Per-pixel index trend over time

Reduce an index stack (from dft_stac_cube()) to a per-pixel trend — the rate at which the index rises or falls across the whole record. Where dft_rast_break() finds an abrupt structural break, this measures the slow, monotonic direction: gradual degradation (a declining stand) or recovery (a restoration planting greening up) that annual categorical labels cannot show.

Usage

dft_rast_trend(cube, min_obs = 6, cores = NULL)

Arguments

cube

A monthly index SpatRaster (the return value of dft_stac_cube()): one layer per time step, with a time value per layer.

min_obs

Integer. Minimum non-NA observations required; pixels with fewer return NA (default 6).

cores

Integer or NULL. Forked workers for the per-pixel reduction. When NULL, uses one fewer than the detected cores.

Value

A two-band terra::SpatRaster with layers trend (index change per year — negative = declining greenness, positive = recovering) and trend_p (Mann-Kendall two-sided p-value; small = a significant monotonic trend).

Details

The slope is a Theil-Sen estimate — the median of all pairwise slopes — which is resistant to a single anomalous season (e.g. a smoke/drought year), unlike an ordinary least-squares fit. Significance is the non-parametric Mann-Kendall test for a monotonic trend.

See also

dft_rast_break() (abrupt breaks), dft_stac_cube().

Examples

if (FALSE) { # \dontrun{
aoi <- sf::st_read(system.file("extdata", "example_aoi.gpkg", package = "drift"))
cube <- dft_stac_cube(aoi, index = "kndvi", datetime = "2017-01-01/2023-12-31")
trend <- dft_rast_trend(cube)
terra::plot(trend[["trend"]])  # negative (red) = declining, positive = recovering
} # }