Zooming farther out provides an even larger view of the groundwater portion of the hydrologic cycle, extending from the continental divide to the coastal ocean (Figure 35). Broadly, there are two end members of the continental scale system:
- the headwaters in mountainous regions near the continental divide; and,
- the lower basins in flat regions near the coast.
Connecting the headwaters and the lower basins are two, large, continental‑scale, terrestrial, water transport systems: one above ground and the other in the subsurface.
- The above ground system is the readily observable stream network, which is concentrated (in channels) and is largely two‑dimensional (hugging the land surface). Small upland streams cascade over the surface and converge down‑gradient in an orderly manner.
- The subsurface system is the difficult‑to‑observe groundwater system, which is diffuse (not in channels nor tunnels, except in karst terrain, but rather in pores and fractures of geologic materials), which, relative to the stream network, is three‑dimensional (with flow paths that do not necessarily follow the shape of the land surface). Patterns of groundwater flow are less orderly than the stream network because groundwater is driven by multi‑scale hydraulic head gradients that are strongly deflected or contorted by complex geologic structures.
Water is exchanged between the surface and subsurface as illustrated by gaining and losing streams and submarine seeps (Figure 35a), and local and regional groundwater flow systems develope (Figure 35b). The local systems tend to be shallow and short, and respond to short‑term rain events and seasonal changes in climatic conditions. The regional systems are deeper and longer. Regional systems tend to “record” the climate conditions from decades to centuries to millenia ago. For example, a gaining stream in the lower reaches of a basin that is supplied by regional groundwater flow may never run dry, even after long droughts because the groundwater was recharged thousands or tens of thousands years ago when the climate was different.
As shown in the large‑scale continental view of Figure 35, groundwater can directly discharge into the ocean along the coast of the continent. This is referred to as submarine groundwater discharge. Such features were understood and utilized by sea‑faring people to locate fresh water long ago as illustrated in Figure 36. Much submarine groundwater discharge occurs close to shore such as in bays and estuaries.
In summary, there are three continental‑scale water transport systems:
- The atmosphere;
- The stream networks; and,
- The groundwater systems.
These continental‑scale systems are not isolated. They do not simply deliver continental rainfall back to the ocean in parallel flow systems, rather they are intimately connected and exchange water many times along the way (Figure 37).
The mechanisms of these exchanges involve water transfer from the atmosphere to the surface and subsurface, and back from the subsurface to the surface and the atmosphere; through precipitation, infiltration, evapotranspiration, and precipitation recycling, as well as through gaining and losing streams. These three transport systems periodically “switch roles” with respect to which one is transporting a particular drop of water from where it first falls on the continent to where it arrives in the ocean. These interactions “power” the global water flow system.