1.1 The Drainage Basin

  • Outputs:

    Evaporation: is the process by which a liquid is changed into gas.

    • Factors affecting evaporation:

      • Meteorological factors : e.g. temperature , humidity and wind speed

      • Amount of water available

      • Vegetation cover

      • Colour of the surface (albedo or reflectivity of surface)

    Transpiration - is the transfer of water vapor from vegetation to the atmosphere, occurring primarily through pores in leaves called stomata.

    Evapotranspiration (EVT)- refers to the combined water loss through both evaporation and transpiration.

    Potential evapotranspiration - is the water loss that would occur if there were an unlimited supply of water in the soil available for vegetation.

    River discharge - is water that flows out of a basin into a ocean

  • Stores

    Interception- refers to precipitation caught and stored by vegetation before it reaches ground

    Soil water/soil moisture - is the subsurface water in soil and subsurface layers above the water table.

    Surface Water- when the infiltration capacity is exceeded, water builds upon the surface

    • Types of surface water stores:

      • Temporary stores: small puddles after a rainstorm and turloughs ( seasonal lakes in Limestone, Ireland)

      • Permanent stores – lakes , wetlands ,swamps ,peatbogs and marshes

    Ground water - refers to the sub surface water that is stored under the surface in rocks

    • Groundwater accounts for 96.5% of all freshwater on Earth. While soil moisture can be recycled into atmospheric moisture within days or weeks through evaporation, groundwater may take up to 20,000 years to complete its recycling process.

    Channel storage -refers to all water that is stored in rivers, streams and other drainage channels

    • Some rivers are seasonal, and some may disappear underground ,either naturally or in areas of carboniferous limestone or Urban areas, where they may be covered (culverted)

  • Above ground flows

    Through fall - water that flows through gaps in vegetation or drops from leaves and twigs

    stemflow - is precipitation that is intercepted and trickles along branches then down the main trunk.

    overland flow - when soil is saturated, or precipitation exceeds infiltration rate, surface runoff occurs – where water flows over the surface.

    • Common in areas of high precipitation and low infiltration

    channel flow / stream flow - is the movement of water in channels such as streams and rivers

    Hortonian Flow - shallow, laminar, fast moving water that causes severe soil erosion when precipitation exceeds the infiltration capacity and depression soil capacity.

  • Below ground flows

    Infiltration - when precipitation soaks/is absorbed into soil.

    • Infiltration Capacity: maximum rate that precipitation can be absorbed by soil in given conditions.

    • Infiltration is inversely proportional to overland runoff. Depends on: rainfall duration, antecedent soil moisture, porosity, slope angle, vegetation.

    Factors affecting infiltration:

    On bare soil : Infiltration on bare soil is slow because the water arrives too quickly to be absorbed by the soil

    Infiltration where there is vegetation: infiltration rate increases where there are vegetation (such as trees),due to throughfall , stemflow and interception slowing the rate that water reaches the store.

    percolation- slow movement of water downwards through the soil into bedrock under gravity.

    • Fast in Carboniferous Limestone.

    • Rate depends on permeability and porosity of bedrock. Chalk and sandstones are porous, spaces allow water to percolate.

    throughflow- when water flows through the soil in natural pipes or percolines. Occurs above bedrock.

    groundwater - is water that has infiltrated the ground, entered the phreatic zone and discharged into the channel.

    Phreatic zone - part of an aquifer (permeable rocks and sediments that can hold groundwater or allow it to pass through) below the water table where all pores are permanently saturated.

    baseflow - where groundwater seeps into the river’s bed and contributes to discharge. Very slow transfer from bedrock and very deep throughflow. Takes months/years.

  • Underground water

    Water tables - is upper level of phreatic zone at which pore spaces and fractures in the ground become saturated

    Ground water - is primarily stored in aquifers, which are underground layers of water-bearing permeable rock, sand, or gravel. The capacity of these aquifers to store water depends on their size, material, and the amount of space between their particles.

    Recharge - is the refilling of water in pores where the water has dried up or been extracted by human activity.- In places where recharge is not taking place, groundwater is considered a non-renewable source.

    • Recharge occurs with:

      • Infiltration – total precipitation at ground surface

      • Seepage –through banks and bed of surface water bodies like ditches and rivers

      • Groundwater leakage & inflow – from adjacent rocks and aquifers

      • Artificial recharge – from irrigation & reservoirs

    • Loss occurs with:

      • Evapotranspiration – especially in low lying areas where water table is close to ground surface

      • Natural discharge – through spring flow and seepage into surface water bodies

      • Groundwater leakage and outflow - along aquicludes to adjacent aquifers

      • Artificial abstraction

    Springs - when waterflow reaches the surface , making a spring – usually occurs when permeable rocks meets impermeable rocks causing infiltrated water to build up and escape as a spring or a source

    Aquifers – are permeable rocks (such as sandstone/limestone) that contain significant quantities of water.

    • Water in Aquifers move slowly and act as a regulator absorbing rainfall that would otherwise reach streams rapidly, and they help to maintain stream flow through dry periods

    Aeration zone / vadose zone – is the zone that is seasonally wetted and seasonally dries

Soil Moisture:

Soil Moisture: water held in sub-surface in soil pores

SM Deficit: available water is being used up.

SM Recharge: precipitation > potential evaporation. Some dry pores refill.

SM Surplus: soil is saturated, water cannot enter, so flows over the surface.

SM Utilisation: evapotranspiration (and other water uses) > precipitation.

Field Capacity: amount of water held once excess has drained away – saturation point.

Wilting Point: the range of soil moisture content at which permanent plant wilting occurs.

Balance = Precipitation – (run off + evapotranspiration + change in soil moisture).

Characteristics:

Sandy soils

  • have many large pores, so is permeable, and has quick infiltration rates.

  • Gaps between sand and pores make it porous

  • Loses water quickly

  • Infiltration rate 6-12 mm/hr

Clay

  • Clay are hygroscopic – clay minerals swell when in contact with water, making it impermeable and unstable.

  • Holds water well

  • Infiltration rate 0-4 mm/hr

Water budget - is the balance between precipitation, potential evapotranspiration and runoff

Equation: S = P – Q – E

S= soil storage

P = precipitation

Q = Channel Flow out of basin

E = Evapotranspiration

  • the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.