•You know that water evaporates from bodies of water, wet soil, and plants, falls as precipitation onto the earth, either evaporates again, or soaks into the ground, or runs off and returns to the ocean.Chapter OneChapter OneHydrologic PrinciplesHydrologic Principles
A watershed or catchment basin is a contiguous area that drains to a common outlet. It is the area around a stream that actually sends water into the stream.
The drainage divide is the locus of points that separates adjacent watersheds. Perhaps the most easily recognized divide in the US is the Continental Divide.On one side, water eventually ends up in the Pacific, and on the other, the Atlantic.
In large watersheds with multiple tributary basins it is sometimes convenient to define sub-basins, provided you have a gauge at each sub-basin outlet, and rain gauges, so you know the lag time between peak rainfall and the time of high water.Watersheds can be defined at a number of different scales. The South Branch of the Raritan has a watershed of its own, and one could consider the South Branch Watershed to encompass the entire drainage around the South Branch. South Branch Watershed would end where it flows into the Raritan River. However, the Raritan also has a watershed. It encompasses the entire South Branch watershed, and also the North Branch Watershed, ending only where Raritan Bay empties into the Atlantic.
Millington GaugeOn the left is the westernmost subwatershed of the Great Swamp watershed.
Some symbolsWater falls onto the earth’s surface as rain or snow, marked P for precipitation. Some of the surface water Evaporates (E) or is transpired