Chapter 2

• Watershed are the areas of land that water flow across through or under before draining into creeks, streams, rivers, wetland, lake or ocean.
• The watershed are separated from each other by highland elevations called the watershed divide (topographic divide) also referred to as watershed boundary.
• The horizontal projection of the area of a watershed is called the drainage area of a stream at that cross section.
• Watershed boundary always follow the highest ridgeline around the stream channels and meet at the bottom and lowest point of the land where water flows out of the watershed.
• Drainage basin an area of land within a physical boundary defined by topographical slopes that divert all runoff to the same drainage outlet.
• A healthy watershed contains some natural features including uplands, floodplains, riparian zones and water bodies
• Upland: area of land located at a higher elevation above a waterbody. Upland typical form watershed boundary or divides.
• Floodplain: flat area of land surrounding a body of water that is subject to periodic flooding after heavy rainfalls, the floodplain holds excess water, allowing it to be slowly released into the river system or seep into groundwater aquifers.
• Riparian zone: is the non-cultivated, vegetated land that touches and immediately surrounds a stream, river, lake or other body of water.
• Watershed are classified based on their sizes and land use.
• Classified by Size

  Small watersheds

  Large watersheds
• Small watersheds - highly sensitive to high-intensity, short duration rainfalls, least heterogeneous and more Nonlinear with size less than 250 km2 .
• Large watershed - less sensitive to highly-intensity rainfall of short duration and are most heterogeneous, large Watershed are more than 250 km2 .
• Classified by Land use

  • Urban,
  • agricultural,
  • forest,
  • mountainous,
  • desert,
  • costal,
  • marsh, old wetland)
• The principal watershed characteristics are:

  1. Physiographical Characteristics
  2. Climatic Characteristics
  3. Socio-economic Characteristics
• A watershed having a fan-shape presents a lower concentration time, and it generates higher flow.
• Aleaf shape watershed generates for the same rainfall, a lower outlet flow, as the concentration time is higher.
• Size of watershed determines the quantity of rainfall received, retained and disposed of (runoff).
• If slope is more:

  • the velocity is more, and hence flood water drains quickly

  • results in higher peak discharge

  • reduces infiltration

  • increases the soil transportation
• The Stream Order, Drainage Pattern, and Drainage Density have a profound influence on watershed as to runoff, infiltration, land management etc.
• It determines the flow characteristics and erosional behavior.
• If drainage density is more, peak runoff is more.
• Physiographical Characteristics of watershed

  Shape,

  Size,

  Slope,

  Elevation,

  Drainage (Stream Order),

  Land Use,

  Vegetation Cover,

  Soils-sand

  Geology,

  Hydrology And Hydrogeology.
• Vegetation cover delay the flow and increases the infiltration and interception, there by reducing the peak flow and protects the soil erosion.
• The more the intensity of rainfall in the watershed, the more is the peak flow disposal from the area.
• If the duration is less, time taken to dispose with lower peak flow is also less.
• If the climatic condition is dry before the rainfall, loss of runoff is more due to infiltration and evapotranspiration.
• A survey of people living in the watershed should be made regarding their:

  ▪ Need & wants

  ▪ Economic condition

  ▪ Health & hygiene

  ▪ Farming practices

  ▪ Precipitation in watershed management.