## 1 Introduction  
The efficient management of water resources allows improving the productivity, stability and quality of crops, achieving a rational use of water and energy [1; 2; 3]. This is especially important in areas with problems of water scarcity, like in semiarid regions of the southeast of Spain where water shortage and rising water costs compromise farms' viability [4]. Moreover, the reduction in the use of water decreases the pollution of the environment [5; 6]. For an efficient use of water, crop evapotranspiration (ET\({}_{\text{C}}\)) is a fundamental data in order to adapt the amount and frequency of irrigation to the demands of each crop [6; 7]. Because of direct determination of plant water needs is complicated, indirect methods have been established based in weather variables and agronomic information. In this way, the FAO Penman-Monteith methodology (FAO, Food and Agriculture Organization of the United Nations) [8] has been profusely used. According to it, the estimation of ETc is calculated multiplying the product of the reference crop evapotranspiration (ET\({}_{\text{O}}\)) by a crop coefficient (Kc). However, accurate estimation of evapotranspiration is a complex task that requires measurement of numerous physical, meteorological and vegetation cover parameters. Moreover, other methods for estimating ETc have been developed in recent years, such as the use of image analysis to measure effective diameter in lettuce crops [1,2,9], which require a personalized calibration process according to the type of crop [10]. Conversely, it is considered that methods directly measuring crop water balance, such as the weighing lysimeter provide the most precise values to estimate the ETc [6,11,12,13,14,15]. To achieve that, a properly calibration of the lysimeters and a representative surrounding field in situ provide small measurements errors. The accuracy of water measured depends on the representativeness of the lysimeter in comparison to the field, the similarity of the plants inside and outside of the lysimeter and the influence of edge effects, boundary conditions, soil properties, fetch and lysimeter surface area [16]. Recently, the development of the load cells and datalogger technology allows one to obtain high-precision weighing lysimeters [17,18]. These lysimeters can be used to determine evapotranspiration with measurement reliability [19], and an accuracy of hundredths of millimeters reducing investment and maintenance costs and making commercial use possible [20,21, 22]. There are two types of lysimeters, weighing and volumetric. The volumetric type estimates the ETc as a residual by measuring all other components of the soil water balance [23]. In weighing lysimeters, the increase or loss of water is measured by the change in mass obtained by weighing the container in which the soil is located. Lysimeters are usually considered difficult to handle, costly to build and their use and maintenance require special care, so their use is usually restricted to research centers [24,25]. Different weighing lysimeters have already been reported in the literature [26, 27, 28, 29, 30, 31]. To avoid the inconveniences there are some little lysimeters like the proposed by [PERSON] et al. [32], of 20 kg of capacity of low cost, but it has a low resolution in ET and it was used in glasshouse. The lysimeter applied in greenhouse to sugarcane pre-sprouted plantlets of [PERSON] et al. [33], and the triangular weighing lysimeter for potted plants of [PERSON] et al. [14], implemented in a lysimetric station. However, these weighing lysimeters are not intended for their use on intensive farms but on research studies. In recent years, small cylindrical weighing lysimeters are being commercialised such as the Smart Field Lysimeter (SFL) and the Ready-To-Go lysimeter. Evapotranspiration studies have been carried out with SFL [34,35]. Using an SFL of 30 cm diameter and 30 cm depth, [PERSON] et al.