Evapotranspiration

Interception

• process by which precipitation falls on vegetative surfaces (the canopy), where it is subject to evaporation  (Fig2.8)
• stemflow (flow along the stem), throughfall (the precip that has avoided interception or dripped off the canopy)
• effects of different kinds of vegetation on capacity and runoff (Fig)
• afforestation could increase evaporation losses by up to 50 to 100%, clearcutting increases runoff

Evapotranspiration

• evapotranspiration summarizes all processes that return liquid water back into water vapor
• water needed and solar energy
• of the water taken up by plants, ~95% is returned to the atmosphere through their stomata (Fig)
•  potential evaporation (PE), i.e. the evaporation rate given an unrestricted water supply - different from actual evaporation
• how can the actual evapotranspiratio be measured?
• water balance
• energy balance
• or combination of both

Estimation of ET from the water balance

• this approach may suffer from the uncertainties in the numbers, example:

p = 10⁷+/- 5 *10⁵ m³/y
r_si = 10⁹ +/- 1.5*10⁸ m³/y
r_so = 9.95*10⁸ +/- 1.5*10⁸ m³/y
ET = p + r_si - r_so

Estimation of ET from the energy balance

• A control volume for energy conservation (Fig2.9), R: solar energy in, H: sensible heat flux  out, El: latent heat flux out, G: sensible heat flux  into ground
• the energy balance for the control volume: dQ/dt = Rn-G-H-E_l
• et = E_l /  (r_w l_v)
• if G, H, and changes in Q over time can be neglected:
• et = Rn/r_w l_v = 200 Wm^-2 / (1000 kg m^-3 * 2.5 10⁶ J kg^-1) = 0.7 cm day ^-1
• in many case, we cannot neglet all the other energy fluxes, see for example measured energy fluxes for a field in CA (Fig 2.10)
• it is not easy to measure the fluxes in the real world, Bowen technique uses the vertical gradient of temperature and vapor pressure
• there are lots of empirical equation describing evapotranspiration, example the Penman equation or combination equation (Fig)
• keep in mind that the actual et depends on
• depth to water table
• soil characteristic
• local heat budget
• precipitation patterns, etc

• Evaporation, global patterns (Fig)
• E higher over oceans than land (except for areas covered by sea ice) (Fig)
• E increases with temperature
• oasis effect
• The Atlantic Ocean is loosing water by evaporation (Fig), this loss is balanced by transport of ocean water from the Pacific ocean (1 Sverdrup = 10⁶ m³ s^-1, equivalent to the discharge rate of all rivers on Earth)

Resources

• Jones, J.A.A. (1997) Global Hydrology. Addison Wesley Longman Ltd., Essex, UK, 399p