Water requirements of evergreen citrus orchards in semi-arid regions of the world are mostly met through irrigation. Definitive knowledge of crop water use is the fundamental basis for sound water management under these production systems. The objectives of this study were two-fold: i) to measure long-term transpiration of well managed micro-irrigated citrus orchards and; ii) to test physically–based models that can be used to predict transpiration across different citrus growing regions. Whole tree transpiration measurements were conducted using the heat ratio method (HRM) sap flow technique in two well-irrigated citrus orchards [Citrus sinensis (L.) Osbeck] in the summer rainfall area of South Africa. The two orchards planted with 'Delta' Valencia and 'Bahianinha' Navel orange trees were located at Moosrivier Farm in Groblersdal. A data set from an orchard planted with 'Rustenburg' Navel orange trees that was located in the winter rainfall area was sourced for model validation. The orchard was located at Patrysberg Farm in the winter rainfall area. All the orchards were drip irrigated and managed according to the industry standards. Weather parameters were monitored using automatic weather stations that were situated close to the fields, for modelling purposes.
The HRM was initially calibrated against eddy covariance measurements in winter (31July to 3 August 2008) and autumn (21 to 25 May 2009) in the 'Delta' Valencia orchard, when soil evaporation was considered negligible. Calibration was done by adjusting the wounding width to ensure that average transpiration of sample trees matched crop evapotranspiration (ETc) measured above the orchard using the eddy covariance method. The wounding width, termed the 'virtual wound width', was obtained by inputting ETc into regression equations of daily transpiration against wound width. Xylem anatomical assessments were conducted on excised stem samples prior to probe insertion and at the conclusion of sap flow measurements in in order to ascertain and explain the virtual wound width determined through calibration. The average virtual wound width that matched average transpiration of sample trees to ETc in both winter and autumn was 3.2 mm. The similarity of the virtual wound width obtained during winter and autumn suggests that wounding did not increase with time and a single field calibration of the HRM using eddy covariance measurements is sufficient for measuring long term transpiration in citrus orchards.
The HRM was used to measure transpiration for periods of 364 days in the 'Delta' Valencia orchard, 301 days in the 'Bahianinha' Navel orchard and 365 days in the 'Rustenburg' Navel orchard. Average transpiration in the 'Delta' Valencia orchard was 1.15 mm day-1 and 2.17 mm day-1 in winter and summer, respectively. On average, 'Bahianinha' Navel orange trees transpired 0.77 mm day-1 and 1.69 mm day-1 in winter and summer, respectively. In the 'Rustenburg' Navel orchard average transpiration was 2.26 mm day-1 for summer and 2.08 mm day-1 for winter. Total transpiration measured in the 'Delta' Valencia, 'Bahianinha' and 'Rustenburg' Navel orchards was 650, 433 and 682 mm, respectively. Transpiration coefficients (Kt) determined in the three orchards ranged between 0.28 and 0.71. The Kt values were almost constant throughout the seasons in the summer rainfall area and significantly higher in the winter months than in the summer months in the winter rainfall area. Derived monthly and seasonal Kt determined for the three orchards were much lower than the standardised values published in the FAO56. Differences among the transpiration coefficients determined in the orchards in this study means that they are orchard specific and can therefore not be directly applied or transferred to different growing regions of the world.
There is a need to develop an easy method for estimating site-specific crop coefficients, in order to improve water management in citrus orchards. The determination of basal crop coefficients based on physical characteristics of the vegetation and an adjustment for relative crop stomatal control over transpiration formulated by Allen and Pereira (2009) was tested. Use of the parameters for generic citrus trees suggested by the authors did not provide good estimates of transpiration in the three study orchards. A good agreement between measured and estimated Kt values was obtained by back-calculating leaf resistances using measured transpiration values, as recommended by Allen and Pereira (2009). The values of leaf resistances obtained through this procedure were higher than the values suggested by Allen and Pereira (2009), but comparable to those reported in literature and measured in the 'Rustenburg' navel orchard. A relationship between mean monthly leaf resistance and ETo in the 'Rustenburg' Navel orchard provided a means of estimating mean leaf resistance which estimated Kt values with a reasonable degree of accuracy in the three orchards. However, this relationship only provided good seasonal estimates of transpiration, which means that it can only be useful for irrigation planning.