RDI for olive trees could be the
solution. The second way in which this study seeks to
contribute is to offer a more straightforward and less
technically-sophisticated variety of RDI that can be easily
adopted by farmers, many of whom do not have the
capacity to engage in more complex schemes and do not
benefit from strong agricultural extension services from the
government. Ultimately, the objective of this research is to
provide farmers with the means to apply RDI within their
daily work, not only to maintain the quantity and quality of
their olive yield but also to aid the nation in decreasing its
water demands in the agricultural sector.
METHODOLOGY
The methodology of this study includes several steps.
After determining the general climatic conditions of the
study location, a soil analysis was conducted to ascertain
the soil texture and water holding capacity that would
eventually guide the irrigation scheduling. Evapotranspira-
tion was then estimated in order to obtain the water
requirements to fit the different RDI treatment levels. The
equipment used for the experiment and the daily
maintenance activities are discussed as well as the fruit
harvesting process. At the end of the experimental period,
olives were tested for their fruit and oil quantities and
qualities. Finally, a statistical analysis was done to
determine the significance of the differences with regard to
olive fruit production and characteristics between the
different treatments.
STUDY AREA
The olive plantation used in this study is located within
the Highlands region of Jordan in the northwestern part of
the country. This particular site is located within the
campus of the Jordan University of Science and
Technology at an elevation of roughly 580 m and at latitude
32°28’36.77” N and longitude 35°58’24.05” E, just to the
east of the city of Irbid (see fig. 1). The olive trees used in
this study are Muhassan olive trees (Olea europaea L.) that
were planted in 1999 and were eight years old at the time of
experimentation in 2007. There are four rows with six trees
per row and 8 m spacing between trees and 8 m spacing
between rows, totaling 24 trees in all and a site area of
roughly 2,240 m2
. The site is characterized by hot summers
and cold winters, with an average winter temperature of
20.5°C and an average summer temperature of 32.6°C, as
recorded in Irbid. The average annual rainfall, again as
recorded in nearby Irbid, is 465 mm. Water used for
irrigation on campus is a mixture of reclaimed wastewater
and rainfall collected in the university lake that has an
annual average electrical conductivity of 1.5 ds/m and a pH
of 8.2.
3. 32(1): 55-62 57
SOIL ANALYSIS
With regard to the soil type, samples were taken at two
depth ranges, 0-30 cm and 30-60 cm, and soil analyses
were run using the Bouyoucos hydrometer method to
determine the soil texture. Results are displayed in table 1
and show that the soil texture for both samples was sandy
clay loam. The water holding capacity (WHC)was
calculated at around 1.5 mm/cm depth of soil based on
Amayreh et al. (2003).
The WHC is then used in the irrigation scheduling as a
guide for adding the appropriate amount of water that the
soil will hold without any deep percolation or loss below
the root zone. The root depth of the olive trees is 1.5 m,
with a maximum storage capacity in the soil of 225 mm.
Therefore, no more than 225 mm of water should be
applied per irrigation event in order to avoid water loss to
deep percolation.
EVAPOTRANSPIRATION ESTIMATE
The evapotranspiration (ETc) is estimated in mm/day. In
order to reach this number, several steps were taken. A
Class A pan was used on-site to determine the daily
evaporation rates (Epan) in mm/day. The pan coefficient
(Kpan) was determined according to the procedures outlined
by the FAO in its guidelines for calculating crop water
requirements (Allen et al., 1998) and was found to be
0.63.With these two pieces of data, the reference
evapotranspiration for the crop (ETo) in mm/day was
calculated by multiplying the Class A pan evaporation
(Epan) by the pan coefficient (Kpan) (eq. 1).
ETo = Epan * Kpan (1)
where
ETo = the reference evapotranspiration (mm/day),
Epan = the pan water evaporation depth in (mm/day), and
Kpan = Class A pan coefficient.
