Function to plot DO as a function of temperature and overlay contours of percentage saturation

CheckPlot_DO

# my_df - data frame with DO and temperature data
# DO - (quoted) name of column of DO values (mg/L) e.g. 'DO'
# temp - (quoted) name of column of temperature values (degrees Celcius) e.g. 'temp'

# outputs a graph

CheckPlot_DO <- function(my_df, DO_col, temp_col) {
  
  DO <- my_df[[DO_col]]
  temp <- my_df[[temp_col]]
  
  Calc_DOsat100 = function(temp){
    
    # temp = temperature in degrees C
    # relationship between temperature and dissolved oxygen as defined by the APHA
    # American Public Health Association (1992)
    # Standard methods for the examination of water and wastewater. 18th ed. Washington DC.
    
    # Required constants
    
    C1 = 139.34411
    C2 = 1.575701e+5
    C3 = 6.642308e+7
    C4 = 1.243800e+10
    C5 = 8.621949e+11
    
    Ta =temp + 273.15
    
    exp(-C1 +  C2/Ta - C3/(Ta^2) + C4/(Ta^3) - C5/(Ta^4))
    
  }
  
  
  temp_seq <- seq(min(temp, na.rm = TRUE), max(temp, na.rm = TRUE), length.out = 101)
  DO_100 <- Calc_DOsat100(temp_seq)
  
  plot(temp, DO, xlab = 'Temperature', ylab = 'DO')
  lines(temp_seq, DO_100, col = 'blue', lty = 2, lwd = 2)
  lines(temp_seq, 2*DO_100, col = 'blue', lty = 2, lwd = 2)
  lines(temp_seq, 3*DO_100, col = 'blue', lty = 2, lwd = 2)
  
  text(x = max(temp_seq), y = DO_100[length(temp_seq)], '100%', col = 'blue', pos = 3)
  text(x = max(temp_seq), y = 2*DO_100[length(temp_seq)], '200%', col = 'blue', pos = 3)
  text(x = max(temp_seq), y = 3*DO_100[length(temp_seq)], '300%', col = 'blue', pos = 3)
  
}