Water balance
The water balance describes the partitioning of water inputs and outputs within a defined control volume — a catchment, a soil column, or an atmospheric column. It is a fundamental constraint in hydrology and climate science, expressing the conservation of mass for water.
The equation
At the land surface, the water balance is:
where is Precipitation (including occult precipitation), is any additional water input (e.g., irrigation, lateral inflow), is evapotranspiration, is runoff (surface and subsurface), and is the change in water storage (soil moisture, groundwater, snow).
On annual time scales and for closed basins, approaches zero and the balance simplifies to .
Role of climate modeling
Dynamically downscaled climate data can close the water balance where observations are sparse. By providing spatially and temporally consistent fields of precipitation, evapotranspiration, and soil moisture, regional climate models fill observation gaps — particularly in remote or complex terrain.
Galapagos water balance
In the Galapagos, the water balance has distinctive features:
- Highland ecosystems receive a large fraction of their water from Occult precipitation during the Garua season, which is not captured by standard rain gauges. This complicates water balance closure from conventional observations alone.
- Lowland areas are arid for much of the year, with episodic convective Precipitation during the hot season and minimal input during Garua.
- ENSO modulation creates large interannual variability in all water balance components.
The DARWIN project addressed the hypothesis that occult precipitation is a significant component of the highland water balance (see DARWIN publication thread).
Qaidam Basin water balance
In a separate research thread, Wang, Schmidt et al. (2021, JGR-Atmospheres) investigated the sensitivity of water balance in the Qaidam Basin (Central Asia) to Mid-Pliocene climate conditions. Using Dynamical downscaling with WRF, they showed that warmer Pliocene conditions shifted the precipitation-evaporation balance sufficiently to sustain the Qaidam mega-lake system — a body of water that no longer exists under present-day climate.
This work demonstrated how regional climate modeling can constrain paleoclimate water budgets in data-sparse environments.
Berlin-Brandenburg precipitation
The Central Europe Refined analysis version 2 (CER v2), described in a 2025 journal article in Meteorologische Zeitschrift, provides three decades of high-resolution Precipitation data for the Berlin-Brandenburg region, enabling detailed water balance studies in an urbanized landscape where convective precipitation variability is high and the observation network has changed over time.
See also: Precipitation, Evapotranspiration, Occult precipitation, Dynamical downscaling, Qaidam Basin