Water shapes natural communities in both obvious and indirect ways.

Of course plants and animals need to consume water to grow and survive. But water also erodes and transports sediments, and plays a crucial role in chemical reactions necessary to create soils and to change them over time.

Water processes occur on all scales. Molecular-scale processes determine whether an individual soil particle is dry or moist (whether it holds water). Large-scale geologic processes determine how much groundwater is available for a community relying on groundwater seepage, or how frequently a particular area is flooded. Planet-wide processes shape huge storms over ocean waters that can spiral onto land bringing rain or creating severe natural disturbances.

A medium-scale way of thinking about the water cycle is the watershed. A watershed is all the land whose precipitation is drained by a particular river (see Watersheds).

Of course, not all rain flows directly to that river or one of the streams feeding into it. If it did, plants would wither, animals living in the uplands would be thirsty, and there would be disastrous flash flooding in the valleys every time it rained. Rather, some of the water evaporates, and some of it soaks into the ground immediately, or after pooling.

Water in the ground slowly percolates down, filling pores between sediments or cracks in bedrock, until it reaches an impermeable layer. Above the impermeable layer some amount of sediment or bedrock is saturated with groundwater. The top of this layer is called the water table.

The water table can rise or fall in response to long-term drought or rainy seasons, but in general it is some distance below our feet. However, the water table does intersect with the surface in some places, and where it does, you’ll see a spring, a swamp, or a stream that’s running despite a lack of recent rain.

Groundwater can return to the surface in other ways as well. Humans pump it out in wells. Plants suck it up with their roots, then return it to the atmosphere through transpiration (giving off water vapor through leaves). Some large trees such as oaks are capable of pumping 200 gallons of water out of the ground on a hot day! (Ever notice land in a low area getting a bit swampy after cutting down a big tree there?)

When water processes are disrupted within a watershed, there can be profound consequences, both above ground (such as increased stormwater runoff) and underground (contaminated groundwater, or diminished groundwater). See Water and Land Use under Stewardship and Ecological Threats.