تبدیل آبیاری سطحی به سامانه‌های آبیاری تحت فشار و اثر آن بر عملکرد درختان مرکبات (مطالعه موردی: منطقه شمال خوزستان)

Introduction: North of the Khouzestan is one of the most important citrus production center. Usually border irrigation is used to irrigate citrus in this area. This system has generally low application efficiency. Several investigations in other arid region have demonstrated in addition to improved irrigation efficiency with low-volume pressurized irrigation systems, citrus trees have adapted with these new irrigation systems. However limited information exists on the performance of mature orchards converted from border surface irrigation to pressurized irrigation systems. Therefore, the current research was conducted to evaluate the feasibility of converting surface irrigation to pressurized irrigation systems on mature citrus trees in climate conditions of North Khouzestan.
Materials and Methods: This study was conducted during three years at Safiabad Agricultural Research Center to evaluate the yield of citrus trees and the quality of fruits for two Marss and Valencia varieties which grow 7 years previously with surface irrigation and converted to pressurized irrigation systems. The treatments consisted of six irrigation methods including Overhead sprinkle irrigation (OHSI), Under tree sprinkle irrigation(UTSI), Trickle irrigation(TI)(six 8 L/h Netafim emitters), Microjet irrigation (MI)(two 180 microjet were located under canopy near of the trunk at opposite sides of trunk),Bubbler irrigation(BI)(a single located under the canopy of each tree)andSurface irrigation(SI) method.Soil texture was clay loam well drained without salinity(ECe=0.69ds m-1), with 1.25 percent organic carbon. The experimental design was completely randomized design. The trees were irrigated during spring and summer seasons. For calculating irrigation water depth in TI, MI and BI systems, daily evaporation from a class A evaporation pan of the Safiabad weather station (nearby the experimental field) was collected, and evapotranspiration of the citrus trees was calculated applying a pan coefficient of 0.8. During the growth season, soil moisture content was measured before irrigation in root zone depth using weighing method at two points of the beginning and the end of the garden to obtain an average showing changes of the field moisture content. Applied water were measured with flow meter for OHSI, UTSI,TI, MI and BI methods and WSC flume for SI treatment. In middle January after fruit ripening, fruit yield was determined by harvesting all the fruits from six trees located in the center of each plot. Weight of fruits from every tree was recorded. Then, 3kilogram fruits per tree were randomly separated and peel thickness, diameter, weight, juice solid percent, total dissolved solids(TSS) and Citric acid were measured.
Results Discussion: The annual precipitation was 385,345, and 336 mm for 2004, 2005, and 2006 years, respectively. The mean temperature of June, July and August (the warmest months) for 2004, 2005, and 2006 was 45.6, 45.2 and 45.8°C. Higher temperature in third year caused to increase heat stress, so fruit yield decreased. Irrigation water consumption in OHSI and UTSI were among 15000 to 17000 m3ha-1. Continues contact of irrigation water contacting with leaves in OHSI causes the accumulation of salts on the leaf surface and leaf drop in harvest season. Consumed water in BI, MI and TI compared with SI method reduced by as much as 48.6%, 57.2%, and 58.4%, respectively. Because soil wetted area in BI, MI and TI methods were low and about 30 to 50 percent of soil area.
There were significant differences in citrus yield, water use efficiency (WUE) and quality in 1% and 5%, so that comparison of means in Mars variety showed that the yield of trees in TI and SI methods were significantly higher than UTSI method. On the other hand, fruit yield was similar in OHSI, MI, TI and SI methods. Valencia variety fruit yield was similar for in BI, MI, TI and SI methods in all 3 years, and significantly more than OHSI and UTSI although BI, MI, TI used only 48% to 58% of irrigation water compared with SI method. WUE under BI, MI and TI methods was enhanced by 2 to 3 times more than SI,OHSI and UTSI methods because consumed water decreased in BI, MI and TI about 50%. Fruit size and fruit weight of Marss variety in the OHSI and fruit size and fruit weight of Valencia variety in the OHSI, MI and SI were better than other systems and had a significant difference in 1% probability.
Conclusion: Overall results of this study indicated that it is possible to convert SI to BI, MI and TI methods in northern khouzestan orchards without decreasing in fruit yield and quality of citrus trees. Salt accumulation on leaf surface in OHSI method was caused to drop leaves in harvest season.