Rivers, Floods and Management: Drainage Basin Hydrological Cycle

Rivers, Floods and Management: Drainage Basin Hydrological Cycle

1. What is a Drainage Basin?

   • A drainage basin is the area of land that is drained by a river and its tributaries.
   • The drainage basin acts largely as a 'closed' system, because it has a very definitive outer edge or boundary called the watershed.
   • Key Words
     • Precipitation: The type, total amount and intensity are key factors in determining the nature of water movement.
     • Evapotranspiration: When water loss from the ground surface to the atmosphere (evaporation), combines with water loss from plants (transpiration) to form the main output from the system.
     • Interception: Vegetation, especially trees, intercepts some precipitation on its way to the ground. Water is then lost back into the atmosphere by evapotranspiration. intercepting plants also use the water for growth.
     • Depression Storage: When water is stored temporarily on the ground surface in the form of puddles
     • Soil Moisture: If soils is saturated, precipitation will flow as overland flow. Clay soils are wet and boggy (leading to overland flow). Sandy soils are much drier so have the opposite effect.
     • Baseflow, or Groundwater Flow: This is a very slow transfer of water through rocks.
     • River Channel: This is the river itself. Also forms the 'exit' for water transferred through the drainage basin
     • Percolation: This is the deeper transfer of water into permeable rocks - those with joints (pervious), or those that are porous.
     • Throughflow: Downhill transfer of water of water through the soil layer to the river.
     • Infiltration: Water moving from the ground surface into the soil.
     • Overland Flow: Rapid form of water transfer over the surface of the ground.

2. The Water Balance

   • In order to get a better understanding of water resources in a drainage basin, we use an equation called the water balance.
   • P = O + E +/- S
     • P = Precipitation
     • O = Total runoff (streamflow)
     • E = Evapotranspiration
     • S = Storage (in soil or rock)
   • An important aspect of the equation is the amount of runoff - expressed as a percentage of precipitation. It is a measure of the proportion of total precipitation that makes its way into streams and rivers

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