## 3 Results
### Principles and Methods to Determine Evaptranspiration in Plum Orchards
[36] was the most used method across the retrieved studies. In this method, ETo is determined using the Penman-Monteith method [34]. In a study by [PERSON] et al. [17], crop water requirements were scheduled daily according to the ETo and Kc reported by [53]. The authors adjusted the Kc values to the tree canopy cover using a reduction coefficient (Kr) suggested by [PERSON] et al. [54]. This method performed well for all the trees studied since they had comparable vigor. The use of Kr yielded satisfactory results for the _Prunus domestica_ cv. [PERSON] [55]. [PERSON] et al. [23] used data from soil moisture sensors to calculate the water requirements of the Red Beaut cultivar using a cloud-hosted web platform called IRRX. This platform also performs various tasks, including irrigation scheduling decision-making without any human intervention. Just like the study by [PERSON] et al. [17], which calculated the water requirements using adjustment factors incorporated into the FAO-56 approach, the IRRIX algorithm described by [56] calculates Etc using the FAO-56 approach fused with an adjustment factor which personalizes the basal crop coefficient (Kcb) to the vigor of the plot. In some instances, some studies calculate Kc from the crop coefficient approach using the SWB method. This was the case in [PERSON] et al. [20], where weekly water requirements were adjusted using the SWB method with the neutron probe measurements described in [PERSON] et al. [19]. The total water requirements of the cv. African Rose were calculated using a theoretical irrigation rate and the Etc obtained using the FAO-56 approach [26]. The eddy covariance (EC) system, which uses the simplified energy balance method, is the most reliable micro-meteorological technique for estimating ET [38]. Due to the system's associated costs, [PERSON] and [PERSON] [52] collected the data for a few days in summer during the optimum irrigation period. This is the probable reason the study did not report the seasonal ET for the cultivar. Studies often use EC-measured ETc data to validate model- or remote-sensing-derived ET data. This was the case in the study by [PERSON] and [PERSON] [57], where the Etc estimates obtained using SEBALI were validated using a network of EC systems. This validation yielded a Root Mean Square Error (RMSE) and a coefficient of determination (R\({}^{2}\)) of 11.53 mm/month and 0.82, respectively. A few studies adopted remote sensing techniques due to their large spatio-temporal coverage under specific resolutions. [PERSON] et al. [58] integrated and harmonized the Normalized Difference Vegetation Index (NDVI) obtained from the Sentinel-2 and Landsat 8 sensors and obtained NDVI time series, used to estimate the ET through fit equations. The results from the study showed a significant correlation between the Etc and the NDVI from Landsat 8 in the plum orchard, reflected by a [PERSON]'s correlation coefficient of 0.8, which is similar to that reported in the literature by [PERSON] et al. [59] and [PERSON] et al. [60]. Additionally, the remotely sensed ET correlated well with the actual measured ET data, producing bias and an RSME of \(-0.4\) mm/day and 0.6 mm/day, respectively. These results allowed them to update the seasonal water balance of the study site, which improved the irrigation water management at the plot and the water distribution system scale. In a separate study, [PERSON] and [PERSON] [57] compared the ET obtained from a SEBALIGEE ET retrieval system with the eddy covariance (EC) values, and the accuracy had an acceptable bias of 0.48 mm/day. An improved bias of 0.32 mm/day and an R\({}^{2}\) value of approximately 80% were obtained after further calibration. Other studies have integrated the FAO-56 approach with remote sensing techniques to achieve more accurate results [18]. The HYDRUS-2D model was used by [PERSON] et al. [24] to calculate ET as the sum of the root water uptake and soil evaporation.