# A Machine Learning Method to Estimate Reference Evapotranspiration Using Soil Moisture Sensors
## Materials and Methods  
### Data Acquisition  
Data collection for this study covered a long period of more than one year, from May 2015 to September 2016. The moisture sensing devices were tested in an experimental plot of 34 m\({}^{2}\) located in the Higher Polytechnic School of Orihuela (EPSO) of the Miguel Hernandez University of Elche (UMH), Spain. Crop rows had a north-south (N-S) orientation. A crop of kikuyu grass (_Pennisetum clandestinum_) with total coverage of the soil was cultivated on this plot. This species of grass was previously used by many authors in different studies on evapotranspiration (ET) [40], and it was employed in our case as a reference model for directly determining the reference ET (ETo). According to [PERSON] et al. [20], the reference crop is \"a hypothetical crop with an assumed height of 0.12 m, a surface resistance of 70 s\(\cdot\)m\({}^{-1}\), and an albedo of 0.23, closely resembling the evaporation from an extensive surface of green grass of uniform height, actively growing and adequately watered\". All these characteristics are approximately met by the selected species. The geographical location of the plot is shown in Figure 1. It has a latitude of 38\({}^{\circ}\)4\({}^{\prime}\)10.17\({}^{\prime\prime}\) N, longitude 0\({}^{\circ}\)59\({}^{\prime}\)6.81\({}^{\prime\prime}\) west (W), and an altitude of 19 m above sea level. The climate in this region is semiarid Mediterranean type, with mild winters and scarce rainfall. Figure 1: Location of the experimental plot in the Higher Polytechnic School of Orihuela (EPSO) of the Miguel Hernández University of Elche (UMH), Spain, and the agroclimatic station of La Murada, Orihuela. Aerial images were extracted from Google Maps. The soil of the experimental plot was loamy in the most superficial layer, with a water field capacity of 0.27 m\({}^{3}\)/m\({}^{3}\) and a permanent wilting point of 0.15 m\({}^{3}\)/m\({}^{3}\). More detailed information of the main soil characteristics is shown in Table 1. The employed irrigation water had an average quality, with a slightly moderate electrical conductivity and a moderate content of total salts. The plot included a pressure irrigation system with a programmer that provided optimal irrigation to the crop during all the experiment, to meet the requirement of \"actively growing and adequately watered\". The agroclimatic data for the study of the water balance were obtained from the website of the Valencian Institute of Agricultural Research ([[http://www.ivia.gva.es/en](http://www.ivia.gva.es/en)]([http://www.ivia.gva.es/en](http://www.ivia.gva.es/en))), which uses information taken from the climatic station of La Murada, Orihuela. This station has a latitude of 38\({}^{\circ}\)10\({}^{\prime}\)51.8\({}^{\prime\prime}\) N, longitude 0\({}^{\circ}\)57'30.8\({}^{\prime\prime}\) W, and it is installed at 96 m above sea level, as shown in Figure 1; the distance from this station to the experimental crop is approximately 9.5 km. It is a Model 3 station, which is the predominant scheme of the SIAR Network (Agroclimatic Information System for Irrigation) adopted by the Spanish Ministry of Agriculture. This model contains a temperature-humidity sensor, a radiation sensor, a wind speed and direction sensor, a pluviometer, and a datalogger. The main variables used in the current research during the study period are shown in Figure 2, including the daily ETo, mean daily temperature, daily rainfall, and daily irrigation by drip and sprinkler. The last two values were directly obtained from flow meters in the experimental plot. The ETo values from the station were estimated using the Penman-Monteith method [20], and the results were considered as the ground truth. It can be observed in Figure 2 that daily temperature and ETo are highly correlated, although the ETo cannot be simply deduced from the temperature.