Wednesday, March 8, 2017

Watersheds! What's the meaning of this and how it works

credit/source: and  CoCoRaHS HQ and Colorado State University

When looking at the location of rivers and the amount of stream-flow in rivers, the key concept 
is the river's "watershed". What is a watershed? Easy, if you are standing on ground right now, 
just look down. You're standing, and everyone is standing, in a watershed.                                                                                                                     A watershed is the area of land where all of the water that falls in it and drains off of it goes to                                                                                                                 a common outlet. Watersheds can be as small as a footprint or large enough to encompass all 
the land that drains water into rivers that drain into Chesapeake Bay, where it enters 
the Atlantic Ocean. This map shows one set of watershed boundaries in the continental
 United States; these are known as National 8-digit hydro-logic units (watersheds).
A watershed is an area of land that drains all the streams and rainfall to a common outlet such
 as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. 
The word watershed is sometimes used interchangeably with drainage basin or catchment. 
Ridges and hills that separate two watersheds are called the drainage divide. The watershed 
consists of surface water--lakes, streams, reservoirs, and wetlands--and all the underlying 
ground water.
 Larger watersheds contain many smaller watersheds. It all depends on the outflow point; 
all of the land that drains water to the outflow point is the watershed for that outflow location. 
Watersheds are important because the stream-flow and the water quality of a river are affected
 by things, human-induced or not, happening in the land area "above" the river-outflow point.

A watershed is a precipitation collector

Map of the northeastern United States showing an outline of the Chesapeake Bay watershed, which drains into the Atlantic Ocean.

Most of the precipitation that falls within the drainage area of a stream's monitoring site
 collects in the stream and eventually flows by the monitoring site. Many factors, some listed
 below, determine how much of the streamflow will flow by the monitoring site. Imagine that
 the whole basin is covered with a big (and strong) plastic sheet. Then if it rained one inch,
 all of that rain would fall on the plastic, run downslope into gulleys and small creeks and then
 drain into main stream. Ignoring evaporation and any other losses, and using
 a 1-square mile example watershed, then all of the approximately 17,378,560 gallons of water 
that fell (you can use our 
as rainfall would eventually flow by the watershed-outflow point.

Not all precipitation that falls in a watershed flows out

To picture a watershed as a plastic-covered area of land that collects precipitation is overly
 simplistic and not at all like a real-world watershed. A career could be built on trying to model 
a watershed water budget (correlating water coming into a watershed to water leaving 
a watershed). There are many factors that determine how much water flows in a stream
 (these factors are universal in nature and not particular to a single stream):                                                                                                                        Precipitation: The greatest factor controlling stream-flow, by far, is the amount of 
precipitation that falls in the watershed as rain or snow. However, not all precipitation                                                                                                                      
 that falls in a watershed flows out, and a stream will often continue to flow where there                                                                                                                  
  is no direct runoff from recent precipitation.

Infiltration: When rain falls on dry ground, some of the water soaks in, or infiltrates                                                                                                                                  the soil. Some water that infiltrates will remain in the shallow soil layer, where                                 
  it will gradually move downhill, through the soil, and eventually enters the stream by                                                                                                                         seepage into the stream bank. Some of the water may infiltrate much deeper,
 recharging groundwater aquifers. Water may travel long distances or remain in storage                                                                                                                    for long periods before returning to the surface. The amount of water that will soak in                                                                                                             overtime depends on several characteristics of the watershed as the following:
  • Soil characteristics: In Georgia, clayey and rocky soils of the northern areas                                                                                                                           absorb less water at a slower rate than sandy soils, such as in Georgia's Coastal                                                                                                                           Plain. Soils absorbing less water results in more runoff overland into streams.
  • Soil saturation: Like a wet sponge, soil already saturated from previous rainfall                                                                                                                                  can't absorb much more ... thus more rainfall will become surface runoff.
  • Land coverSome land covers have a great impact on infiltration and rainfall                                                                                                                             Impervious surfaces, such as parking lots, roads, and developments, act as                                                                                                                                      a "fast lane" for rainfall - right into storm drains that drain directly into streams.                                                                                           Flooding becomes more prevalent as the area of impervious surfaces increase.
  • Slope of the land: Water falling on steeply-sloped land runs off more quickly than                 
  • Evaporation: Water from rainfall returns to the atmosphere largely through evaporation.                                                                                                                 The amount of evaporation depends on temperature, solar radiation, wind, atmospheric                                        
  • Transpiration: The root systems of plants absorb water from the surrounding soil                                                                                            in various amounts. Most of this water moves through the plant and escapes into                                                                                                                             the atmosphere through the leaves. Transpiration is controlled by the same factors as                                                                                                                    evaporation, and by the characteristics and density of the vegetation. Vegetation slows                                                                                                                    runoff and allows water to seep into the ground.
  • Storage: Reservoirs store water and increase the amount of water that evaporates and                                                                                                        infiltrates, storage and release of water in reservoirs can have a significant effect on the                                                                                                                     stream-flow patterns of the river below the dam.
  • Water use by people: Uses of a stream might range from a few homeowners and                                                                                                                   businesses pumping small amounts of water to irrigate their lawns to large amounts of                                                                        water withdrawals for irrigation, industries, mining, and to supply populations with                                                                                                                           drinking water.                                                                                                                                                                                                        credit/source:

What is a Watershed?

A watershed is an area of land that catches rain and snow and drains or 
seeps into a marsh, stream, river, lake or groundwater. Homes, farms,
 cottages, forests, small towns, big cities and more can make up watersheds.                                                                                                 
Some cross municipal, provincial and even international boarders. 
They come in all shapes and sizes and can vary from millions of acres,
like the land that drains into the Great lakes, to a few acres that drain into
a pond.
The interactive presentation shown below uses Adobe© FLASH PLAYER
All articles are credit to original writer/writers references including the images and 
youtube video.