The ETo was then equated with the average daily
consumptive use rate of a mature crop with full canopy
(Ud) in mm/day. To determine the shaded area (Pd), the plot
area for each tree came to 64 m2
(8 m spacing between
trees and rows) and the shaded area per tree at midday was
seen to be 12.5 m2
(the radius of the shaded area under each
tree was 2 m). Hence, the total shaded area percentage was
20%, or 12.5 m2
/64 m2
.Ud is corrected for by the Pd as seen
in equation 2 to calculate the average daily transpiration
rate for a crop under trickle irrigation (Td) (Keller and
Bliesner, 1990).
Td = Ud (0.1 * Pd
0.5
) (2)
where
Td = the average daily transpiration rate for a crop under
trickle irrigation (mm/day),
Figure 1. Map showing the location of Jordan University of Science and Technology (JUST), where the study site is located, within Irbid
Governorate and Jordan at large.
Table 1. Soil analysis results.
Sample
T[a]
(°C)
EC[b]
(ms/cm) pH
Hydrometer
40 s
(solution
density)
Hydrometer
2 h
(solution
density)
T
(°C)
Soil
Texture
0-30 cm 26.5 0.89 8.59 45 31 25
Sandy clay
loam
30-60 cm 26.6 0.76 8.59 47 30 25
Sandy clay
loam
[a]
T=temperature.
[b]
EC=electrical conductivity.
4. 58 APPLIED ENGINEERING IN AGRICULTURE
Ud = the average daily consumptive use rate of a mature
crop with full canopy (mm/day), and
Pd = the percentage of soil surface area shaded by crop
canopies at midday (%).
Finally, the actual amount of water used by a crop (ETc)
was calculated, as seen in equation 3, by multiplying Td by
the crop coefficient (Kc), which was determined through
the above-mentioned FAO guidelines (Allen et al., 1998)
and is 0.65 for olives.
ETc = Td * Kc (3)
where
ETc = the crop water requirement (mm/day),
Td = the average daily transpiration rate for a crop under
trickle irrigation (mm/day), and
Kc = the crop coefficient.
FIELD AND IRRIGATION MANAGEMENT
The field in which the olive trees in this study are
located was plowed using a disk plow. Weeds were
controlled with the application of herbicides two times
during the study period. Regular fertilization, spraying, and
pruning measures were taken during the study period and
care was taken not to affect the experiments.
A drip irrigation system was installed with each tree
fitted with a 220 L/h self-compensated pressure bubbler.
The system included a main valve, a main water meter, a
pressure regulator, pressure gauges, and a 120 mesh in.2
disk filter. Each lateral was equipped with a valve to stop
and/or control water flow at anytime and with water meters
to measure the desired water flow according to the
irrigation schedule. Irrigation events were scheduled twice
per week and irrigation pipes and disk filters were flushed
weekly to avoid clogging of any of the orifices.
The experiment was conducted over one season, as has
been the experimental period in similar studies on olive
trees (Motilva et al., 2000; Tovar et al., 2001; Romero
et al., 2002; Tognetti et al., 2005; Garcia et al., 2013).
There were a total of 43 irrigation events in three phases:
pre-RDI, RDI, and post-RDI. Irrigation of the olive trees in
the pre-RDI phase began on 3 June and all trees continued
to receive the same irrigation quantities until 3 July,
totaling 8 irrigation events in this phase. RDI began
thereafter at the start of the pit hardening stage and
included 23 irrigation events, lasting until 23 September.
The post-RDI phase began subsequently and 12 irrigation
events were conducted during this time period, with all
trees receiving the same water quantities. Four treatments
were applied across the entire plot, with each treatment
being applied randomly and separately on a row of 6 trees
within the plot. The first row acted as the control (T1),
receiving the full crop water requirement of 100% of the
evapotranspiration (ETc) for the duration of the experiment.
The other three rows received treatments that provided the
trees with 75%, 65%, and 50% of the ETc (T2, T3, and T4,
respectively) from 3 July to 23 September.
HARVESTING AND TESTING
The olive fruits were harvested on 18 November and a
random sample of 30 olives from each tree was taken for
physical testing, meaning a sample size of 180 olives per
treatment or a total sample size of 720 for all treatments.
