This article is part of a series on ET-based irrigation scheduling for agriculture. The rest of the series can be found at https://edis.ifas.ufl.edu/topic_series_ET-based_irrigation_scheduling_for_agriculture.
The first step to using evapotranspiration (ET) for irrigation scheduling is to estimate reference ET (ETo). This document lists some of the public sources of ETo in Florida.
Evapotranspiration: Basic Concepts
ET is the process through which water is lost to the atmosphere from the soil by evaporation and from plants by transpiration. ET of a specific crop (also referred to as "crop ET" or "actual ET") is affected by several factors including weather, the crop under consideration, its management, and environmental variables (Table 1). The more information available about factors affecting ET, the more accurate the ET prediction will be. Generally, ET is not directly measured but estimated using mathematical or empirical models that have been developed over time and selected site-specific factors listed in Table 1. More information on basic ET concepts can be found in EDIS document ABE 343, Evapotranspiration: Potential or Reference (https://edis.ifas.ufl.edu/ae256).
Crop ET (ETc) is calculated using Equation 1 by multiplying ETo by the crop coefficient (Kc). ETo refers to ET from a well-watered hypothetical grass surface of known characteristics (height of 4.72 in, surface resistance of 70 sec 3.2ft-1, and an albedo of 0.23) (Irmak and Haman 2017). It expresses the evaporative demand of the atmosphere at a given location from a well-watered reference grass that is fully shading the ground.
Etc = ETo x Kc
Equation 1. Formula used to calculate ETc for a specific crop based on reference ET and crop coefficient.
Reference ET can be calculated using different equations. The different mathematical equations used for ETo estimation are based on different concepts, and the variables (inputs) to include depend on the equation selected. ETo may be determined using a complex equation (i.e., Penman-Monteith) or simpler equations (i.e., Hargreaves). It is important to know which radiation or temperature-based method to use in the calculation of ETo because some equations are more accurate than others depending on the location where they are applied (Table 2). Basic information on how to estimate ETo can be found in EDIS document AE446, Smart Irrigation Controllers: Operation of Evapotranspiration-Based Controllers (https://edis.ifas.ufl.edu/ae446).
The Kc component of Equation 1 integrates the crop characteristics (e.g., crop height, the fraction of net radiation absorbed at the land surface, canopy resistance, and evaporation from bare soil surface) into the ETc estimation equation to account for the difference in transpiration from the actual crop compared to that from the reference grass. Typical Kc values for some Florida crops can be found in EDIS document AE456, Evapotranspiration-Based Irrigation for Agriculture: Crop Coefficients of Some Commercial Crops in Florida (https://edis.ifas.ufl.edu/ae456). General information on estimating crop water requirements for irrigation from ETc can be found in EDIS document AE457, Evapotranspiration-Based Irrigation Scheduling for Agriculture (https://edis.ifas.ufl.edu/ae457).
Sources of ETo Data for Implementing ET-Based Irrigation Scheduling in Florida
Two types of ETo data can be used in ET-based irrigation scheduling: historical ETo and real-time ETo. Historical ETo should represent long-term daily, monthly, or seasonal ETo averages for a long record of data that includes yearly and 10-year variations. Real-time ETo used to schedule irrigation is updated daily, which provides an advantage over the historical ETo-based approach because it accounts for daily variations in weather conditions. Florida growers can easily obtain real-time ETo and monthly average ETo data from the Florida Automated Weather Network (FAWN) website at http://fawn.ifas.ufl.edu/, where ETo is estimated using the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) (1984) modified Penman equation. Daily, average daily, and historic monthly ETo can be obtained from the FAWN database for numerous locations throughout Florida using the following steps:
Go to http://fawn.ifas.ufl.edu/.
Click Tools on the top menu.
Click Evapotranspiration (ET) menu under Irrigation.
A table with daily ETo for the past 7 calendar days and the 7-day average ETo for each of the FAWN weather station sites will appear. A graph with the past 14 days' ETo for selected FAWN sites is also available.
Historical data for FAWN stations can also be obtained from FAWN by clicking on the Data Access menu tab and selecting Report Generator. Historical data can be 15-minute observation; or hourly, daily, or monthly averages.
Actual ET data in Florida can be obtained from the United States Geological Survey (USGS). Data can be accessed by following the steps below.
Click Learn more link under Evapotranspiration Network to view a map of monitoring stations and site names with numbers.
Click on the Site number of the station closest to you. This will take you to the USGS NWIS Web Interface.
From the list of available parameters, select Evapotranspiration (Mean).
Choose Output format.
Enter the Begin date and End date.
Daily ET data are then presented in your choice of output format.
Of the two public sources of ET data, data from USGS have the greatest quality control in estimating actual ET over the different land covers where USGS has ET monitoring stations; however, the available data are limited to only a 10-year period (1995–2005) at some sites. FAWN has a wider coverage of weather stations and a more continuous period of record.
Obtaining ETo or actual ET values from the above public weather data sources will improve the estimation of crop water requirements, which are key to implementing crop-specific ET-based irrigation schedules. For ETo estimation using radiation- or temperature-based methods, always select the method most suitable for your area from Table 2.
Dukes, M. D., M. L. Shedd, and S. L. Davis. 2019. Smart Irrigation Controllers: Operation of Evapotranspiration-Based Controllers. AE446. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/ae446
Irmak, S., and D. Z. Haman. 2017. Evapotranspiration: Potential or Reference? ABE343. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/ae256
Jacobs, J. M., and S. R. Satti. 2001. Evaluation of Reference Evapotranspiration Methodologies and AFSIRS Crop Water Use Simulation. Final Report. Palatka, FL: St. Johns River Water Management District. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.629.9639&rep=rep1&type=pdf
Kisekka, I., K. W. Migliaccio, M. D. Dukes, J. H. Crane, B. Schaffer, S. M. Guzman, and H. K. Bayabil. 2019. Evapotranspiration-Based Irrigation for Agriculture: Crop Coefficients of Some Commercial Crops in Florida. AE456. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/ae456
Kisekka, I., K. W. Migliaccio, M. D. Dukes, B. Schaffer, J. H. Crane, H. K. Bayabil, and S. M. Guzman. 2019. Evapotranspiration-Based Irrigation Scheduling for Agriculture. AE457. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/ae457
Factors that influence ET.
Examples of simpler radiation-based equations that can be used to estimate ETo for different locations in Florida.