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)
#> Warning: SPEI fitting failed: L-skewness out of range. Returning NAs.
#> Warning: SPEI fitting failed: L-skewness out of range. Returning NAs.
#> Warning: SPEI fitting failed: L-skewness out of range. Returning NAs.
#> Warning: SPEI fitting failed: L-skewness out of range. Returning NAs.
#>        period       value index          unit
#> 3  2020-03-01  0.09350209  spei dimensionless
#> 4  2020-04-01  0.65413050  spei dimensionless
#> 5  2020-05-01 -0.13016494  spei dimensionless
#> 6  2020-06-01  0.34906431  spei dimensionless
#> 8  2020-08-01 -0.71839509  spei dimensionless
#> 9  2020-09-01 -0.88961909  spei dimensionless
#> 10 2020-10-01 -0.32470178  spei dimensionless
#> 11 2020-11-01  0.15364327  spei dimensionless
#> 15 2021-03-01  1.29161793  spei dimensionless
#> 16 2021-04-01  1.56001783  spei dimensionless
#> 17 2021-05-01  1.56249955  spei dimensionless
#> 18 2021-06-01  1.58397556  spei dimensionless
#> 20 2021-08-01  1.07585476  spei dimensionless
#> 21 2021-09-01 -0.06492128  spei dimensionless
#> 22 2021-10-01 -0.60473222  spei dimensionless
#> 23 2021-11-01 -1.01052347  spei dimensionless
#> 27 2022-03-01 -1.10655643  spei dimensionless
#> 28 2022-04-01 -0.64567489  spei dimensionless
#> 29 2022-05-01 -0.79241883  spei dimensionless
#> 30 2022-06-01 -1.05023289  spei dimensionless
#> 32 2022-08-01  0.71280617  spei dimensionless
#> 33 2022-09-01  1.46483020  spei dimensionless
#> 34 2022-10-01  1.74668897  spei dimensionless
#> 35 2022-11-01  1.60458580  spei dimensionless
#> 39 2023-03-01  0.12202701  spei dimensionless
#> 40 2023-04-01 -0.41140329  spei dimensionless
#> 41 2023-05-01  0.54261255  spei dimensionless
#> 42 2023-06-01  0.18422503  spei dimensionless
#> 44 2023-08-01 -0.86533087  spei dimensionless
#> 45 2023-09-01  0.45254686  spei dimensionless
#> 46 2023-10-01  0.67032696  spei dimensionless
#> 47 2023-11-01  0.38821492  spei dimensionless