The remaining quantity of olives was sent directly to an
olive press. The fruits were pressed by cold pressing
machines, in which olives are pressed at low temperatures
(between 28°C-32°C) to preserve both the flavor and the
nutritional components of the oil. The total quantity of fruit
and oil produced, the percentage of total weight that was
pressed into oil, the oil acidity, and the peroxide were
measured for each treatment group.
After the harvest, physical tests were also run to
determine the effects of RDI on fruit weight, major and
minor diameter, pit weight, pit-to-fruit ratio, and the
ripeness index. The ripeness index is calculated according
to the method used by the National Institute of Agronomi-
cal Research in Spain, which is based on a subjective
evaluation of the olive skin and pulp colors (Motilva et al.,
2000). Ripeness index values range from 0 to 7. The
procedure consists of distributing a randomly taken sample
of 100 fruits into eight groups: intense green (group N=0),
yellowish green (group N=1), green with reddish spots
(group N=2), reddish brown (group N=3), black with white
flesh (group N=4), black with less than 50% purple flesh
(group N=5), black with 50% or more purple flesh (group
N=6), and black with 100% purple flesh (group N=7). The
index is expressed as (Ni ni)/100, where N is the group
number and the n is the fruit number in that group.
STATISTICAL ANALYSIS
Statistical analyses were run on the above-mentioned
physical parameters (total fruit weight, major diameter,
minor diameter, pit weight, and pit-to-fruit ratio) using the
statistical software package STATA (StataCorp., College
Station, Tex.). Regression analyses using clustered
sampling (based on individual trees) and F-tests were used
to determine if there were overall significant differences
between the means of each parameter among the
treatments. Regression analyses also showed levels of
significance for the differences between the means of the
control treatment and the means of the three RDI level
treatments. Further F-tests were run to check for significant
differences between the three RDI treatments themselves.
The same regressions were run using cluster bootstrapping
procedures in order to obtain a higher level of accuracy
with cluster-robust standard errors and the same levels of
significant were achieved. With regard to the data on fruit
and oil production, data was only available per tree for fruit
production. Regression analyses and F-tests were used on
this data to determine if there were significant differences
between the control treatment and RDI treatment trees for
fruit production.
RESULTS AND DISCUSSION
The water volumes used in the four test treatments T1-
T4 are listed in table 2, with evapotranspiration levels of
100%, 75%, 65% and 50%. These quantities include the
periods before and after RDI during which all treatments
were receiving the same quantities of water. Thus, for
5. 32(1): 55-62 59
example, while T2 was supplied with 75% of the water that
T1 was supplied with during the RDI period, the total water
used in T2 for the entire irrigation period is 85% of the
water used for T1. The water use per hectare is also given
in order to scale-up the water use calculations for the
broader picture in Jordan later (the plot area for each
treatment was 560 m2
, which is 0.056 ha).
Observations were made regarding the fruit characteris-
tics and vegetation growth of the stressed and non-stressed
trees throughout the season. Fruits in T1, undergoing no
stress, and T2 appeared green in color throughout the RDI
period while in the other treatments (T3 and T4) the fruit
began to change color on 16 September during the fruit
development phonological stage 79, according to the
Biologische Bundesanstalt, Bundessortenamt, Chemische
Industrie (BBCH) phonological scale (Sanz-Cortés et al.,
2002). The dimensions of the olive fruits were also
noticeably affected by the level of RDI. Fruits in T4 were
smaller and had more defects while fruits in the other
treatments were larger and had fewer defects. No fruit drop
was recorded in any of the four treatments during the
season.
The results of the measurements at harvest of the olive
fruits and oil, the percentage of oil in the total fresh weight,
the oil acidity and the amount of peroxide are listed in table
3 for all four treatments. The olive fruits production for T1,
T2, T3, and T4 in kg per overall treatment area (560 m2
)
are listed in table 3 and their corresponding production
amounts in kg per ha, respectively, are: 2,985,000,
2,195,000, 2,280,000, and 2,230,000. Similarly, olive oil
production in kg per treatment area is listed in table 3 and
the amounts in kg per ha for T1, T2, T3, and T4 are,
respectively: 453,000, 468,000, 513,000, and 423,000.
