INTRODUCTION
Yearly total rainfall received on the surface in Iran is about 400 Billion
Cubic Meters (BCM), out of which 280 BCM is lost through evapotranspiration
and the remaining 120 BCM either runs on the surface or infiltrates to raise
soil moisture or joins to groundwater. A third of the total surface water of
the country (about 34 BCM) flows through rivers passing the vast Khuzestan plain.
The soil and climate of Khuzestan is suitable for diverse agronomic and horticultural
farming throughout the year, KWPA (2003). One of the major
crops in this province is date palm. Although this crop is adapted to dry conditions;
higher yields can be obtained with irrigation. With about 30 million palm trees
covering a total area of 216 thousand ha, the total date production in Iran
is about 900 thousand tons. In spite of the importance of this crop, so far
little is known about the potential of increasing of yield for this crop through
modern irrigation methods, To determine the proper irrigation depth and interval
for Estamaran date palm trees grown on Omidiyeh experimental field, this field
research experiment is designed. Date palms are more able to adapt to scarce
of water supply. Furr and Armstrong (1955) reported
that prolonged periods of severe water shortage during the growing season had
adverse effects on the growth of leaves and on size, grade and yield of fruits.
Hussein and Hussein (1982) suggested that an irrigation
interval of 4 weeks applying 71 mm per irrigation was the most suitable in date
palms. Hilal et al. (1986) showed that the low
frequency periods and high volume of water per irrigation were more favorable
for date palms growth. Comparing drip versus bubbler irrigation was more favorable
for date palms growth. Comparing drip versus bubbler irrigation for 5 years
on mature Deglet Noor date palms, Reuveni (1975) found
that drip was superior to bubbler irrigation in term of total yield and other
growth parameters. He related this effect to higher water availability with
drip compared to bubbler. In the same experiment as reported here, Bacha
et al. (1998) showed that the average yield was higher in trickle
irrigation system than in other two systems (basin and bubbler). Also, Al-Amoud
et al. (2000) showed that trickle irrigation system gave the best
water use efficiency followed by basin and then bubbler and that an annual water
volume of 100 m3 per tree have produced the highest water use efficiency.
The date palms are considered highest salt tolerant fruit crop and expected
to give its 100% yield potential at soil electrical conductivity (saturation
extract, ECe) value of 4.0 dS m-1 and at irrigation water electrical
conductivity (ECw) value of 2.7 dS m-1 and still can give 50% of
its yield potential at ECe value of 18 dS m-1 and ECw value of 12
dS m-1, (Ayers and Westcot, 1985). Hassan
and El-Azayem (1990) tested 11 fruit species for salinity tolerance and
found that date palms were the most salinity tolerant of all. Salt tolerance
of date palms varies with cultivars. For example, Akhlas cultivar has lower
soil salinity tolerance compared to Ruzaiz at Al-Hassa oasis of Saudi Arabia
by Abderrahman and Abdelhadi (1990). However, Nimah
(1985) stated that salt accumulation was higher in the surface layer of
the soil for trickle irrigation system compared to bubbler. Date palms are more
able to adapt to scarce of water supply.
MATERIALS AND METHODS
This investigation is part of large study on the combined effects of
irrigation systems and water regimes on date palms in an oat-stubble field
at the research farmland, located southeast Khuzestan Province of Iran
at 49° 42’ 30” E and 30° 50’ N, during the period
of July (2006) through November (2008). The experiments were designed
as split plot method based on Complete Randomized Blocks (CRB) by 12 treatments
and 3 replications (total number of 36 trees). The main variable factor
was irrigation frequency period, taken at 3 levels; daily (A1), two days
(A2) and three days (A3) intervals. The secondary variable factor was
the method of estimating of depth of applied irrigation based on Penman
Monteith method 100% (B1), Penman Monteith 70% (B2) Class A Pan 100% (B3)
and Class A Pan 70% (B4). Irrigation water was carried through a 90 mm
sub main, feeding a number of 32 mm manifolds. Laterals took water from
these manifolds and delivered water to the bubblers besides palm trees
grown at a distance of 8×8 m.
The soil of the experimental site was sandy loam with pH of about 8 and
ECe of 1.8 dS m-1. Final intake rate of soil was 6-8 mm h-1
which improved by the addition of organic matter to basins.
Preparations before starting irrigation included:
| • |
Separating the selected trees |
| • |
Separating female pods (in order to restriction of the time of the
experiment and the need for having significant vegetative growth results,
in the mentioned time, female pods were pruned. Consequently the present
study has not any yield data whereas vegetative growth was the most
important factor to focus) |
| • |
Weeding |
| • |
Pruning and cutting |
| • |
Maturing with fertilizer application |
| • |
Marking and tagging leaflets and thorns for growth measurement |
| • |
Calibration of bubblers for a fixed discharge of 222 L h-1 with an inlet pressure of 2.5 bars |
Table 1 shows the average of water qualitative analysis
of the irrigation, from July 2006 through November 2008.
