A very classic and crude definition of a watershed is the surface area of the land that results in the discharge of water to one specific downgradient point or, the upgradient area that contributes to water in the stream at a downgradient point on a given stream. In fact, the actual watershed is a combination of the surface and groundwater subsurface areas (not necessarily the same) that results in the movement of water through the terrestrial ecosystem or landscape to a downgradient point. It includes both surface water and groundwater features and areas that lose water through transpiration, sublimation, and evapotranspiration; it is not just the groundwater recharge areas.
Typically, water enters the watershed through a combination of precipitation, snowfall, dew, snow, ice, hail, and condensation, and then leaves the watershed system area through streamflow, groundwater withdrawal, evaporation, transpiration, and sublimation. Water is stored in the watershed as soil moisture, biological organisms, minerals, and in various unconsolidated and confined aquifers.
Watersheds come in all shapes and sizes and this is a function of the local geology, erosion, climate, biological community, temperature, and rainfall quantity and distribution throughout the year. A very typical watershed area will start at the highest elevation for a given landscape and then, at some point, a number of gullies will go from having seasonal flow to year-round flow with connected or disconnected wetland systems that will begin to interconnect downgradient to create a drainage pattern. As the drainage network grows, the smaller gullies combine to form a stream, i.e., a First Order Stream. Two First Order Streams combine to form a 2nd Order Stream which combines with another 2nd Order Stream to form a 3rd Order Stream and so on. This process repeats until at some point a larger river system will form that will ultimately discharge into the ocean (some streams in arid regions never reach an ocean). In addition to the water we can see at the surface in wetlands, lakes, ponds, streams, and rivers, there is also a subsurface flow system or the groundwater system where the water moves through aquifers from recharge zones to local or regional discharge zones.
