Climate

• rivers are fed by precipitation (P) of which some is used for evapotranspiration (ET) and the remainder is called effective precipitation
• most important fluxes in catchment: precipitation, evapotranspiration and discharge
• forms of precipitation: rain, snow, hail, fog, dew
• stratiform/frontal precipitation: characterised by prolonged periods of extensive areas with low precipitation intensity
• stratiform clouds: wide and relatively thin blankets with homogeneous grey and white colours
• convective precipitation: characterised by short-term, intensive showers on local scale
• convective clouds: small and towering with vertical structures formed by upward movement of water droplets
• spatial variability in precipitation caused by

  • short term: size and shape of clouds
  • long term: high downwind of areas where much water evaporates and windward side of mountains
• rain shadow: smaller amount of precipitation on downwind side of mountains
• average annual precipitation for world: 1000 mm/year
• average annual precipitation for Europe: 300-2500 mm/year
• average annual precipitation in NL: 750-950 mm/year
• precipitation events (showers/storms) last several minutes or hours or days with dry spells in between
• in NL it rains 10% of time
• precipitation varies seasonally and yearly
• climate change affects both average annual precipitation sum and type of precipitation
• rain measured with rain gauges (strongly influenced by wind) at one location or with weather radar which senses the location and severity of precipitation event tens of km around instrument (but measures high above the ground)
• evaporation: transition from liquid water to water vapour and requires much energy (heat of vaporisation)
• sun provides short-wave radiation
• reflected sunlight is called albedo and depends on land cover
• earth emits long wave radiation which is partly reflected back to earth by clouds
• net radiation: difference between incoming and outgoing radiation at earth surface (both short and long wave radiation)
• sensible heat flux: radiation heating land surface and air
• latent heat flux: radiation leading to evaporation
• soil heat flux: radiation heating ground
• total evapotranspiration (ET) consists of transpiration, interception evaporation, soil evaporation and open water evaporation
• transpiration: water lost through stomata of plants
• interception evaporation: evaporation of precipitation which falls on vegetation and does not reach soil (short term)
• soil evaporation: evaporation from moist soil
• open water evaporation: evaporation from lakes or rivers
• net radiation and meteorological factors impact amount of evapotranspiration

  • high air temperature, low air humidity, high wind speeds increase evapotranspiration
• land use impacts evapotranspiration

  • more water evaporates from land surface
  • large differences between vegetation types
• spatial variability in evapotranspiration caused by spatial variability in controlling factors (meteorological factors, land use, soil moisture) which are influenced by water management
• evaporation pan: container with water in which level is measured with the decrease giving open water evaporation
• lysimeter: container filled with local soil and vegetation which is either weighed or outflow at bottom is measured
• reference evapotranspiration (ETref): ET that theoretically could take place for actual meteorological conditions over well-watered grassland
• potential evapotranspiration (ETpot): ET that theoretically could take place for actual meteorological conditions and local vegetation
• $ETpot =$$f*$$ETref$

  • f = crop factor
• crop factor depends on type of vegetation and varies seasonally
• actual evapotranspiration (ETact): ET that occurs in reality and determined from potential evapotranspiration and current soil moisture content in rooting zone

  • wet areas: ETact = ETpot