Description Usage Arguments Details Value Author(s) References Examples

Potential evapotranspiration (PET) is the amount of evaporation and transpiration that would occur if a sufficient water source were available. Reference evapotranspiration (ET0) is the amount of evaporation and transpiration from a reference vegetation of grass. They are usually considered equivalent. This set of functions calculate PET or ET0 accordind to the Thornthwaite, Hargreaves or Penman-Monteith equations.

See hargreaves

See hargreaves

1 2 3 4 5 6 7 8 9 10 11 12 13 | ```
thornthwaite(Tave, lat, na.rm = FALSE)
hargreaves(Tmin, Tmax, Ra = NA, lat = NA, Pre = NA, na.rm = FALSE)
penman(Tmin, Tmax, U2, Ra = NA, lat = NA, Rs = NA, tsun = NA,
CC = NA, ed = NA, Tdew = NA, RH = NA, P = NA, P0 = NA,
z = NA, crop='short', na.rm = FALSE)
penman(Tmin, Tmax, U2, Ra = NA, lat = NA, Rs = NA, tsun = NA, CC = NA,
ed = NA, Tdew = NA, RH = NA, P = NA, P0 = NA, z = NA,
crop = "short", na.rm = FALSE)
thornthwaite(Tave, lat, na.rm = FALSE)
``` |

`Tmin` |
a numeric vector, matrix or time series of monthly mean daily minimum temperatures, ºC. |

`Tmax` |
a numeric vector, matrix or time series of monthly mean daily maximum temperatures, ºC. |

`Ra` |
optional, a numeric vector, matrix or time series of monthly mean daily external radiation, MJ m-2 d-1. |

`lat` |
a numeric vector with the latitude of the site or sites, in degrees. |

`Pre` |
optional, a numeric vector, matrix or time series of monthly total precipitation, mm. |

`na.rm` |
optional, a logical value indicating whether NA values should be stripped from the computations. |

`U2` |
a numeric vector, matrix or time series of monthly mean daily wind speeds at 2 m height, m s-1. |

`Rs` |
optional, a numeric vector, matrix or time series of monthly mean dialy incoming solar radiation, MJ m-2 d-1. |

`tsun` |
optional, a numeric vector, matrix or time series of monthly mean daily bright sunshine hours, h. |

`CC` |
optional, numeric a vector, matrix or time series of monthly mean cloud cover, %. |

`ed` |
optional, numeric a vector, matrix or time series of monthly mean actual vapour pressure at 2 m height, kPa. |

`Tdew` |
optional, a numeric vector, matrix or time series of monthly mean daily dewpoint temperature (used for estimating ed), ºC |

`RH` |
optional, a numeric vector, matrix or time series of monthly mean relative humidity (used for estimating ed), %. |

`P` |
optional, a numeric vector, matrix or time series of monthly mean atmospheric pressure at surface, kPa. |

`P0` |
optional, a numeric vector, matrix or time series of monthly mean atmospheric pressure at sea level (used for estimating P), kPa. |

`z` |
optional, a numeric vector of the elevation of the site or sites, m above sea level. |

`crop` |
optional, character string, type of reference crop. Either one of 'short' (default) or 'tall'. |

`Tave` |
a numeric vector, matrix or time series of monthly mean temperatures, ºC. |

`thornthwaite`

computes the monthly potential evapotranspiration (PE) according to the
Thornthwaite (1948) equation. It is the simplest of the three methods, and can be used when only
temperature data are available.

`hargreaves`

computes the monthly reference evapotranspiration (ET0) of a grass crop based
on the original Hargreaves equation (1994). However, if precipitation data `Pre`

is provided
a modified form due to Droogers and Allen (2002) will be used; this equation corrects ET0 using
the amount of rain of each month as a proxy for insolation. The Hargreaves method requires data
on the mean external radiation, `Ra`

. If such data are not available it can be estimated
from the latitude `lat`

and the month of the year.

`penman`

calculates the monthly reference evapotranspiration (ET0) of a hypothetical
reference crop according to the FAO-56 Penman-Monteith equation described in Allen et al. (1994).
This is a simplification of the original Penman-Monteith equation, and has found widespread use.
By default the original parameterization of Allen et al. (1994) is used, corresponding to a short
reference crop of 0.12 m height. Parameterization for a tall reference crop of 0.5 m height due
to Walter et al. (2002) can also be used, by setting the `crop`

parameter to 'tall'. The
