Standardized Precipitation Index (SPI)

Compute the SPI by fitting a parametric distribution to rolling monthly precipitation accumulations and transforming to standard normal deviates. Two distributions are supported: the two-parameter gamma (default; WMO-1090 standard) and the three-parameter Pearson III. The Pearson III tail is heavier and is preferred in arid regions where the wet-day distribution is highly skewed (Stagge et al. 2015).

Usage

ck_spi(precip, dates, scale = 3, distribution = c("gamma", "pearsonIII"))

Arguments

precip

Numeric vector of daily precipitation (mm).

dates

Date vector of the same length as precip.

scale

Integer. Accumulation period in months (default 3).

distribution

Character. Either "gamma" (default) or "pearsonIII".

Value

A data frame with columns period, value, index, and unit.

References

McKee, T. B., Doesken, N. J., & Kleist, J. (1993). The relationship of drought frequency and duration to time scales.

Stagge, J. H., Tallaksen, L. M., Gudmundsson, L., Van Loon, A. F., & Stahl, K. (2015). Candidate distributions for climatological drought indices (SPI and SPEI). International Journal of Climatology, 35(13), 4027-4040. doi:10.1002/joc.4267 .

Examples

dates <- seq(as.Date("2020-01-01"), as.Date("2023-12-31"), by = "day")
set.seed(42)
precip <- rgamma(length(dates), shape = 0.5, rate = 0.1)
ck_spi(precip, dates, scale = 3)
#>        period       value index          unit
#> 3  2020-03-01 -0.11011769   spi dimensionless
#> 4  2020-04-01 -0.05373019   spi dimensionless
#> 5  2020-05-01 -0.63179704   spi dimensionless
#> 6  2020-06-01 -0.34386911   spi dimensionless
#> 7  2020-07-01 -1.45546229   spi dimensionless
#> 8  2020-08-01 -0.90296848   spi dimensionless
#> 9  2020-09-01 -0.97445477   spi dimensionless
#> 10 2020-10-01 -0.69220945   spi dimensionless
#> 11 2020-11-01 -0.48619603   spi dimensionless
#> 12 2020-12-01  0.92316398   spi dimensionless
#> 13 2021-01-01  1.17686170   spi dimensionless
#> 14 2021-02-01  1.19675188   spi dimensionless
#> 15 2021-03-01  1.53862378   spi dimensionless
#> 16 2021-04-01  1.64855339   spi dimensionless
#> 17 2021-05-01  1.68464304   spi dimensionless
#> 18 2021-06-01  1.69319000   spi dimensionless
#> 19 2021-07-01  1.30681187   spi dimensionless
#> 20 2021-08-01  1.23614871   spi dimensionless
#> 21 2021-09-01 -0.55620266   spi dimensionless
#> 22 2021-10-01 -0.74501028   spi dimensionless
#> 23 2021-11-01 -0.87901274   spi dimensionless
#> 24 2021-12-01  0.02292495   spi dimensionless
#> 25 2022-01-01  0.09089535   spi dimensionless
#> 26 2022-02-01  0.05428237   spi dimensionless
#> 27 2022-03-01 -1.26200269   spi dimensionless
#> 28 2022-04-01 -0.83629965   spi dimensionless
#> 29 2022-05-01 -0.84903114   spi dimensionless
#> 30 2022-06-01 -0.89901194   spi dimensionless
#> 31 2022-07-01  0.35982039   spi dimensionless
#> 32 2022-08-01  0.72803083   spi dimensionless
#> 33 2022-09-01  1.65083132   spi dimensionless
#> 34 2022-10-01  1.70049667   spi dimensionless
#> 35 2022-11-01  1.69640022   spi dimensionless
#> 36 2022-12-01  0.68912675   spi dimensionless
#> 37 2023-01-01 -1.26757630   spi dimensionless
#> 38 2023-02-01 -1.25093754   spi dimensionless
#> 39 2023-03-01 -0.16684883   spi dimensionless
#> 40 2023-04-01 -0.76134150   spi dimensionless
#> 41 2023-05-01 -0.20469462   spi dimensionless
#> 42 2023-06-01 -0.45403613   spi dimensionless
#> 43 2023-07-01 -0.21079630   spi dimensionless
#> 44 2023-08-01 -1.06138507   spi dimensionless
#> 45 2023-09-01 -0.12378988   spi dimensionless
#> 46 2023-10-01 -0.27006092   spi dimensionless
#> 47 2023-11-01 -0.33551191   spi dimensionless
#> 48 2023-12-01 -1.63473901   spi dimensionless