Irrigation water requirement of palm trees was calculated using Penman
Monteith method and potential evapotranspiration of reference crop (ET0),
estimated with Pan Evaporation record, as followed:
where, Kp is pan coefficient and Epan is pan evaporation (mm day-1).
ET0 was also estimated by Penman Monteith method, using computer
software (Cropwat 7.0, FAO’s FTP-server). A crop coefficient (Kc)
of 0.9 was used for date palm. Effective rainfall was estimated using
80% probable mean monthly rainfall.
Pe = [1.252496×Pm(0.82416)-2.93522]×10(0.00095512×ETc) |
where, Pe is effective rainfall (mm month-1), Pm is mean rainfall
(mm month-1) and ETc is crop evapotranspiration.
Considering that in drip irrigation, soil surface evaporation losses are minimal
and almost all the water consumed is lost by transpiration, calculation of mean
daily transpiration of date palm is given by American Soil
Conservation Service (1992):
| Table 1: |
Average of water qualitative analysis |
 |
| Table 2: |
Average of water requirement of treatments |
 |
| Table 3: |
Average of the net depth of irrigation (mm) for all
treatments for peak month’s data |
 |
| Table 4: |
Average of gross depth of irrigation (mm) for all treatments
for peak month’s data |
 |
Td = Ud [Ps/100+0.15 (1-Ps/100)] |
where, Td is mean maximum daily transpiration (corrected with 80% effective
rainfall), Ud is mean maximum daily consumptive use and Ps is percent
of plant coverage (Ps = 7.7%).
Table 2 shows that during Nov.-Dec. and Dec.-Jan.,
80% probable monthly effective rainfall exceeds mean maximum monthly transpiration
for date palm; therefore there was no irrigation for this period.
The net depth of applied irrigation (In) is derived as:
where, Fi is frequency period of irrigation.
Table 3 shows the average of the net depth of irrigation
(mm) for all treatments for peak month’s data (July- Nov) (2006-2008).
Because all variables (net depth, gross depth, etc.) were maximum in July
through November (2006-2008), in order to abbreviation, putting data of
other months in the tables are ignored. So data of July through November
(2006-2008) were presented as sample.
Gross depth of irrigation (Ig) is estimated by:
where, Tr is transpiration ratio during peak period (in this plan estimated
1.15) and Ea is irrigation efficiency, beside Ea is given as per:
where, Eu is uniformity of dripping (in this plan estimated 90%) and
Et is crop water use efficiency (in this plan estimated 1).
| Table 5: |
Average of daily gross water requirement for all treatments
for peak month’s data |
 |
Table 4 shows the average of gross depth of irrigation
(mm) for all treatments for peak month’s data (July- Nov.) (2006-2008).
Gross daily requirement of a crop in liters per day, estimated by the
following equation:
where, Sp is distance between trees in each row and Sr is distance between
rows of trees.
Time of water application, Ta (h), is calculated by the following equation:
where, Np is number of bubblers surrounding a tree and qa is bubbler
discharge (l/h).
Table 5 shows the average of daily gross water requirement
for all treatments for peak month’s data (July- Nov.) (2006-2008).
RESULTS AND DISCUSSION
According to the results of the variance analysis for all the treatments
in crop characteristics including number of leaves, thorns and length
of leaflets, in about two years period of experiment, it was determined
that A1B2 treatment (Penman Monteith 70% and daily irrigation), at index
of number of leaves, has significant difference at 5% level and recommended
as the best treatment in this plan. Data in Table 6
are the number of leaves for all the treatments before start of study.
Data in Table 7 are the number of leaves for all the
treatments in end of study.
From the point of view of water saving, recommendation of A1B2 treatment
is the most appropriate one, because it can leads to saving about 7344
m3 water per hectare per month (only for period of July through
November (2008) compared to uncontrolled bubbler irrigation by local farmers,
in the part of experimental farm of Shahid Rajaee, that had 3.87 hectare
area, 610 date palm trees and bubblers with discharge of about 750 L h-1
and daily irrigation interval at July-Sep, two days at Sep-Oct and three
days at Oct-Nov. (Table 8).
| Table 6: |
No. of leaves measured for any treatments |
 |
In a series of research conducted by the Work Group of Agricultural Products
of Iran (WGAPI), Penman Monteith method was compared with FAO-24 (Corrected
Penman, Radiation and Blany Criddle method) for many weather stations (IRNCID,
2002). Penman-Monteith was selected as the most appropriate method. Therefore,
it concludes that ET0 by Class A Pan method can be used more advantageously
in Omidiyeh which is a dry region. It can be concluded, for Estamaran date palm
in Omidiyeh region, if enough water was available, treatment A1B2 is the most
appropriated treatment, but if less water was available, treatments using Class
A Pan method are more appropriate also Comparison of reference potential evapotranspiration
by Penman Monteith and Class A Pan in the Omidiyeh for peak months is shown
in Table 9.
| Table 7: |
No. of leaves measured for any treatments |
 |
| Table 8: |
Comparison of the water requirement of A1B2 treatment
with uncontrolled bubbler irrigation in the part of experimental farm |
 |
| Table 9: |
Comparison of potential evapotranspiration for peak
month’s data |
 |
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