Derive the lake-and-river fraction of each 0.5-degree cell from GLWD

Aggregates the Global Lakes and Wetlands Database rasters to the 0.5-degree grid as a fraction of the whole cell. This is the one implementation of that derivation in WHEP: read_glwd_water() calls it for build_polycell_support(), and inst/scripts/prepare_spatialize_all.R calls it to write LPJmL's lakes_rivers input. It used to live only in that script, so the polycell producer read a hand-made .clm artefact of an LPJmL run instead and the two answers were free to diverge.

Usage

glwd_water_fraction(glwd_dir, cells = NULL)

Arguments

glwd_dir

Directory holding the GLWD rasters, as inst/scripts/download/download_hydrology.R lays them out: GLWD_v2/ for v2, or glwd_3/hdr.adf / glwd_3.tif for v1.

cells

Optional tibble of lon/lat cell centres to sample at. NULL (default) returns every cell of the 0.5-degree grid the rasters cover.

Value

A tibble with lon, lat and water_frac, carrying a "glwd_version" attribute of "v1" or "v2".

Which classes count as inland water

GLWD v2 is a 33-class wetland map, not a water fraction, so a subset has to be chosen and the choice is a judgement rather than a lookup. Taken here: lakes as classes 1-3 (freshwater lake, saline lake, reservoir) and rivers as class 7 (small streams). Everything else – palustrine and riverine wetland, peatland, mangrove, saltmarsh, rice paddies – is excluded: those are land that is wet, not surface water, and build_polycell_support() books them under land_area_ha.

Under GLWD v1 the equivalent classes are 1 (lakes) and 3 (rivers). The two vintages are not interchangeable and give totals about 20% apart; see read_glwd_water().

Class membership is multiplied by the companion area_pct raster where one is present, so a partially covered source pixel contributes its own fraction rather than counting whole.

Examples

# Requires the GLWD download; not run without it.
if (nzchar(Sys.getenv("WHEP_LPJML_INPUT_DIR"))) {
  glwd_water_fraction(
    file.path(Sys.getenv("WHEP_LPJML_INPUT_DIR"), "GLWD")
  )
}