Software application for real-time ET o /ET c calculation through mobile devices
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In the southeast of Spain, farmers usually resort to the Agricultural Information Service of Murcia (SIAM) to get the data needed for an estimation of their water use and needs. Through 48 automatic stations, the SIAM provides data about temperature, wind speed, solar radiation or humidity, on a not very user-friendly web page which is not adapted to mobile devices and shows some shortcomings in downloading data. In addition, the key reference parameter in irrigation, the daily reference evapotranspiration (ET o ), is not provided on the current day and the hourly ET o and the crop evapotranspiration (ET c ) are not offered either. This paper presents a new software application for mobile devices capable of providing the required data by an easy-to-use and friendly app. This tool employs the GPS co-ordinates of a place under study to get the data required from the closest SIAM stations and interpolates station data to increase accuracy. The hourly and daily ET o are automatically calculated using the Penman–Monteith equation, declared by the FAO in 1990 as the only valid method for calculating evapotranspiration. The ET o computation entails the calculation of the net solar radiation, the heat soil flux or the psychometric constant. A new extension of the tool also allows calculation of the ET c for a set of typical crops of the southeast of Spain. All the computed data can be stored and edited, as well as those geographical positions of interest. To the best of the authors’ knowledge, this is the first application of this kind available.
KeywordsEvapotranspiration FAO Penman–Montheith Mobile application Android GPS
Fortunately, the emergence of smartphones has increased exponentially in the last decade (Device Atlas 2016), and most farmers use them daily for diverse professional and personal purposes, as supported by the scientific literature on mobile applications developed for agriculture (Molina-Martinez et al. 2011; Jonoski et al. 2012, Bueno-Delgado et al. 2015). This suggests that smartphones could be a good tool to obtain the data required to know the water needs or water use in a place of interest or for a specific crop. Thus, this paper presents a new software application (aka app) for mobile devices that allows calculation, storage and management of the hourly and daily ET o as well as the ET c for a set crop, in an easy-to-use and friendly tool. With this aim, the app allows the user to indicate the place under study through its co-ordinates using the GPS integrated in the mobile device through Google Maps tool (Google 2005) or by hand. After that, the app downloads the data required from the SIAM stations closest to these co-ordinates transparently to the user. If the co-ordinates are close to two or more stations, the app interpolates the data to increase accuracy. Then, the hourly and daily ET o are automatically calculated using the Penman–Monteith equation, declared by the FAO in 1990 (Allen et al. 1998) as the only valid method for calculating evapotranspiration. Note that the calculation of ET o involves computing other necessary parameters that the SIAM does not offer, such as the net solar radiation, the soil heat flux, the psychometric constant or the vapor saturation pressure curve. The app also allows calculation of ET c for a set of crops. All data computed can be stored and edited in the app, as well as those geographical positions of interest. The app is executed in Android Operating System (OS) (Android 2009) because Android devices compose 82.8% of all smartphones and tablets on the market (IDC corp., 2016). This new app has details like elements in the screen designed to be compatible with tablets and other mobile devices; graphics are included to increase application usability and information of interest can be sent by text messages, bluetooth, whatsapp, e-mail or social networks. To the best of the authors’ knowledge, this is the first application of this kind available.
Checking every single station in real-time: the user searches the station of interest, and a pop-up window is displayed with the data stored in the last reading. Although the SIAM apparently offers these data in “real-time”, in fact the data are delayed by 2 or 3 h.
Plotting the current day: this option displays a table with the average, maximum and minimum values of the data stored by all stations in the current day (until the last read). Every row is a single station, and every column is a meteorological parameter: average, maximum and minimum values of temperature and humidity, rainfall, and the average and maximum wind speed.
Plotting any previous day: the SIAM offers the possibility of plotting a table similar to the previous one for any previous day since January 1996. This table also includes absolute maximum and minimum values of temperature and humidity, average wind direction, hours of sun, average radiation, reference evapotranspiration, and maximum and average rainfall.
The SIAM also offers irrigation and fertilization schedulers for the most com- mon varieties of fruit and vegetable crops in Murcia Region, that is, lemon, orange, mandarin, grapefruit, apricot, peach, nectarine, almond, apple, pear, olives, grapes, artichokes, broccoli, lettuce, melon, pepper and tomato.
