Lecture4 2 interception and ET dung

VFU- 2016 Lecture #4 Evapotranspiration Principles of Watershed management Dr Bui Xuan Dung- Department of Environment Management Evapotranspiration Returning water to the atmosphere Evapotranspiration (ET) • Composed of two subprocesses – Evaporation occurs on surfaces of open water or from vegetation and ground surfaces – Transpiration is the removal of water from the soil by plant roots, transported through the plant into the leaves and evaporated from the leaf’s stomata • Typically combined in mass balance equations because the components are difficult to partition Evapotranspiration Evaporation Open Water Soil Vegetation Surfaces Transpiration Plants The Processes • Free-water evaporation – Open water surfaces • Lakes, rivers; – Other sources of water • Vegetation surfaces, soil surface • Transpiration • Roots  Stem  Leaves  Stomata  Atmosphere Control on Evaporation Transpiration  Transpiration is the loss of water in the form of vapor from plants  Factors that affect transpiration rates  Type of plant  Wind  Temperature  Humidity  Water supply Transpiration coefficient (TC): Water amount of transpiration/dry mass production TC: 200 đến 1.000 Potential Evapotranspiration (PET) A measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply Actual Evapotranspiration (AET) The quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration ET for watershed scale Potential evapotranspiration (PET) Definition by Penman (1948) The amount of water transpired in unit time by a short green crop completely shading the ground, of uniform height and never short of water Therefore, PET based on atmospheric conditions and a specific vegetation type More general definition: Potential evapotranspiration is a water loss from the soil surface completely covered by vegetation Rate of energy use for evaporating water Rn = (L)(E) + H + G + Ps LE = Rn - H - G - Ps If we assume Ps is very small relative to the other energy LE = Rn - H - G Rate of water loss from surface as ET LE Eeb  pw Lv W/m2 Watt per square meter Pw: density of water (1000 kg/m3) Lv: latent heat of vaporization (2.47x106 J/kg) Net radiometer Measurement of flux of short and longwave Lysimeter- Thẩm kế • A device to measure the quantity or rate of downward water movement through a block of soil usually undisturbed, or to collect such percolated water for analysis as to quality • Defined as: – A small unit of soil on which water balance values can be obtained  Lysimeters account for change in water storage  i.e Measure actual evapotranspiration Lysimeter for measuring potential evapotranspiration input (Rainfall R and Additional water A) and output (Percolated water P) collected in the receiver, then PE can be estimated from the equation: PE = R + A – P R A P Principles of a Lysimeter • A tank filled with soil is weighted on a scale – The difference in weight between the beginning and the end of the day indicates how much water was lost during the day, or, – how much water the crop used • At midnight (or some standard time) – water tank below the lysimeter is filled with water that can be used for irrigation during the day – ie no weight change as a result of irrigation during the day Three tank method Tanks A and B: no bottom side How measure ET and Percolation? Example for Lysimeter Good method, but expensive for installation!! Mass balance measurement using lysimeter EvaporativeFLUX  m1  m2  w  Dt  A m1 and m2: mass at the beginning and end of time interval Dt : duration of time interval A: Cross sectional area of lysimeter Priestley-Taylor equation for calculation of Potential Evapotranspiration s Rn  G E p  1.26 s    w Lv S: slope of saturation vapor density and air temperature : psychrometric constant Figure Evapotranspiration over grass at Vancouver Airport 0.7 0.6 Elys Eeb Ep Estimated ET E (mm/hr) 0.5 0.4 0.3 0.2 0.1 Actual ET based on lysimeter 0700-0800 1000-1100 1300-1400 Time 1600-1700 Penman Method Using Meteorological data for estimation of PET Slope of saturation vapor curve PET  Net radiation Wind function DRn   (es  ea ) f (u ) (D   ) L Latent heat vaporization Psychometric constant Penman-Monteith Method Using Meteorological data, but including some Vapor pressure of vegetation factors Air density Specific heat of surface and air air PET  DRn  C p (es  ea ) / D   (1  rst / ) Stomatal resistance Aerodynamic resistance Bordeaux (Le Bray site): Eddy covariance installations at canopy level in a French pine forest Evapotranspiration at various locations in forested areas Annual ET (mm) Tropical forest Japan Annual rainfall (mm) ... as ET LE Eeb  pw Lv W/m2 Watt per square meter Pw: density of water (1000 kg/m3) Lv: latent heat of vaporization (2. 47x106 J/kg) Net radiometer Measurement of flux of short and longwave Lysimeter-... measure ET (evapotranspiration); • Usually determine ET by: ET = P - Q Example: Elk River Catchment Precipitation – Runoff = ET (assuming no DS) 1000 mm - 400 mm = 600 mm ET ~ 600 mm Precipitation ET. .. coefficient that varies with seasons and lake Its annual average over the US is about 0.7 ET measurement ET measurement • Water balance (water budget): Q = P - ET – Can measure Q (runoff); – Can