From the statistical analyses on fruit production, there are
statistically significant differences between each of the RDI
treatments and the control treatment (T1). The differences
among the RDI treatments with regard to fruit production
were not found to be significant.
It can be seen that the highest oil production was
achieved in T3 at an ETc level of 65% while for fruit
production the highest amount was seen in T1 at an ETc
level of 100%. These results are in agreement with previous
studies that have found that deficit irrigation increases oil
production but reduces fruit production (Motilva et al.,
2000; Garcia et al., 2013). In T1, in which olive trees were
not put under water stress, oil accumulation was delayed
and at the time of pressing, most fruits were not ready in
terms of coloring and ripeness, indicating that these trees
would have needed to be irrigated for a longer period of
time if oil production were the goal. On the other hand, in
T2 and T3, oil acidity was lower and the percentage of oil
was higher, in comparison to T1 and T4, signaling the
appropriateness of water stress during the pit hardening
stage in these treatments for the production of oil. These
results suggest that over-stressing (T4) or preventing the
stress of olive trees (T1) will result in a lower quantity of
oil with higher oil acidity.
The results of the physical tests on the ripeness index,
fruit weight, major and minor diameter, pit weight, and pit-
to-pulp ratio are listed in table 4. The calculated ripeness
indices show that it is lowest for T1 and this supports the
finding that this treatment has the lowest oil productivity.
The pit weight in T1 is also the lowest, again indicating that
the fruits were not in the oil accumulation stage because
most of the flesh was still water and not oil. In T2 and T3,
the highest ripeness indices were recorded, also supporting
the finding that the highest oil yield as a percentage of fresh
weight is found in these treatments. With regard to fruit
dimensions, overall, the fruits in T4 were smaller than in
the other treatments, signaling that this treatment method is
not appropriate for either fruit or oil production.
Statistical analyses revealed that the overall differences
between the parameter means among all four treatments
were significant at a 95% confidence level except for with
regard to the pit weight. The analyses also showed that
some of the specific differences between the control
treatment (T1) and the RDI treatments (T2, T3, and T4)
were significant. These results are shown in table 5 (T1
serves as the baseline to which the other three treatments
are compared in the regressions). In particular, T4 showed
the most statistical difference between itself and T1 on all
parameter measures. Otherwise, only T3 displayed a
statistically significant difference with T1 with regard to pit
weight. As for differences among the RDI treatments, T4
showed statistically significant differences with both T2
and T3 for all parameters except pit weight. T2 and T3 did
not show any statistically significant differences between
each other. All of the results, while not showing that T2
and T3 actually enhanced the physical parameters of olive
Table 2. Total water used by each treatment during the study.
Treatment
Water Use per Treatment
(mm)
Water Use per ha
(m3
/ha)
T1 100% ETc 131 1,305
T2 75%ETc 111 1,111
T3 65%ETc 103 1,030
T4 50%ETc 92 918
Table 3. Total olive fruits, oil production, and
chemical properties per irrigation treatment.
Peroxide
(meq.O2/kg)
Oil
Acidity
(oleic
acid)
Percentage
of Oil
(% fresh
weight)
Olive Oil
Production
(kg)
Olive
Fruits
Production
(kg)Treatment
50.4815.545.3298.5100% ETT1
50.4521.946.8219.575% ETT2
50.4523.151.3228.065% ETT3
50.4819.542.3223.050% ETT4
Table 4. Mean and standard deviations of the physical parameters of the olive fruits for each treatment.
Treatment Ripeness Index Fruit Weight (g) Major Diameter (mm) Minor Diameter (mm) Pit Weight (g) Pit-Pulp Ratio
T1 2.36 2.52 ±0.88 20.27 ±2.35 15.00 ±1.91 0.64 ±0.14 0.28 ±0.12
T2 3.07 2.90 ±1.16 21.47 ±2.28 15.48 ±2.23 0.74 ±0.23 0.27 ±0.08
T3 2.98 2.38 ±0.85 20.20 ±2.57 14.94 ±2.07 0.73 ±0.18 0.32 ±0.07
T4 2.94 1.93 ±0.58 18.97 ±1.88 13.52 ±1.86 0.70 ±0.13 0.38 ±0.07
6. 60 APPLIED ENGINEERING IN AGRICULTURE
fruits, at least point to there being no detrimental effect
from these treatments. On the other hand, the negative
effects in T4 are proven to be of note.