Although the SIAM discloses a lot of information, it does not supply solar radiation or the daily/hourly ET o in the current day. In fact, daily ET o , and the maximum and minimum radiation are provided only for previous days. Besides, the SIAM server does not provide a responsive web page to be displayed on mobile devices. Another important flaw on the SIAM web page is the way in which data are displayed. All data are recorded in a non-intuitive table or in a pop-up window, which usually takes a long time to load, being not possible to get data in a fast, comfortable and friendly way. In addition, the SIAM forces export of data as a .csv, .xls, .rtf or .pdf file, and most of the times the download fails and the data are not well recorded. Finally, the SIAM does not allow sharing the data by email, whatsapp or social networks.
The theoretical base of the formulas to estimate the water needs of crops considers, among others, the daily/hourly ET o , the net solar radiation and the ET c . These are computed by the application presented in this work, transparently to the user, simply applying some data recorded by the SIAM or extracted from the GPS co-ordinates.
A brief description of the equations computed by the application is given in the following sub-sections.
As can be seen, the hourly and daily ET o depend on some parameters that can be obtained by the automatic stations in real time, or daily, such as wind speed or air temperature. However, other parameters such as R n , G, γ, e a or e s , must be calculated through the equations reviewed in the following subsections.
Net solar radiation
The net solar radiation (R n ) is the balance between that absorbed, reflected and emitted by the earth’s surface energy, which is calculated as R n = R ns −R nl , that is, the difference between the net radiation of short wave (R ns ) and long wave (R nl ). Its value is positive during the day and negative overnight. Its daily value is usually positive except in cases of extreme latitude.
Soil heat flux
Since the variation of the ground temperature is delayed in comparison with the air temperature changes, a period of time longer than a day should be considered in order to determine G. The depth of penetration of the temperature wave is determined by the time interval. Then, ∆ z is 0.10–0.20 m for an interval of one or a few days or may be greater than 2 m for monthly periods.
As a consequence, for daily and shorter periods, the FAO suggests different approaches: for one day period or a few days G day = 0, whereas for hourly periods of light, G hr = 0.1R n , and during night periods, G hr = 0.5R n .
Slope of the vapor saturation pressure curve
Saturation pressure vapor
For normal irrigation planning and management purposes, defining basic irrigation schedules, and according to most studies of water balance, average crop coefficients are appropriate and more convenient than those coefficients K c that are calculated daily, considering soil and crop coefficients separately (Allen et al. 1998).
To determine the duration of each stage of crop development (beginning, crop development, mid-season, end season) and to select the corresponding values of K c .
To adjust the value of K c according to the stage, identifying K c,ini , K c,mid and K c,end .
To calculate ET c following Eq. (13), using the K c value obtained in the previous steps.
The software tool developed in this work is a mobile app, designed with the aim of offering farmers and technicians a friendly and easy-to-use tool that allows computation of key parameters in many tasks of water management in agriculture, by using their mobile phones or tablets and in just a couple of clicks. Specifically, this tool allows (i) to obtain weather data in a place of interest, taking these data from a network of weather stations strategically installed in Murcia Region (Spain); (ii) to calculate the daily and hourly evapotranspiration reference for a given location; (iii) to calculate the crop evapotranspiration for a crop of interest; and (iv) to store, edit, delete or share the data computed. All these features are executed almost automatically and transparently to the user.
The app has been designed to be executed only on devices with Android OS (Android 2009) and GPS, but the latter is not mandatory. Android was selected as OS because it dominated the global mobile market in 2015, being installed in 82.8% of all smartphones worldwide and it is expected to follow the same tendency till 2018 (IDC corp., 2016). The app has been designed to be compatible with most Android versions from 9 API level, that is, Android 2.3. This means that the app works in 99.7% of mobile devices using Android OS worldwide. It has been also designed with an adaptive design, to be compatible with any tablet or smartphone.
ET o computation
The data are downloaded as a .csv file, and the app parses it to extract the data of interest, automatically and transparent to the user. Note that the list displays all stations, highlighting those closest to the location entered in the previous step (Fig. 5b). The user can select one or more stations. If more than one station is selected, the downloaded weather data are interpolated in order to increase accuracy. Note that selecting stations is an optional step. The user can enter weather data manually, for instance, those obtained by a sensor network installed in the place of interest.
