In Tunisia (36.5˚N, 10.2˚E, Alt. 10 m), rainfall is the major factor governing olive production. It is characterized by large variability in time and space, making yields of olive trees highly dependent on the amount of water received and timing. Thus, improvement of olive productivity by irrigation is necessary. This study aimed to determine the crop water needs of olive orchards and the rainfall frequencies at which they are covered following age and sites of olive production. For this purpose, the rainfall distribution functions are established for different cities of Tunisia (Tunis, Bizerte, Béja, Nabeul, Sidi Bouzid, Gabes and Sousse). For all sites and growing periods, the reference evapotranspiration (ET0) was computed by using several formulas. Their performance against the Penman Monteith (PM) method was evaluated graphically and statistically in all considered cities in order to evaluate their accuracy for better adapting them to the existing environmental conditions, particularly when data are missing to compute ET0-PM. Results presented herein show that the estimated ET0 values strongly correlate with ET0-PM at all sites and formulas with r values up to 0.88. Particularly, the methods of Turc and Ivanov appropriately predict the ET0-PM in all climatic regions of Tunisia and may constitute an appropriate alternative for ET0 estimation when data are missing to compute ET0-PM. However, although the Turc method performs well with all climatic zones, arid and semi-arid, in western, northern and coastal areas of Tunisia, the Ivanov method appears to be more appropriate to the northern areas (Béja and Bizerte) characterized by semi arid climate and having annual rainfall of up to 450 mm, though a poorer agreement was found when using the Eagleman formula. Estimates of ET0 by using the Hargreave-Samani (HS) formula for the east-southern area (Gabes) characterised by arid climate show satisfactory agreement with ET0-PM estimates, corroborating previous findings reporting

The study was carried out in north Tunisia (36.5˚N, 10.2˚E) in order to get a comprehensive view of the growth dynamic of young olive orchards (Olea europaea L.,). The experiment involved irrigated trees of cultivars Chétoui, Manzanille, Meski and Picholine, planted at 6 × 6 m 2 spacing. Tree height, shoot length and canopy, fruit and trunk diameters were monitored regularly after plantation on 12 trees per variety. Root development was analyzed on Chétoui trees, only. Growth patterns were established annually for each variety before proposing an average model for each growth parameter. Results showed that tree height, shoot length and trunk diameter grew following an S-shaped curve with maximum annual increases occurring on the 4 th year for tree height and a year later for canopy. The minimum gain coincided with the highest fruit load year, indicating that competition for assimilates concerns also young trees. Average growth patterns for tree height and shoots showed sustained rates all over the growing season with seven distinct periods of growth. Rapid growth occurred in April, July, and September, with similar growth trends observed for productive and less productive cultivars. However, the studied varieties behaved differently. Picholine cv., provided the most important increases and was the best water user. Results also showed that most roots were confined to the top soil layers and developed nearby the trunks. High root densities and important water depletion were observed in this area and thus, water and fertilizers should be supplied for young trees at these depths and distances from trunks. Root and canopy development were highly correlated (r = 0.94) and interfered with fruit growth. When trees set their first productions, the root-canopy ratio approximated the unit. An optimum ratio between root length and leaf area was found (2.3 km·m −2 ) for the 6-year-old tree, indicating good equilibrium between the above and the underground parts. On the basis of these results, a mathematical model was developed allowing a precise estimation of water requirements of olive trees during a period, where ground cover rarely exceeds 30%. We can conclude that all these models, graphic and mathematic give precise information on the occurrence of the various phenophases of young olive trees and may be used for a quantitative appraisal of the performance of olive varieties under a given environment. However, some aspects would be probed deeper and particularly the influence of climatic data on growth dynamic.

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