In Tunisia (36.5oN, 10.2oE, Alt.10
m), rainfall is
the major factor govering olive production. Annual and seasonal falls are
variable following years and regions, making yields of olive trees
fluctuating consistently. Irrigation
was applied since the 70th in the intensive olive orchards to
improve and stabilize olive production. This study aimed to determine the crop
water needs of olive orchards and the rainfall frequencies at which they are
covered following age and site of olive production. For this purpose, the
rainfall distribution functions were 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 methods. Their performance against the
PM-ET0 (Penman-Monteith) estimates was
evaluated graphically and statistically for a better adaptation them to the
existing environmental conditions, particularly when data
are missing to compute ET0-PM. Results show that ET0 estimates strongly correlate with ET0-PM with r values of up to
0.88. Particularly, the methods of Turc and Ivanov appropriately predict the ET0-PM
in all climatic regions of Tunisia, constituing an appropriate alternative for
determining ET0 when data are missing to compute ET0-PM.
However, although the Turc method performs well with all climatic zones of
Tunisia, the Ivanov method appears to be more appropriate to the northern areas
(Béja and Bizerte), though a poorer agreement was found when using the Eagleman
method. Estimates of ET0 by using the Hargreave-Samani (HS) formula
for the east-southern area (Gabes, arid climate) show satisfactory agreement
with ET0-PM estimates. It appears also that at a given site, the
most appropriate method for ET0 estimation at annual scale may be
different from that giving the best value of ET0 when considering
the growing stages of the olive tree, for example, the method of Turc, although
it was appropriate when estimating the annual ET0 value for Sousse,
it wasn’t adequate at seasonal scale. In opposite, although the method of BC is
suitable for stages 1, 2, 4 and 5 at Sousse, the appropriate method for the
overall cycle is that of Turc. This indicates that there is no weather-based
evapotranspiration equation that can be expected to predict evapotranspiration
perfectly under every climatic situation due to simplification in formulation
and errors in data measurement. However, we can say that when data are missing,
ET0 can be estimated with a specific formula; that of Turc can be
appropriately used for Tunis, Sidi Bouzid, Sousse and Béja at annual scale
despite of their appartenance to different climatic regions, while the method
of Ivanov is quite valuable for Bizerte and Nabeul. Results show also that
values of P-ETc recorded during the irrigation period are negative even for
young plantations, with lowest and highest deficits observed at Béja and Gabes
cities, respectively. The driest
period is that of July-August for all sites with F values exceeding 0.9 in most
cases. Only 10% of water needs are supplied by rainfall during this period of
fruit development. Therefore, irrigation is needed all time for adult trees
even at the rainiest locations. For young plantations, irrigation becomes
necessary beginning from the second period of tree development, i.e. April-June
for Bizerte, Béja, Nabeul and Tunis and since the early spring period for both
young and old plants for Gabes and Sidi-Bouzid. It appears from this analyze
based on the seasonal rainfall frequencies and water needs computed with the
PM-method, that there is a need for irrigating olive plantations aging more than 5
years in most case studies and especially when olive is cultivated in the
western areas of Tunisia. Results indicate also that the use of no adequate
method to estimate ET0 allowed overestimating or underestimating of
irrigation water needs. So it is desirable to have for Tunisia a method that
estimates ET consistently well and future research is needed to reconcile which
should be the standard method of calculating the change in the crop coefficient
over time. However, despite a quite good performance of the PM-equation in most
applications, particularly when it is used for irrigation scheduling purposes,
some problems may appear because of lack of local information on Kc-values and
determination of the effective rainfall. Additional research is needed on
developing crop coefficients that use the Penman-Monteith equation when
calculating ET. In conclusion we can say that on the basis
of the results produced, we can decide for each region and growing period if
complementary irrigation is needed or not. Indicative amounts are given for
each case study.