Pressing the “Compute ET o ” button, the daily and hourly ET o are calculated using the equations described before. The results are shown filling the table under the button. Positively, all results in the table can be edited by the user, for instance, if partial data of temperature, humidity and wind are known in advance.
Finally, the “Access Database” button allows storage of the computed data together with all agro-meteorological data in a second database. The content is accessible by pressing the “View Results” button (Fig. 5c).
ET c computation
To load an ET o value from the database. The ET o must come from the location where the crop has been or will be planted.
To select the type of crop from a list given by the application, which contains the most common 18 varieties of fruits and vegetables in Murcia Region: lemon, orange, mandarin, grapefruit, apricot, peach, nectarine, almond, apple, pear, olives, grapes, artichokes, broccoli, lettuce, melon, pepper and tomato.
To indicate the stage of the crop development, given by three options: initial, middle and ending. The application loads automatically the K ini,(DB), K mid,(DB) or K end,(DB) values recommended by the FAO (Allen et al. 1998) for the crop and the stage under study. Note that these data are loaded from a non-editable database.
To select the type of irrigation: rainfall, sprinkling, swimming holes, furrow, drip, etc. Note that each irrigation mode has a specific f w value recommended by the FAO (Allen et al. 1998), which are also stored in the application.
Once all data are set, the user has to press the “Compute ET c ” button in order to calculate the ET c through Eq. (13) and with the assumptions described in Crop Evapotranspiration section. The result is automatically displayed and the user can store it in a database, together with the configuration set to compute it. These data, can be edited, deleted and shared.
As stated in the previous sections, the app uses the following databases: Geolocation database, Daily/hourly ET o database, ET c database and Crop-FAO database. All of them are stored in the mobile device, retaining their values between sessions. The first three are designed to easily filter results by name and edit data. To do this, the user has to press on an entry. A new menu is displayed with options: delete, modify any data or share the information of an entry. In addition, the menu provides the option to delete the whole database. The crop-FAO database, in turn, stores the data about the information required to compute ET c for different crops. Such data are those recommended by the FAO about types of crops, stages of development, crop coefficients, irrigation methods and f w values. This database cannot be edited or deleted.
The app developed in this work is in its first version and new features are being investigated in order to improve its functionality and accuracy, particularly new methods of correction on radiation, since the different degrees of cloudiness may alter the final value; to include charts using the data stored in the databases in order to have a better view about the evolution of evapotranspiration; or to expand the list of fruits and vegetables and include all those studied by the FAO as well as their parameters about the ET c computation. Finally, other Spanish weather stations networks are being studied in order to extend its use to other Spanish regions. The final goal is to offer a tool that permits computation of the ET o and ET c anywhere in the world and for a wide range of crops.
In this paper, a mobile application has been presented that permits farmers and agricultural technicians to work with their mobile devices for managing those agricultural tasks related to optimizing water resources and irrigation. The app computes daily/hourly ET o and ET c in an easy and friendly framework and in a couple of clicks, saving time and, therefore, money. This tool computes the required parameters by accessing the SIAM weather stations network, placed in Murcia Region (Spain), to collect real weather data. However, access to the weather station is not mandatory if the farmer knows the weather data. Additionally the app applies the equations recommended by the FAO to compute the parameters. It also uses databases to store, edit and share the data about geographical locations, their ET o , weather data, ET c and crop information.
The app has been designed to be an attractive product for the end user: it is developed for Android OS, which is a free, stable and the most widely used OS in 2015; it has an adaptive design and the screen elements have been introduced in order to be executed in tablets and all types of smartphones; it includes methods for capturing the input of Cartesian co-ordinates through the display of maps with Google Maps tool, by the selection on the map, including the height given in this co-ordinate by web service. Furthermore, the app provides graphics that facilitate the tool usability, allowing the computed data to be sent by text message, email, whatsapp or social networks. It also permits the use of wireless communication technologies for data transfer, such as Bluetooth or IrDA.
This work was supported by the research Projects 3639/13TIC and 3703/13TIC. Thanks to Telenatura S.L. for its technical support.
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