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