Hydrological Zones

Water input

Precipitation

Mean annual precipitation traces the great wet belts — the tropics, monsoon Asia, and the maritime mid-latitudes — against the deserts of the subtropics and continental interiors. This is the primary forcing of every other variable on this page.

Source: NASA GPM IMERG Final daily precipitation (0.1°) — Version 06 (2000–2020) spliced with Version 07 (2021–2025). Basin means are area-weighted over the 0.1° grid. Huffman et al., GPM IMERG, NASA GES DISC.

Energy

Surface temperature

Average surface skin temperature, the energy signal behind evaporative demand. The diverging scale is centered on 0 °C, separating basins that spend the year above freezing from the cold high-latitude and high-altitude watersheds.

Source: NASA GLDAS-2.2 Catchment Land Surface Model (CLSM), daily 0.25° (AvgSurfT_tavg), converted from kelvin to °C and area-weighted to each basin. Rodell et al., GLDAS, NASA GES DISC.

Water output

Evapotranspiration

The water returned to the atmosphere by evaporation and plant transpiration. ET is high where both water and energy are abundant (humid tropics) and suppressed by either cold (poleward basins) or dryness (deserts, where it is limited by supply rather than demand).

Source: GLDAS-2.2 CLSM Evap_tavg [kg m⁻² s⁻¹], converted to mm/day (×86 400) then summed to an annual total. Physically impossible cell values (< −1 mm/day condensation) were screened as artifacts before aggregation.

Water output

Runoff

Total runoff — surface storm runoff plus subsurface baseflow — is the water that leaves the basin as streamflow. It concentrates in wet, steep, and snowmelt-fed regions and effectively vanishes across arid interiors.

Source: GLDAS-2.2 CLSM Qs_tavg (storm surface runoff) + Qsb_tavg (baseflow), each [kg m⁻² s⁻¹] → mm/day → annual sum, area-weighted per basin.

Storage

Groundwater storage

The modeled groundwater held in each basin — a state variable, so it is averaged rather than summed. It reflects the long-term balance of recharge and drainage and the water-holding capacity of the landscape.

Source: GLDAS-2.2 CLSM GWS_tavg [mm], area-weighted basin mean.

Terrain (static)

Stream gradient

A morphometric descriptor rather than a climate variable: the basin relief (highest minus lowest elevation) divided by the length of its main stem, expressed dimensionlessly as m/m. Because it spans several orders of magnitude, it is shown on a logarithmic scale — high in mountainous headwaters (yellow) and low across floodplains and cratonic lowlands (dark), which helps explain why some basins are flashy while others are slow.

Stream gradient is a static basin property — no time series.

Source: HydroSHEDS void-filled DEM (3 arc-sec ≈ 90 m where available, else 15 arc-sec ≈ 500 m) for basin relief, and HydroRIVERS v1.0 for the main stem — traced from the outlet upstream, following the maximum upstream-drainage-area reach at each confluence. Lehner & Grill (2013), HydroSHEDS.

Classification

Hydrological zoning thresholds

To turn the continuous variables above into hydrological zones, each is binned by a small set of thresholds chosen for hydrological meaning and grounded in the climate-classification literature — the Köppen-Geiger backbone (temperature × precipitation), UNEP/FAO aridity bands, and the Budyko energy-vs-water framework. Crossing the temperature and precipitation axes reproduces the familiar Köppen A/B/C/D/E families; the runoff, evapotranspiration, terrain, frost and groundwater axes add the hydrology Köppen omits.