The Global Water Cycle

How Water Moves

The water cycle describes the movement of water over, above and below the Earth’s surface.  Water can easily change between any of its three states: vapor, liquid and ice. Its phase transitions among the gaseous, liquid and solid states dominate the behavior of the weather, climate and environmental systems. The way water moves between all three phases is a powerful vehicle for rearranging Earth’s energy budget. In addition, the bulk movement of water by precipitation, infiltration, transpiration, runoff and subsurface flow redistributes water around the globe.

Diagram of Earth's water cycle, showing evaporation, transpiration, condensation, and preciptiation.
Illustration of Earth's water cycle.

Source: NOAA National Weather Service Jetstream

Key to the connection between water and energy cycles is how the solar radiation affects the atmosphere. The direct contribution from the sun explains only about 25% of the energy in global atmospheric dynamics. The other 75% is transferred to the atmosphere through the evaporation of water from the surface, primarily from the oceans. This water vapor then condenses into clouds and in doing so, releases its latent heat into the atmosphere. This latent heat drives atmospheric circulation, playing a major role not only in cloud formation and storm development, but in the large-scale movement of air around the world. TRMM created the first reliable global latent heating estimates ever made by measuring the profile of rain as it falls through the sky, as a function of altitude.

GPM provides combined radar/radiometer estimates that extend beyond the tropics. It measures the intensity and variability of three dimensional latent heating structures of precipitation systems as well as microphysics and surface water fluxes. The enhanced measurement and sampling capabilities of GPM are helping us to understand how precipitation patterns change over time across local to regional and global scales. These patterns translate into changes in hydrologic fluxes and states (e.g., runoff, evapotranspiration, soil moisture and groundwater recharge) both directly and in combination with land process models.

By providing more accurate estimates of the rate of transfer of water from the atmosphere to the surface, TRMM and GPM reduce a significant source of uncertainty in the global water/energy budget. Scientists are combining GPM observations with land surface data to provide better estimates of soil moisture, leading to better predictions of vegetation cover, weather forecasts and integrated hydrologic models.

For more information on TRMM and GPM Applications, please see: