% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/drought.R
\name{ck_pet_pm}
\alias{ck_pet_pm}
\title{Reference Evapotranspiration (FAO-56 Penman-Monteith)}
\usage{
ck_pet_pm(
  tmin,
  tmax,
  lat,
  dates,
  elev = 0,
  wind = 2,
  rh_min = NULL,
  rh_max = NULL,
  rs = NULL,
  albedo = 0.23,
  krs = 0.16
)
}
\arguments{
\item{tmin}{Numeric vector of daily minimum temperatures (degrees C).}

\item{tmax}{Numeric vector of daily maximum temperatures (degrees C),
same length as \code{tmin}.}

\item{lat}{Numeric. Latitude in decimal degrees.}

\item{dates}{Date vector of the same length as \code{tmin}.}

\item{elev}{Numeric. Elevation above sea level in metres
(default 0).}

\item{wind}{Numeric vector or single value. 2-m wind speed (m/s).
Default \code{2} (FAO-56 fallback for unmeasured wind).}

\item{rh_min, rh_max}{Optional numeric vectors of daily minimum and
maximum relative humidity in percent. Both must be supplied
together; otherwise vapour pressure falls back to \code{e0(tmin)}.}

\item{rs}{Optional numeric vector of daily incoming solar
radiation (MJ m^-2 day^-1). If \code{NULL}, Hargreaves-Samani
estimate is used.}

\item{albedo}{Numeric. Surface albedo (default 0.23).}

\item{krs}{Numeric. Hargreaves-Samani coefficient for the Rs
fallback (default 0.16 for inland sites; 0.19 for coastal).}
}
\value{
A data frame with columns \code{date}, \code{value}, \code{index}
(\code{"pet_pm"}), and \code{unit}.
}
\description{
Compute reference evapotranspiration ETo using the FAO-56
Penman-Monteith equation (Allen et al. 1998), the international
standard for ETo estimation. Required inputs are daily Tmin and
Tmax; optional inputs (humidity, wind speed, incoming solar
radiation, elevation) increase accuracy. Where humidity, wind, or
solar radiation are missing, FAO-56 fallback estimators are used.
}
\details{
Inputs and units:
\itemize{
\item \code{tmin}, \code{tmax}: daily minimum and maximum temperature (degrees C).
\item \code{lat}: latitude in decimal degrees, used for the
extraterrestrial-radiation calculation.
\item \code{elev}: elevation above sea level in metres (default 0).
\item \code{wind}: 2-metre wind speed (m/s). Default 2 m/s (the
FAO-56 fallback value when wind data are unavailable).
\item \code{rh_min}, \code{rh_max}: minimum and maximum daily relative
humidity (\\%). If both are NULL (default), actual vapour
pressure is estimated as \code{e0(tmin)} (FAO-56 Eq. 48).
\item \code{rs}: daily incoming solar radiation (MJ m^-2 day^-1).
If NULL, estimated by Hargreaves-Samani: \code{Rs = krs * Ra * sqrt(Tmax - Tmin)} with \code{krs = 0.16} for inland sites.
\item \code{albedo}: surface albedo (default 0.23, the FAO grass
reference).
}
}
\examples{
dates <- as.Date("2024-07-01") + 0:9
tmin <- c(15, 16, 14, 17, 15, 13, 16, 14, 15, 16)
tmax <- c(30, 32, 28, 33, 31, 27, 34, 29, 30, 32)
ck_pet_pm(tmin, tmax, lat = 45, dates = dates)
}
\references{
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M.
(1998). Crop evapotranspiration: guidelines for computing crop
water requirements. \emph{FAO Irrigation and Drainage Paper 56}.
}
