Glossary

The whole drainage area of one outlet, at the coast or in an inland sink. Basins are numbered by area from 1, so basin 1 is the largest of the continent (the Amazon in South America, the Mississippi in North America) and the row of a basin in the basin table is its id.

In the data: the basin mask bsn, the basin table bat; continental_basin_id, global_basin_id (−9999 until a numbering across continents exists), basin_grid_cell_count, basin_area_km2.

See outlet, basin mask, basin table.

The pixel a basin drains to: a river mouth on the coast or an inland sink. Every terminal pixel of the flow directions is an outlet, so a continent has as many basins as outlets — 30,077,037 in North America, most of them single coastal pixels — and the released tables keep those of at least 1 km².

In the data: outlet_row, outlet_col, outlet_lon, outlet_lat; outlet_dir 0 at the coast, 255 in an inland sink.

See basin.

A unit of HydroBASINS (Lehner and Grill, 2013) at its second or third level, whose division and codes were defined manually. It is used only to gather whole basins that lie together into a basin group; nothing is computed on it. The region number carries the code of the Level-03 unit in front: 62211 is the eleventh region of unit 622, the Amazon.

In the data: level2_code, level3_code in the basin group tables; level1, level2, level3 in the region tables.

See basin group, region.

One or more whole basins gathered under one HydroBASINS Level-03 or Level-02 unit. A group has no capacity: it need not share an outlet, need not be connected, and can be any size. A group that fits the capacity becomes one region as it is; one that does not is divided along whole-degree lines, or its basin too large on its own is taken out and cut into pieces. South America has 33 Level-03 groups, 10 of them over the capacity of 160 square degrees. Islands are gathered into their own groups.

In the data: the basin group tables gat_<continent>_30m_l2.csv, gat_…_l3.csv and outlines grp_<continent>_l2.gpkg, grp_…_l3.gpkg; group_id, group_kind (1 one basin, 2 several basins, 3 islands), basin_count, land_grid_cell_count, window_grid_cell_count; group_l2, group_l3 in the basin table.

See HydroBASINS unit (Level-02, Level-03), region, bounding rectangle, window.

A connected part of one basin cut at tributary outlets, with one outlet and possibly several inlets; not a complete subtree. A basin too large for one region is cut into pieces, and its pieces become regions. The piece table has one row per piece with its outlet, its parent piece (the piece it drains into) and the step between the two.

In the data: the piece table pat; piece_id, parent_piece_id (0 for the piece holding the basin outlet), outlet_row, outlet_col, parent_inlet_row, parent_inlet_col, piece_grid_cell_count, piece_area_km2, downstream_depth, crossing_length_m.

See cut basin, region.

At a cut, the exit pixel is the pixel whose downstream neighbour lies in another part — a piece has at most one, its outlet; the inlet is the pixel in the downstream part that receives that water. The piece table gives, for each piece, the inlet of its parent piece that its outlet flows into.

In the data: parent_inlet_row, parent_inlet_col in the piece table (−1 for the piece holding the basin outlet, whose water leaves to the sea or an inland sink); crossing_length_m, the distance from the outlet pixel to that inlet.

See piece, region order.

What is computed at once: a set of whole basins, or the pieces of one cut basin, within the capacity, read together as one window of the continental grid. The region mask gives every land pixel its region; the region table has one row per region with its window; the region order says which regions come before which. The region number is the Level-03 code followed by a serial number: 62211 is the eleventh region of unit 622.

In the data: the region mask rgn, the region table rat; region_id, kind, basin_count, piece_count, cut_basin_id, region_grid_cell_count, window_grid_cell_count, topological_level.

See partition, bounding rectangle, window, region order.

One division of a continent into regions. The site shows two groupings and three partitions for HydroSHEDS v2: the groupings gather whole basins into basin groups along the HydroBASINS Level-02 and Level-03 units, with no limit on size; the partitions, at capacities of 160, 80 and 40 square degrees, start from the Level-03 groups and cut where a region exceeds the capacity. MERIT Hydro at 90 m has one partition: 96 regions, the Level-02 groups divided into their Level-03 children where a group exceeded the 32-bit limit.

In the data: group_l2, group_l3, region_160deg2, region_80deg2, region_40deg2 in the basin table: the basin's group in each grouping and its region in each partition, 0 where the basin is cut over several regions.

See basin group, capacity.

The largest whole-degree rectangle a region may have, stated in square degrees. It bounds the rectangle, not the land inside it, so that a region is read from the continental file as one window and addressed with a 32-bit index: 160 square degrees is 2,073,600,000 pixels at 1 arc-second, just under 2³¹. The three capacities of 160, 80 and 40 square degrees suit workstations with roughly 64, 32 and 16 GB of memory.

See partition, bounding rectangle, window.

A basin that exceeds the capacity on its own and is cut into pieces at tributary outlets; the four largest basins of South America at 160 square degrees. In a capacity partition such a basin has no single region (its region column reads cut), and the region table names it in cut_basin_id. In the basin groups no basin is cut.

See piece, capacity.

