ui.R

# Let user put in latitude, then calculate Ra
# let user put in mean temperature, low, high
# let user pick crop coefficient
# then print the reference ET and the ETk



library(shiny)

shinyUI(fluidPage(
  
  # Application title
  titlePanel("Reference and crop evapotranspiration  (ET) calculator"),
  withMathJax(),

  # explain what this is
  
  tags$div(
    HTML(paste("This calculates the reference evapotranspiration (ET",
               tags$sub("o"),
               ") in mm for a single day based on latitude, date, and the minimum and
               maximum air temperature. Multiplying ET",
               tags$sub("o"), " by a user-selected crop coefficient gives
               the crop evapotranspiration, ET",
               tags$sub("c"), ". These calculations are based on the Hargreaves ET",
               tags$sub("o"), " equation (equation 52) in the ", 
               a("FAO Crop Evapotranspiration",
               href = "http://www.fao.org/docrep/X0490E/x0490e07.htm#an%20alternative%20equation%20for%20eto%20when%20weather%20data%20are%20missing"),
               " book. More details at the end.",
               sep = ""))
  ),

  # Sidebar to input necessary data
  
  verticalLayout(
   
      
      
      
      dateInput("date", "Date:"),

      
      numericInput("latitude",
                  "Latitude in degrees N or S of the equator; + for northern hemisphere, - for southern hemisphere",
                  min = -70,
                  max = 70,
                  value = 13.4,
                  step = 0.1),
      
 
      
      numericInput("high", "Maximum temperature (°C)", 33,
                   min = -20, max = 60, step = 0.1),
      
   
      
      numericInput("low", "Minimum temperature (°C)", 19,
                   min = -50, max = 35, step = 0.1),
      
   
      
      numericInput("kC", HTML(paste("Crop coefficient (K", 
                                    tags$sub("c"), ")", sep = "")),
                              0.8, min = 0, max = 1.5, step = 0.01),
      
        
    
    
    # Show the output
   
      h3(textOutput("text1")),
    
    # explain what this is
      hr(),
      helpText('Equation 52 is \\(ET_{o} = 0.0023(T_{avg} + 17.8)(T_{max} - T_{min})^{0.5}R_{a}\\),
               where \\(ET_{o}\\) is the reference evapotranspiration, \\(T_{avg}\\) is the average air temperature,
               \\(T_{max}\\) and \\(T_{min}\\) are the maximum and minimum air temperatures, respectively,
               and \\(R_{a}\\) is the extraterrestrial radiation expressed in units of mm (equivalent evaporation) rather than
               energy units of megajoules per square meter. The date and the latitude allow one to calculate
               \\(R_{a}\\), and the difference between the maximum and minimum air temperature is related to the
               amount of cloud cover, thus this difference can be used to estimate the fraction of extraterrestrial
               radiation reaching the surface of the earth.'),
    
     helpText(HTML(paste('If you would like to check these calculations, you can find tables of values for 
                         extraterrestrial radiation in ',
                    a("Table 2.6 of Crop evapotranspiration.",
                      href = "http://www.fao.org/docrep/x0490e/x0490e0j.htm#annex%202.%20meteorological%20tables"),
                    ' Convert the extraterrestrial radiation to equivalent evaporation in mm by multiplying by 0.408,
                    and that is the \\(R_{a}\\) value to use in Equation 52. For more information about this calculator, 
                    see the code on ',
                    a("GitHub.",
                      href = "https://github.com/micahwoods/ETo"),
                    sep = "")))
    )
  )
)