## 3 Results
### Principles and Methods to Determine Evaptranspiration in Plum Orchards  
Over the years, various ET estimation methods have been developed and validated over a diverse range of land cover types (i.e., grasslands, agricultural land and forests). ET can be measured directly using instrumentation or empirically derived based on its relationship with other atmospheric and ground-based parameters [38]. The methods include meteorological methods (e.g., Penman-Monteith, Blaney-Criddle, Priestley-Taylor, Thronthwaite), micro-meteorological methods (e.g., the Bowen ratio, eddy covariance, scintillometery), eco-physiological techniques (e.g., sap flow, the isotope method, lysimeters) and more recently remote-sensing-based methods [47]. In many studies, the method selection is often motivated by data availability, the aim and proposed spatial and temporal scale of the study, the required accuracy and the associated costs of the study (available funds). The main difference between these methods is their spatial footprint (the spatial scale at which they can accurately estimate ET). Eco-physiological methods, specifically the sap flow method, were developed to provide plant scale estimates of ET. In contrast, meteorological and micro-meteorological methods can measure the ET for a homogeneous surface (i.e., an orchard) at the local scale. These methods provide point estimations of ET, which, assuming a site's homogenous nature, represent the whole study area. A significant drawback of these methodologies, apart from scintillometry, is their inability to measure the spatial variability of ET fluxes over an area of interest. A network of stations is required for the application of meteorological and micro-meteorological methodologies to heterogeneous landscapes. However, several challenges are associated with this network, including the cost associated with the establishment, operation and maintenance of the stations and the costs of employing a team of experienced technicians needed to operate said stations. Another challenge relates to the interpolation of point ET estimates. [PERSON] and [PERSON] [48] noted that interpolation introduces errors into the estimation of ET, particularly in mountainous areas (where meteorological conditions vary over short distances), as well as areas where stations are unevenly distributed, which is often the case. Remote-sensing-based models have been proposed as alternatives to traditional methods, as they can measure the spatial variation in ET across an area with relatively high accuracy [49]. [PERSON] [47] echoed the importance of accurately estimating ET levels and their spatio-temporal variability for improved water resource management and monitoring, particularly in water-scarce environments. According to the results obtained in this study, the FAO-56 crop coefficient approach [36] is the most widely used approach for estimation of the water requirements of plum cultivars [50] (n = 10). Thirteen (13) methods and technologies were noted in the reviewed literature. A significant number of studies (49%) adopted the FAO-56 approach in their estimation of the water requirements of plums. The Soil Water Balance (SWB) model was only used in a study conducted on the cv. Angeleno by [PERSON] et al. [21]. The Treder, Grabarczyk and Rzekanowski and Press methods were all tested in one (4%) study conducted in Poland by [PERSON] et al. [51]. Soil moisture sensors were used in conjunction with the IRRIX software to establish the water requirements of the Red Beau cultivar [23]. The HYDRUS-2D and CROPWAT models were used in only one (4%) study each, by [PERSON] et al. [24] and [PERSON] et al. [25], respectively. Although the eddy covariance (EC) system has been used over the years to estimate the ET of various crops, it was only used in one (4%) study on the African-Delight cultivar by [PERSON] and [PERSON] [52]. Remote sensing (RS) techniques (SEBALI and NDVI time series) were among the adopted methods (4%). The FAO-56 coefficient approach by [PERSON] et al.