Standardised Precipitation-Evapotranspiration Index (SPEI)

Compute the SPEI by fitting a log-logistic distribution to the monthly climatic water balance (precipitation minus potential evapotranspiration) accumulated over a rolling window.

Usage

ck_spei(precip, pet, dates, scale = 3, distribution = c("log-logistic", "gev"))

Arguments

precip

Numeric vector of daily precipitation (mm).

pet

Numeric vector of daily potential evapotranspiration (mm).

dates

Date vector of the same length as precip and pet.

scale

Integer. Accumulation period in months (default 3).

distribution

Character. Either "log-logistic" (default, Vicente-Serrano et al. 2010) or "gev" (Generalised Extreme Value, fitted by L-moments; preferred for water-balance series with heavy upper or lower tails).

The three-parameter log-logistic is fitted by L-moments, which requires an L-skewness in (0, 1). A calendar month whose water balance is symmetric or left-skewed falls outside that range; the fit warns and returns NA for that month rather than reporting a value from a distribution that does not describe the data. Use distribution = "gev", which accommodates both tails, when this happens often in your series.

Value

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

References

Vicente-Serrano, S. M., Begueria, S., & Lopez-Moreno, J. I. (2010). A multiscalar drought index sensitive to global warming: the Standardized Precipitation Evapotranspiration Index. Journal of Climate, 23(7), 1696-1718.

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)
pet <- rep(3, length(dates))
ck_spei(precip, pet, dates, scale = 3)
#>        period        value index          unit
#> 3  2020-03-01  0.093502089  spei dimensionless
#> 4  2020-04-01  0.654130498  spei dimensionless
#> 5  2020-05-01 -0.130164941  spei dimensionless
#> 6  2020-06-01  0.349064306  spei dimensionless
#> 7  2020-07-01 -1.131718122  spei dimensionless
#> 8  2020-08-01 -0.718395087  spei dimensionless
#> 9  2020-09-01 -0.889619094  spei dimensionless
#> 10 2020-10-01 -0.324701783  spei dimensionless
#> 11 2020-11-01  0.153643269  spei dimensionless
#> 12 2020-12-01  0.798306865  spei dimensionless
#> 13 2021-01-01  0.875630337  spei dimensionless
#> 14 2021-02-01  0.903167756  spei dimensionless
#> 15 2021-03-01  1.291617930  spei dimensionless
#> 16 2021-04-01  1.560017834  spei dimensionless
#> 17 2021-05-01  1.562499545  spei dimensionless
#> 18 2021-06-01  1.583975560  spei dimensionless
#> 19 2021-07-01  1.088059255  spei dimensionless
#> 20 2021-08-01  1.075854758  spei dimensionless
#> 21 2021-09-01 -0.064921279  spei dimensionless
#> 22 2021-10-01 -0.604732222  spei dimensionless
#> 23 2021-11-01 -1.010523471  spei dimensionless
#> 24 2021-12-01 -0.549632124  spei dimensionless
#> 25 2022-01-01 -0.021811213  spei dimensionless
#> 26 2022-02-01 -0.002107092  spei dimensionless
#> 27 2022-03-01 -1.106556426  spei dimensionless
#> 28 2022-04-01 -0.645674890  spei dimensionless
#> 29 2022-05-01 -0.792418831  spei dimensionless
#> 30 2022-06-01 -1.050232892  spei dimensionless
#> 31 2022-07-01  0.228939468  spei dimensionless
#> 32 2022-08-01  0.712806165  spei dimensionless
#> 33 2022-09-01  1.464830196  spei dimensionless
#> 34 2022-10-01  1.746688969  spei dimensionless
#> 35 2022-11-01  1.604585801  spei dimensionless
#> 36 2022-12-01  0.290547628  spei dimensionless
#> 37 2023-01-01 -0.937710126  spei dimensionless
#> 38 2023-02-01 -0.909162220  spei dimensionless
#> 39 2023-03-01  0.122027007  spei dimensionless
#> 40 2023-04-01 -0.411403289  spei dimensionless
#> 41 2023-05-01  0.542612546  spei dimensionless
#> 42 2023-06-01  0.184225032  spei dimensionless
#> 43 2023-07-01 -0.260915796  spei dimensionless
#> 44 2023-08-01 -0.865330867  spei dimensionless
#> 45 2023-09-01  0.452546865  spei dimensionless
#> 46 2023-10-01  0.670326962  spei dimensionless
#> 47 2023-11-01  0.388214919  spei dimensionless
#> 48 2023-12-01 -1.402827757  spei dimensionless