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

Abstract

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 that the HS method performs well in most climatic regions, with the exception of humid areas where it tends to overestimate ET0. 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. The formula of Turc, although it was appropriate when estimating the annual ET0 value for Sousse, it wasn’t adequate at seasonal scale. Adversely, 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. Results also show that the average annual value of ET0-PM calculated by using full datasets from January to December is well correlated to the maximum and minimum daily estimates with r values ranging between 0.70 and 0.83. These variations indicate 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. Nevertheless we can say that when data are missing, ET0 can be estimated with a specific formula, suggesting that of Turc 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. Determination of the crop evapotranspiration (ETc) on the basis of ET0-PM computations following age and the growing periods show positive values of (ETc-P) at annual scale for Tunis, Nabeul, Sousse Bizerte and Beja when young olive plantations are considered, but for old trees, values are positive only for Tunis, Bizerte and Béja. Seasonal differences between ETc and P (rainfall) recorded during the irrigation period are negative even for young plantations. The lowest and highest deficits are observed at Béja and Gabes cities, respectively. The rainiest periods are December-February for Béja, Tunis and Bizerte and September-November for the other sites with similar trends for rainfall frequency (F). 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 even for one year old orchards. 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 since the early spring period for both young and old plants for Gabes and Sidi-Bouzid. It appears from this analysis based on the seasonal rainfall frequencies and water needs computed with the PM-formula, 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. Irrigation is needed during the growing fruit period but also during the other seasons, when shoots grow. Results also indicate that the use of no adequate method to estimate ET0 allowed overestimating or underestimating of water requirements. So it is desirable to have 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 values and determination of the effective rainfall. 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. Also, it appears to our best knowledge that this work with that of Nasr (2002) [1] is the only one that has estimated ET0 for the whole country using PM-ET0 compared to other empirical methods, but we didn’t test the possible advantages in using calibrated values for the radiation adjustment coefficient or temperature adjustment for dew point temperature estimation as proposed by Allen (1996) [2]. This calibration is therefore a line to be explored. Additional research is needed on developing crop coefficients that use the Penman-Monteith equation when calculating ET and a standardized method of calculating the time base for the crop coefficients preferably based on a growing degree day concept.