#' @title Hamon Potential Evapotranspiration Equation
#' @description The Hamon method is also considered as one of the simplest estimates
#' of potential Evapotranspiration.
#' @param par proportionality coefficient (unitless)
#' @param tavg vector of mean daily temperature (deg C)
#' @param lat latitude ()
#' @param jdate a day number of the year (julian day of the year)
#' @return outputs potential evapotranspiration (mm day-1)
#' @details For details see Haith and Shoemaker (1987)
#' @examples
#' \dontrun{
#' if(interactive()){
#' #EXAMPLE1deg
#' }
#' }
#' @rdname hamon
#' @export
hamon <- function(par, tavg, lat, jdate) {
var_theta <- 0.2163108 + 2 * atan(0.9671396 * tan(0.0086 * (jdate - 186)))
var_pi <- asin(0.39795 * cos(var_theta))
daylighthr <- 24 - 24/pi * acos((sin(0.8333 * pi/180) + sin(lat * pi/180) * sin(var_pi))/(cos(lat *
pi/180) * cos(var_pi)))
esat <- 0.611 * exp(17.27 * tavg/(237.3 + tavg))
return(par * 29.8 * daylighthr * (esat/(tavg + 273.2)))
}
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