## 3 Results
### Evaportranspiration in Plum Orchards  
The amount of water applied to a field is referred to as water use, and it either is consumed by crops through evapotranspiration (ET) or is not consumed [34]. Water consumption in a field is beneficial if it is consumed by crops through transpiration (T);  
Figure 5: Frequency of studies published on the estimation of plum water requirements and the cumulative analysis of these publications. otherwise, it is non-beneficial if it is consumed as soil evaporation (Es) or by weeds. Therefore, there is a need to develop methods and technologies to estimate precise ET volumes due to the increasing incidence of water scarcity globally. Accurate estimation of ETc is essential to establish the maximum crop water requirements and therefore understand how water use is related to yield quality and quantity [34]. ETc is a primary component of the agricultural water balance. It represents the combination of the T (plants' water absorption from the root zone) and Es (vaporization from the soil surface) processes [35]. These processes are illustrated in Figure 6. While also contributing to the water cycle and surface energy balance, ETc can be used to compute the water requirements in a particular field. Since ETc rates are the same as crop water requirements, they can be used to measure crop water stress. The rate of ET is determined by the collective influence of meteorological factors, crop characteristics and orchard management practices. The primary meteorological factors influencing ET are solar irradiance, relative humidity, air temperature and wind speed. The energy required to convert liquid water into vapor is provided by the incident solar radiation (and to a lesser degree, the ambient temperature of the surrounding air), whilst the atmospheric demand for vapor determines the rate, also referred to as the reference evapotranspiration (ETo) [36]. The influence of solar radiation on the ET rate is a function of the available radiation [which varies depending on the region, time of day, season, cloud conditions, etc.] and the fraction of incident radiation intercepted by the crop canopy. Stomata [mouth-like pores found in the epidermis of leaves and stems] regulate the gas exchange between a plant and the atmosphere [37]. The stomata open and close in response to a light stimulus (in this case, solar radiation), and as such, they are typically open during the day and closed at night. They facilitate the diffusion of carbon dioxide (CO\({}_{2}\)) from the atmosphere into a leaf and the movement of water from the leaf into the atmosphere (transpiration) [38]. [PERSON] et al. [39] highlighted the strong correlation between solar radiation and the observed transpiration rates given that water availability is not a limiting factor. The water holding capacity of the air changes with temperature, where warmer air has a higher holding capacity than colder, denser air. Therefore, the atmospheric demand for water vapor would be higher on warm, dry, sunny days than on cold, humid and cloudy days. The influence of wind speed on the ET rate can vary depending on the prevailing conditions. Generally, wind and air turbulence function to transfer water vapor from the evaporative surface to the overlying air. If the air at the  
Figure 6: Illustration of water balance in an orchard setting. evaporative surface is not replaced by drier air, the driving force for evaporation decreases due to the overlying air becoming saturated. The rate of ET increases with wind speed to the point where solar radiation or water availability becomes a limiting factor. At that point, a further increase in the wind speed will not increase the evaporation rate. Relative humidity (RH) and vapor pressure deficit (VPD) indicate the amount of water vapor in the atmosphere. RH has a negative relationship with ET, where increasing RH values decrease the ET over land surfaces.