# Aridity indices

### Description

Calculates aridity according to several indices.

### Usage

1 2 |

### Arguments

`clim_norm` |
climatic normals |

`coeff_rad` |
mean monthly solar radiation; used only for Thornthwaite's annual index Im. Default is |

`coeff_Hargr` |
(vector of monthly) correction coefficient(s) for Hargreaves' equation |

`monthly` |
logic. Sets calculation to the monthly mode if |

`indices` |
set of aridity indices to be listed. Default is all indices (1 to 6 for annual, 1 to 2 for monthly). |

### Details

`clim_norm`

is a monthly data frame of climate normals, with column names: "P", "Tn", "Tx", "Tm" (precipitation, minimum, maximum and mean temperature, respectively). It can be the output of function `climate`

.

Monthly potential evapotranspiration (PE) is calculated via the Hargreaves' formula (Hargreaves and Samani, 1985):

PE = (0.0023*(clim_norm$Tx - clim_norm$Tn)^(0.5)*(clim_norm$Tm+17.8)*coeff_rad)* lmv * coeff_Hargr

where Tn, Tx, Tm are min, max, and mean temperatures, respectively, and lmv is the number of days in any month.

`coeff_rad`

and `coeff_Hargr`

are needed only by Thornthwaite's annual index `Im`

and UNEP's `Ai`

index, whose PE term is calculated via Hargreaves' equation.

`coeff_rad`

corresponds to the mean monthly extra-atmospheric radiation (see function `ExAtRa`

).

`coeff_Hargr`

is either a single value or a vector of 12 coefficients to adjust Hargreaves' estimation of potential evapotranspiration (implemented in `Im`

and `Ai`

indices). From calibration in 6 stations from the same network of `Trent_climate`

, its average value is 0.75.

When `monthly`

is `TRUE`

, a data frame with monthly detail is generated for one station, instead of a synthetic single-line data frame.

`indices`

' values are the following:

1 De Martonne - Ia (annual or monthly). De Martonne, 1925.

2 Thornthwaite - Im (annual or monthly). Thornthwaite, 1948.

3 Emberger - Q (annual only). Emberger, 1955.

4 Lang - R (annual only) - only for positive mean annual temperature. Lang, R., 1920.

5 Rivas-Martinez - Io (annual only). Rivas - Martinez, website http://www.globalbioclimatics.org/

6 UNEP - Ai (annual only). UNEP, 1997.

A reference for the aridity degree for any index is given in the list object `arid_ind_tables`

(see `Trent_climate`

.

### Value

Either a single-line data frame (when `monthly = FALSE`

) with the desired aridity index(es), or a data frame (`monthly = TRUE`

), with monthly values of the desired index(es).

### Author(s)

Emanuele Eccel

### References

De Martonne E., 1925: Traite de Geographie Physique: 3 tomes, Paris.

Emberger, L., 1955. Une classification biogeographique des climats. Receuil des travaux des laboratoires de botanique, geologie et zoologie de la faculte des sciences de l'universite de Montpellier (Serie Botanique), Fascicule 7, 3-43.

Hargreaves, G.H., and Samani, Z.A., 1985. Reference crop evapotranspiratin from temperature. Applied Engineering in Agriculture, 1(2):96-99

Lang, R., 1920. Verwitterung und Bodenbildung als Einfuehrung in die Bodenkunde. Schweizerbart Science Publishers, Stuttgart

Rivas-Martinez - http://www.globalbioclimatics.org/

Thornthwaite, C. W., 1948: An Approach toward a Rational Classification of Climate. Geographical Review, Vol. 38, No. 1(Jan.):55-94.

UNEP (United Nations Environment Programme), 1997. World atlas of desertification 2ED. UNEP, London.

### See Also

`climate`

, `ExAtRa`

### Examples

1 2 3 4 5 6 | ```
data(Trent_climate)
# clima_81_10 is a list of data frames having climatic means of temperature and precipitation
# as required by the aridity indices algorithms, each one referring to one station.
# It can be the output of function climate.
# coeff_rad is a monthly vector of average daily extra-atmospheric solar radiation,
# calculated e.g. by function ExAtRa.
``` |

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