spi: Standardized Precipitation Index (SPI)

In spi: Compute SPI index

Description

The SPI function computes the SPI index (McKee et al., 1993) from a predefined time scale (1 month, 3 months, 24 months, etc.) to a period choosen by users.

Usage

spi(nargs, filename, id, fd, title, output, txlab, tylab)

Arguments

nargs	number of arguments (minimum = 3)
filename	name of datafile The datafile in ASCII format must have the following layout: Months 2005 2006 2007 2008 2009 2010 Jan 28.1 5.8 22.9 64.2 70.1 85.9 Feb 41.4 85.1 149.2 31.0 142.1 36.9 Mar 153.2 145.9 101.6 308.8 171.8 57.5 Apr 57.0 212.4 170.3 244.5 278.9 132.9 May 154.9 119.9 57.8 128.8 212.7 55.6 jun 158.6 81.3 160.8 94.0 115.3 63.3 Jul 22.6 27.2 29.4 80.9 82.7 30.9
id	initial data
fd	final data
title	data title
output	output type ( 1 - graph, 2 - results matrix )
txlab	the X axis title
tylab	the Y axis title

Details

Positive SPI values indicate greater than median precipitation and negative values indicate less than median precipitation. Drought periods are represented by relatively high negative deviations. Normally, the 'drought' part of the SPI range is arbitrary split into moderately dry (-1.0 > SPI > -1.49), severely dry (-1.5 > SPI > -1.99) and extremely dry conditions (SPI < -2.0). A drought event starts when SPI value reaches -1.0 and ends when SPI becomes positive again (McKee et al., 1993).

This function use the SPI range defined by National Climatic Data Center (NCDC):

exceptionally moist:	SPI >= 2.0
extremely moist:	1.60 <= SPI < 1.99
very moist:	1.30 <= SPI < 1.59
moderately moist:	0.80 <= SPI < 1.29
abnormally moist:	0.51 <= SPI < 0.79
near normal:	-0.50 <= SPI <= 0.50
abnormally dry:	-0.79 <= SPI < -0.51
moderately dry:	-1.29 <= SPI < -0.80
severely dry:	-1.59 <= SPI < -1.30
extremely dry:	-1.99 <= SPI < -1.60
exceptionally dry:	SPI <= -2.0

Value

Returns a data matrix or a graphic with SPI values.

Author(s)

Josemir A Neves ([e-mail]/[e-mail])

References

McKee, T.B., Doesken, N.J., Kleist, J., 1993. The relationship of drought frequency and duration to time scales. In: Preprints, Eighth Conference on Applied Climatology, January 17 e 22, Anaheim, California, pp. 179 e 184.

Examples

##load data

data(spi_data) 

##write to file

write.table(spi_data,file="spi.txt",quote=FALSE,row.names=TRUE)	  

## Standard format with the output in the text format

spi(3,"spi.txt",1963,2010)

## A full output in graphical format

spi(7,"spi.txt",1963,2010,"Standardized Precipitation Index - Rio Grande do Norte State",1,"years","months")

spi documentation built on May 2, 2019, 2:42 a.m.