Rainfall Distribution Functions for Irrigation Scheduling: Calculation Procedures Following Site of Olive ( Olea europaea L.) Cultivation and Growing Periods

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, Beja, 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 (Beja 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 Beja 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 Beja 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, Beja, 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.