The results of the multifactor analyses of variance are discussed in the following sections.
Influence of the Irrigation Method and Amount of Irrigation Water on Olive Yield
Table
3 shows the multifactor analysis of variance results obtained for olive yield and Fig.
5 depicts the least-squares means for olive yields and the 95% confidence intervals obtained for the three years of study for both irrigation methods and irrigation water applied.
The multifactor analysis of variance decomposes the variability of the dependent variable into contributions due to various factors. The contribution of each factor is measured having removed the effects of all other factors. The statistical analysis did not show any significant interaction between the factors of irrigation system and water amount for any year at the 5% significance level () as the differences in yield between the irrigation methods were almost the same for any irrigation water amount applied.
The irrigation water amount had a statistically significant effect on olive yield at the 5% significance level for the first two years of experiment. However, in the third year of study, the influence was not statistically significant. This result can be explained considering that rainfall was higher than expected during that year, especially in the winter season. For this reason, the actual irrigation cutoffs were smaller than those previously planned. Furthermore, the olive yields in that year were especially low due to the severe pruning and alternate bearing of the olives trees.
Based on the multiple comparisons of means analysis, two homogeneous groups have been identified (Fig.
5). There were no statistically significant differences between the control treatment (100% NIR) and the 80% NIR treatment in any year, but differences were always significant for the 60% NIR treatment except in the last year of study due to the previously mentioned reasons.
The olive yields in the first two years of study were similar for the control and 80% NIR treatments (20.4 and in the first year and 25.7 kg for both treatments in the second year). A clear olive yield reduction was observed for the 60% NIR in these first two years of study (16 and , respectively), which corresponded to 21% and 11% reductions.
Regarding the irrigation method, the olive yield with the SDI system was always greater than the one obtained using the traditional DI system for any year and irrigation water applied. The differences between these two irrigation methods were 2, 3.24, and for the three years of study (10, 12, and 8.6%, respectively) and they were statistically significant for the second year of study.
Influence of the Irrigation Method and Amount of Water on Olive-Oil Yield
The results obtained in this experiment for the olive-oil yield were similar to the ones obtained for the olive yield. The irrigation water depth applied was statistically significant for the first two years of study but not for the third.
Again, two homogeneous groups can be observed. Oil yield was very similar for the control and 80% NR treatments (approximately 5.9 and in the first and second year, respectively), while the oil yield dropped to 4.6 and in the more restrictive treatment (reductions of 22 and 11%, respectively).
In the third year the oil yield reduction for the 60% treatment was 18% with respect to the control treatment (), although in that year the oil yield in the 80% treatment was abnormally low.
Regarding the irrigation method, the SDI system produced greater olive-oil yield every year, although the differences were not statistically significant. The increases in oil yield with respect to the SDI were 12, 6, and 7% for the three years of study, respectively. The increases in oil yield were always lower than the increases in olive yield. This is due to the inverse relationship between the olive yield and the oil content observed in this research and in previous works (
Lavee and Wodner 2004). Several factors contribute to this inverse relationship. Among them, the maturity of the fruits, which is usually more advanced for low yields, and the mesocarp/endocarp (pulp/pit) ratio, which is also greater for low yields due to the greater size of the fruits.
The results of this experiment show that the use of the SDI system with a uniform cutoff of 20% of the calculated NIR seems to be a good strategy to save water and achieve an appropriate productive level.
Influence of the Irrigation Method and Water Amount of Water on the Irrigation Water Use Efficiency
Table
4 shows the result of the multifactor analysis of variance obtained for
, and Fig.
6 depicts the mean values of the
and the 95% confidence intervals as a function of the irrigation method and irrigation water amount for the three years of study.
The results of the analysis show that the irrigation water amount was a statistically significant variable in the first two years but not in the third year, although there were clear differences. There is an inverse relationship between the irrigation water amount and the irrigation water use efficiency.
The maximum olive productivity ratio was obtained in the second year (7.3 kg of
). The values of this index were lower in the first and third year (4.2 and
, respectively). This was due to the higher yield in the second year and the higher precipitation that diminished the irrigation needs. Similar
values can be derived from the data obtained by Moriana et al. (
2003) in an experimental essay carried out in a
mature olive orchard in the province of Córdoba, Spain. From their biennial yields and irrigation water applied data,
values can be calculated for two consecutive biennial periods (1997–1998 and 1998–1999).
values ranging from 2.4 kg of
(for a 100%
irrigation treatment) to 7.6 kg of
(for a 75%
irrigation treatment) can be obtained for the first biennial period. However, it must be taken into account that precipitation was much higher than that occurred in the experiment presented in this paper.
were significantly lower for the second biennial period. Lower average
values have been reported for low-density olive orchards in Portugal (
Ramos and Santos 2010).
The irrigation method also had a relevant influence on the irrigation water use. This influence was only statistically significant in the second year. The efficiency of the irrigation water use was higher with the SDI system than with the DI system, especially in the second year. In that year the productivity values were in the SDI system versus in the DI system. This corresponds to a percentage increase in productivity of 13.1% with the SDI system with respect to the DI system. No significant interactions were found between the irrigation system and the amount of water applied.
The inverse value of the water use efficiency represents the amount of water applied per unit of product yield. The calculated values of this variable indicate that 0.247, 0.146, and of water were needed to produce one kilogram of olives in the three years of study respectively, with the DI system, while only 0.226, 0.129, and of water were used with the SDI system. This means that water savings of (8.5%), (11.6%), and (8%) were achieved with the new SDI system. These water savings were even larger for the full irrigation (100% NIR) treatment in which the water savings reached approximately 20%.
Similar results were found when the was considered. The most significant factor affecting the irrigation water use efficiency was the amount of irrigation water. A statistically significant relationship was found in the first two years of study.
The irrigation method also has a relevant influence on the irrigation water use (), although this relationship was not statistically significant. The SDI system was more efficient in the irrigation water use than the DI system for any irrigation water amount or year of study.