method requires data on the incoming solar radiation, `Rs`

; since this is seldom available,
the code will estimate it from data on the bright sunshine duration `tsun`

, or alternatively
from data on the percent cloud cover `CC`

. Similarly, if data on the saturation water
pressure `ed`

are not available, it is possible to estimate it from the dewpoint temperature
`Tdew`

, from the relative humidity `RH`

or even from the minimum temperature `Tmin`

(sorted from least to most uncertain method). Similarly, the atmospheric surface pressure `P`

required for computing the psychrometric constant can be calculated from the atmospheric pressure at
sea level `P0`

and the elevation `z`

, or else it will be assumed to be constant (101.3 kPa).
The code will produce an error message if a valid combination of input parameters is not provided.

If the main input object (`Tave`

, `Tmin`

, `Tmax`

) is a vector or a matrix, data will
be treated as a sequence of monthly values starting in January. If it is a time series then the
function `cycle`

will be used to determine the position of each observation within the
year (month), allowing the data to start in a month different than January.

See hargreaves

See hargreaves

A time series with the values of monthly potential or reference evapotranspiration, in mm. If the input is a matrix or a multivariate time series each column will be treated as independent data (e.g., diferent observatories), and the output will be a multivariate time series.

A time series with the values of monthly potential or reference evapotranspiration, in mm. If the input is a matrix or a multivariate time series each column will be treated as independent data (e.g., diferent observatories), and the output will be a multivariate time series.

A time series with the values of monthly potential or reference evapotranspiration, in mm. If the input is a matrix or a multivariate time series each column will be treated as independent data (e.g., diferent observatories), and the output will be a multivariate time series.

Santiago Beguería

Thornthwaite, C. W. (1948). An approach toward a rational classification of climate.
*Geographical Review* **38**: 55–94. doi:10.2307/2107309.

Hargreaves G.H. 1994. Defining and using reference evapotranspiration.
*Journal of Irrigation and Drainage Engineering* **120**: 1132–1139.

Droogers P., Allen R. G., 2002. Estimating reference evapotranspiration under inaccurate data conditions.
*Irrigation and Drainage Systems* **16**: 33–45.

Allen R. G., Smith M., Pereira L. S., Perrier A., 1994. An update for the calculation of reference
evapotranspiration. *ICID Bulletin of the International Commission on Irrigation and Drainage*, 35–92.

Allen R.G., Pereira L.S.,Raes D., Smith, M. 1998. *JCrop evapotranspiration - Guidelines for
computing crop water requirements - FAO Irrigation and drainage paper 56*. FAO, Rome. ISBN 92-5-104219-5.

Walter I.A. and 14 co-authors, 2002. The ASCE standardized reference evapotranspiration equation. Rep. Task Com. on Standardized Reference Evapotranspiration July 9, 2002, EWRI–Am. Soc. Civil Engr., Reston, VA, 57 pp.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | ```
# Load data for Tampa, lat=37.6475N, elevation=402.6 m. a.s.l.
# Data consists on monthly values since January 1980
data(wichita)
attach(wichita)
names(wichita)
# PET according to Thornthwaite
tho <- thornthwaite(TMED,37.6475)
# Hargreaves
har <- hargreaves(TMIN,TMAX,lat=37.6475)
# Penman, based on sun hours, ignore NAs
pen <- penman(TMIN,TMAX,AWND,tsun=TSUN,lat=37.6475,z=402.6,na.rm=TRUE)
# Penman, based on cloud cover
pen2 <- penman(TMIN,TMAX,AWND,CC=ACSH,lat=37.6475,z=402.6,na.rm=TRUE)
# Plot them together
plot(cbind(tho,har,pen,pen2))
# Now consider the data started in June 1900
thornthwaite(ts(TMED,start=c(1900,6),frequency=12),37.6475)
# Comparison with example from Allen et al. (1998), p. 69, fig. 18:
# Data from Cabinda, Angola (-5.33S, 12.11E, 20 m a.s.l.)
data(cabinda)
pen.cab <- penman(cabinda$Tmin,cabinda$Tmax,cabinda$U2,
Rs=cabinda$Rs,tsun=cabinda$tsun,RH=cabinda$RH,lat=-5.33,z=20)
plot(cabinda$ET0,pen.cab)
abline(0,1,lt='dashed')
summary(lm(pen.cab~cabinda$ET0))$r.squared
``` |

sbegueria/SPEI documentation built on May 8, 2019, 9:57 p.m.

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