AGRICULTURE WATER DEMAND MANAGEMENT
In scaling-up these results beyond a single on-farm
impact, implementing RDI on all olive trees in Jordan
could have a noteworthy impact on the country’s usage of
its limited water resources. For the purpose of oil
production,T3 at 65% of ETc has proven to be the best
method. Its water usage was calculated to be 1,030 m3
/ha
for the entire irrigation period necessary for olive trees
during the year. This is in comparison to the 1,305 m3
/ha
needed for the irrigation of the olive trees at 100% of ETc.
As noted earlier, irrigated olive trees cover about 15,525 ha
of cultivated land in Jordan. Under 100% of ETc irrigation,
that means that 20,260,125 m3
(20 MCM) of water are
needed annually for irrigated olive trees. In comparison,
under 65% ETc irrigation, only 15,990,750 m3
(16 MCM) of
water are needed annually, representing a reduction in
demanded irrigation water by olive trees of 4,269,375 m3
(4 MCM), a drop of 21%.
Considering that the water used to irrigate olive trees in
the Highlands is all groundwater, this savings would aid in
lowering Jordan’s overdraft of this precious resource. As
has been calculated by Jordan’s Ministry of Water and
Irrigation (2013), the agricultural sector’s overall surface
and groundwater usage comes to 475 MCM per year, 53%
or 250 MCM per year of which is groundwater. The
groundwater that would be saved by using RDI on all areas
with irrigated olive trees in Jordan at the 65% ET level
(4 MCM) would represent a savings of 2%.
CONCLUSIONS AND RECOMMENDATIONS
The results indicate that RDI is an appropriate method of
irrigation for olive trees that does not necessarily have any
negative impact on fruit or oil yield and quality if applied
in the correct amount. The suitable level of RDI for olive
trees depends on the ultimate goal of production, whether
that be for olive fruits or olive oil. For the purpose of oil
production, the highest percentage achieved is at an RDI
level of 65%, as seen in T3, while for fruit production, the
highest level is achieved with no RDI, as seen in T1. The
positive results of RDI seen with the purpose of oil
production is even more advantageous considering that
there is a higher demand for olive oil than fresh olives in
Jordan and olive oil has a longer shelf-life, both factors that
should play favorably into the pockets of farmers.
Moreover, if RDI at a 65% level is used on olive trees, a
21% cut in irrigation water use could be achieved without
negative impacts on fruit and oil production provided that
irrigation scheduling is conducted in the manner described
in this study.
With regard to the overall groundwater savings that
could be achieved in Jordan with the use of RDI on all
irrigated olive trees, while it is minimal in comparison to
the country’s overall water usage, the use of RDI could be
expanded beyond just olive trees. Research suggests
positive results with other irrigated trees as well. For
example, in studies on the impact of deficit irrigation on
apple production, Mpelasoka et al. (2001) found no
negative effect on yield and some improvement in fruit
quality, and Leib et al. (2006) found no impact on the yield
or size with partial root-zone drying irrigation. Research on
grape production (Acevedo-Opazo et al., 2010; Santesteban
et al., 2011) suggests improvements in quality and no large
negative effect on yield from an RDI strategy. With regard
to several studies on the use of deficit irrigation on citrus
fruits, once again there is evidence to suggest no significant
negative effect on yield but quality parameters are sensitive
and the timing and phasing of water stress has to be precise
(García-Tejero et al., 2010a; García-Tejero et al., 2010b;
Ballester et al., 2011; García-Tejero et al., 2011; Ballester
et al., 2013). And finally, studies on tomatoes have signaled
toward the potential for use of deficit irrigation with no
negative yield or quality effects but much depends on the
particular soil characteristics and the timing and phasing of
the water deficits (Mitchell et al., 1991; Savić et al., 2009;
Jensen et al., 2010; Wang et al., 2012). There is thus the
potential for Jordan to explore applying RDI to its other
irrigated tree and even vegetable crops, a strategy that
could give way to yet more water savings.