The smallest rectangle on whole-degree lines that holds a region or a basin group, read from the continental file in one go: rows row_min to row_max and columns col_min to col_max, all multiples of 3,600 pixels; the region mask picks the region's pixels out of it. window_grid_cell_count counts the whole window, fill_percent is the share of it the region actually covers. The bounding box (bbox_row_min … bbox_col_max) is the tighter rectangle of the region's own pixels, not rounded to degrees.

See region, continental grid.

The order in which regions are computed: a region after every region upstream of it, so that every value that crosses a cut has arrived before it is needed. topological_level counts from the region that holds the basin outlet (0) upstream; for pieces, downstream_depth counts the same way from the piece with the outlet.

See region, piece.

A pixel is one square of the native grid: 1 arc-second, about 30 m at the equator, for HydroSHEDS v2; 3 arc-seconds, about 90 m, for MERIT Hydro. Every number in the tables is computed on that grid. A cell is one square of a coarser grid: the simplified grids the maps and vectors are drawn from (1/400, 1/200, 1/120 degree) and the whole-degree cells the windows are aligned to. Grid cell is the neutral word used only in column names, because one table layout serves both resolutions: every column that counts squares is named <what>_grid_cell_count.

In the data: basin_grid_cell_count, region_grid_cell_count, piece_grid_cell_count, land_grid_cell_count, window_grid_cell_count count native pixels; coarse_grid_cell_count and cells_per_degree describe the simplified grid of a vector file.

The direction each pixel drains to, one of its eight neighbours, as distributed by the data producer — HydroSHEDS v2 (Lehner et al., 2022) and MERIT Hydro (Yamazaki et al., 2019). Everything on this site is computed on top of it. The flow directions themselves are not redistributed here.

See tile.

The producer's unit of distribution: HydroSHEDS v2 comes in 10° × 10° tiles, MERIT Hydro in 5° × 5° tiles packed in 30° packages. A tile boundary cuts flow paths wherever the terrain takes them, which is why the work is organised by basins and regions instead; the tile grids are drawn on the maps because they say which files to fetch.

See region.

The flow directions of one continent assembled into one grid, never held in memory as a whole: North America 424,800 × 277,200 pixels, South America 219,600 × 259,200. Rows and columns in the tables count from its top-left corner.

See bounding rectangle, window.

The raster in which every land pixel carries the id of its basin (the basin raster of the paper). One per continent, at the native resolution.

In the data: bsn_<continent>_30m.tif, bsn_<region>.tif at 90 m.

See basin.

The raster in which every land pixel carries the id of its region, one per partition. Released at 30 m for the three partitions; the basin groups are given by their tables and outlines.

In the data: rgn_<continent>_30m_<partition>.tif.

See region, partition.

One row per basin in id order, with its outlet, pixel count (basin_grid_cell_count), area, bounding box, its group in each grouping and its region in each partition; the row of basin k is row k. The site releases the basins of at least 1 km²; the map draws those of at least 10 km².

In the data: bat_<continent>_30m_ge1km2.csv.gz.

See basin.

One row per region of a partition: its window, the number of basins and pieces it holds, what kind of region it is (whole basins, or the pieces of one cut basin), its topological level, and a point inside it where its number goes on a map. The basin group table (gat) is the same for a grouping: one row per group, with its window and its unit code.

In the data: rat_<continent>_30m_<partition>.csv, gat_<continent>_30m_l2.csv, gat_…_l3.csv; label_lon, label_lat.

See region.

One row per piece of a cut basin in a partition: the region it lies in, its outlet pixel, the inlet pixel of its parent piece that its outlet flows into, and the step between them. The region order follows from it.

In the data: pat_<continent>_30m_<partition>.csv.

See piece, region order.

The basin polygons, their boundaries as lines and the region outlines as GeoPackages, simplified for use as a mask and on the map. The outlines are simplified; the numbers are not — every attribute is computed on the native grid and never measured from these outlines.

In the data: bsn_<continent>_ge1km2.gpkg, bnd_<continent>_ge1km2.gpkg, grp_<continent>_l2.gpkg, grp_…_l3.gpkg, rgn_<continent>_<partition>.gpkg; every row carries coarse_grid_cell_count, the cells of the simplified grid it covers; vector tiles bsn_rgn_<continent>.pmtiles for the map.

See color_id.

A colour number carried by every polygon of the basin, basin group and region files: 1 to 8, chosen so that no two neighbouring polygons share a number, and 0 where a row has no polygon. Any map — this site, QGIS, matplotlib — colours the polygons by it without choosing colours itself; the GeoPackages carry the palette as their default style. The numbers are given per file, so the same basin may carry another number at another resolution. Lines and boundaries do not have it.

In the data: color_id in bsn_, grp_ and rgn_ GeoPackages and in the map tiles; the palette: 1 orange, 2 sky blue, 3 bluish green, 4 yellow, 5 blue, 6 vermillion, 7 reddish purple, 8 grey (Okabe and Ito), with four reserve entries.

See vectors, simplified.

The longest path of the flow directions inside a basin, from its most distant headwater pixel to its outlet. The map draws it, with arrows, for a cut basin, so that the way the water passes from region to region can be followed.

See cut basin.