WATERSHED (CATCHMENT/DRAINAGE BASIN)
“It is a landmass, which drains into a stream system, circumscribed by a boundary technically known as divide, and having an outlet which is to be considered as measuring point or mouth of the stream system.”
J.E Reader- Roizsch defines watershed in his lectures delivered.
“Watershed is a land body which drains into a stream system, it is delineated by a circumscribing boundary called divide and has an outlet which is either the mouth of the stream or designated point of interest.”
THE HYDROLOGICAL CYCLE:
The Global Stock of Water:
There are nearly 1400 km3 of water on earth – enough (if it were all above ground at a uniform depth) to cover the earth’s surface to a depth of some 3 km. most of this water is, however, is relatively inaccessible and is not active in the annual cycling of water that supplies rivers, lakes and living things.
More than 97% is found in oceans (see figure) where the water is not only salty but has on the residue of several thousand years. Only the near surface oceans layers take part in annual water cycling.
An additional 2% of the water is glaciers and ice caps of mountains with similarly long residence times (about 10, 000 yrs).
Some 0.7% is found in groundwater, most of which is more than 750 m underground. Except where it is trapped for pumping by the human, it too has a long residence of several thousand years or more.
The most active cycling water is in the surface layers of the oceans in the shallower underground, in lakes, rivers in the atmosphere and in the soil (see above fig). Here, the average residence time for water in the atmosphere is 10 days, that for the longest river is 20 days or less, and that for soil moisture is about 1 month.
The Hydrological Cycle/ Water Cycle:
Following fig illustrates the whole Water/ Hydrological cycle in detail.
Water first evaporates from the hydrosphere, then it condenses and forms raindrops in the atmosphere, then falls on the ground in the form of precipitation to the lithosphere and then again it rejoins the hydrosphere in the form of rivers, springs, runoff, etc. as shown above.
For much detail of hydrological cycle, consider the following diagram.
Forms and passages of Water Cycle
- These forms and passages can be listed as (from figure):
- Evaporation from land
- Evaporation from ocean
- Evaporation from precipitation
- Evaporation from reservoirs
- Evaporation from water surfaces
- Moisture over land
- From Vegetation
- From Soil
- Precipitation on ocean
- Precipitation on land
- Water movement
- Deep percolation
- Overland Flow
- Surface runoff
- Groundwater Flow/ movement
- Underground runoff
- Sub-surface flow
- Soil Moisture
- Water table
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