In considering the use of RDI among farmers in Jordan,
and while the most simple method has been advocated in
this article, there are still concerns with regard to
implementation on the ground. If one is not already in
place, a drip irrigation system has to be bought and
installed and this is a significant up-front investment for
many farmers without the financial means or backing. Drip
irrigation systems also require regular maintenance and
replacement of parts, something that needs constant
attention and awareness of issues that can arise with drip
irrigation systems. It is common that the people working on
the farms on a daily basis are workers who might not be
well-educated or have knowledge of how to attend to RDI
in the manner laid-out in this article so some training would
be required. This is where a type of agricultural extension
services agency would be beneficial to give farmers this
training and also act as a consultant when problems arise.
All of these logistical matters would, in the end, also have
to be prefaced with building trust among farmers that RDI
can reap successful results with olive trees. This study will
aid in these efforts of proving to and encouraging farmers
to undertake this strategy. As the agricultural sector
worldwide is put under further pressure by the higher-
prioritized and more economically valuable municipal and
Table 5. Statistical results of regressions with cluster sampling
testing the difference in means of the physical parameters
of the olive fruits between the RDI treatments with T1
acting as the comparison treatment.
Fruit
Weight[a]
Major
Diameter
Minor
Diameter
Pit
Weight
Pit-Pulp
Ratio
T2
0.38
(0.43)
1.20
(0.87)
0.48
(0.84)
0.10
(0.07)
-0.01
(0.04)
T3
-0.14
(0.32)
-0.07
(0.82)
-0.06
(0.62)
0.09**
(0.04)
0.04
(0.04)
T4
-0.59**
(0.28)
-1.30*
(0.70)
-1.49**
(0.58)
0.06*
(0.04)
0.09**
(0.04)
[a]
* Significant at a 90% confidence level.
** Significant at a 95% confidence level.
7. 32(1): 55-62 61
industrial sectors, there will be a need to find appropriate
ways to achieve greater efficiency in water use for
irrigation. This will require developing, testing, and
implementing a wider range of alternative approaches to
the current methods of irrigation. In this study, adopting
RDI as an alternative approach has proved to be a
promising strategy for minimizing irrigation water demand
in olive orchards while avoiding any undesired effects on
crop production. Achieving a suitable level of RDI in olive
orchards, though, will depend on farmers accepting and
trusting this strategy and continuing studies like the present
one. With the application of RDI, farmers’ profits can be
maintained or even increased and the country at large can
reap water savings, allowing resources to meet the growing
demands elsewhere.
ACKNOWLEDGMENTS
This research was funded by the USAID-funded Water
Demand Management Master’s Program in the Civil
Engineering Department at the Jordan University of
Science and Technology. The program provided students
with the funds for the master’s degree program as well as
their research. This work is in part the outcome of this
opportunity, without which the research would not have
been possible.
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![56 APPLIED ENGINEERING IN AGRICULTURE
While Jordan has witnessed the advent of modern,
pressurized irrigation systems, these networks are not
realizing their full potential and it has become ever-more
necessary to exploit this potential through proper
management techniques and improved irrigation
scheduling. Regulated deficit irrigation (RDI) is one of the
promising management techniques that can be applied to
Jordan’s agricultural sector. RDI, in essence, maximizes
water use efficiency by purposely reducing water
application during certain stages of growth that are less
sensitive to water stress. In these stages, water is supplied
at a rate lower than the evapotranspiration rate but still at a
rate high enough to avoid any negative impacts on crop
yield or quality (Fereres et al., 2003; Kijne, 2003; Velez
et al., 2007). As a result, water is saved and crops are
maintained.
