Aridity indices


Calculates aridity according to several indices.


arid(clim_norm, coeff_rad = NULL, coeff_Hargr = rep(0.75, 12),
  monthly = FALSE, indices = 1:6)



climatic normals


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


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


logic. Sets calculation to the monthly mode if TRUE. Default is FALSE.


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


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

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.


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).


Emanuele Eccel


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 -

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


# 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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