Several studies have already been carried out to
investigate the impact of RDI on crop yields in countries
with arid environments such as Australia, Spain, Turkey,
Morocco, the United States, and China [Food and
Agriculture Organization (FAO), 2002], with some also
focusing on olive trees. Dabbou et al. (2010), in their
experiment on olive trees in Tunisia, found that the best oil
and fresh fruit production occurred with a 75% RDI
scheme. Ramos and Santos (2010) likewise found the
highest oil and fruit yields occurring with a 60% sustained-
deficit irrigation regime within their olive tree test case in
Portugal. And the olive trees tested by García et al. (2013)
in Spain with RDI treatments saw higher oil yields and no
difference in quality as compared to control cases. On the
other hand, Gómez-del-Campo (2013) reports that while
the control treatment with no RDI on olive trees in Spain
still had the highest oil content, one of the RDI treatments
had an oil content very close to the control and just as
importantly reaped significant water savings, unlike the
control group.
With regard to the particular timing of RDI application,
it has been found that the second phase of fruit develop-
ment in the olive tree, when pit hardening occurs, is the
most resistant to water deficit (Goldhammer, 1999), while
the blooming growth stage is extremely sensitive to water
stress (Moriana et al., 2003). The third phase of olive fruit
development, oil accumulation, while difficult to pinpoint,
is also said to be sensitive to water stress (Lavee and
Wodner, 1991). Tognetti et al. (2005) have recommended
that RDI be applied to olive trees after the pit hardening
stage to cover 66% of the crop evapotranspiration. It has
been seen elsewhere that olive trees show only a slight
reduction in fruit production and oil yield under severe
water stress during this stage of growth (Goldhammer
et al., 1994; Moriana et al., 2003).
While these studies reveal useful information with
regard to olive trees and deficit irrigation, the present study
seeks to add to this body of knowledge in two ways. First,
the specific suitability of RDI for olive trees within
Jordan’s unique and local climatic conditions is newly
examined. In particular, the study is conducted in the
Highlands region, which uses limited groundwater
resources as the main source for irrigation and also suffers
from high soil salinity. Farmers have shifted to olive trees
and away from other crops in past years due to their more
drought-resistant and saline-tolerant nature (Chebaane
et al., 2004). But the sustainability of olive tree cultivation
will still depend on farmers’ abilities to use less water and
this study’s support of RDI for olive trees could be the
solution. The second way in which this study seeks to
contribute is to offer a more straightforward and less
technically-sophisticated variety of RDI that can be easily
adopted by farmers, many of whom do not have the
capacity to engage in more complex schemes and do not
benefit from strong agricultural extension services from the
government. Ultimately, the objective of this research is to
provide farmers with the means to apply RDI within their
daily work, not only to maintain the quantity and quality of
their olive yield but also to aid the nation in decreasing its
water demands in the agricultural sector.
METHODOLOGY
The methodology of this study includes several steps.
After determining the general climatic conditions of the
study location, a soil analysis was conducted to ascertain
the soil texture and water holding capacity that would
eventually guide the irrigation scheduling. Evapotranspira-
tion was then estimated in order to obtain the water
requirements to fit the different RDI treatment levels. The
equipment used for the experiment and the daily
maintenance activities are discussed as well as the fruit
harvesting process. At the end of the experimental period,
olives were tested for their fruit and oil quantities and
qualities. Finally, a statistical analysis was done to
determine the significance of the differences with regard to
olive fruit production and characteristics between the
different treatments.
STUDY AREA
The olive plantation used in this study is located within
the Highlands region of Jordan in the northwestern part of
the country. This particular site is located within the
campus of the Jordan University of Science and
Technology at an elevation of roughly 580 m and at latitude
32°28’36.77” N and longitude 35°58’24.05” E, just to the
east of the city of Irbid (see fig. 1). The olive trees used in
this study are Muhassan olive trees (Olea europaea L.) that
were planted in 1999 and were eight years old at the time of
experimentation in 2007. There are four rows with six trees
per row and 8 m spacing between trees and 8 m spacing
between rows, totaling 24 trees in all and a site area of
roughly 2,240 m2
. The site is characterized by hot summers
and cold winters, with an average winter temperature of
20.5°C and an average summer temperature of 32.6°C, as
recorded in Irbid. The average annual rainfall, again as
recorded in nearby Irbid, is 465 mm. Water used for
irrigation on campus is a mixture of reclaimed wastewater
and rainfall collected in the university lake that has an
annual average electrical conductivity of 1.5 ds/m and a pH
of 8.2.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)