1. Hydrologic Losses - Evaporation
Learning Objectives
Be able to calculate Evaporation from a lake or reservoir using the following methods
Energy Balance (p304)
Aerodynamic Method (p307)
Combined Method (p309)
Priestley-Taylor (p309)
Be able to select a method appropriate for the information available

2. Why do we care about Evaporation and Transpiration (ET) ?
70% of precipitation that falls on the US is evaporated
Water vapor role in atmospheric circulation, land feedback to atmosphere, understanding and predicting climate change
Q=P-ET. P-ET is the water available for use
ET "loss" supports ecosystems and agriculture
Reservoir losses
The antecedent "wetness" that determines what happens to runoff depends on ET

3. Energy Balance at the Earth Surface
Rn = G + Hs+ LE
Latent Heat of Vaporization
J/kg
LE = lvwE
J/m2/s or W/m2
From Mays, 2011, Ground and Surface Water Hydrology

4. Evaporation Example (7.3.1)
Given net radiation of 185 W/m2, air temperature 28.5 C, RH=55%, wind speed = 2.7 m/s at height of 2 m
Calculate Evaporation by
Energy balance (assuming Hs and G are zero)
Aerodynamic method
Combined method
Priestley-Taylor method

5. Aerodynamic Method (p307)
Evaporation rate as a function of water vapor pressure gradient and turbulent transport
Primary inputs
u – wind speed typically at 2 m
es – saturation vapor pressure at the surface
ea – vapor pressure of the air, depends on temperature and relative humidity
B(u) depends on wind and surface roughness
Does not require Energy information

6. Boundary Layer Aerodynamics
From Chow et al., 1988

7. Combined Method (Penman) (p309)
Vapor pressure deficit (drying power of the air)
Equation in text missing exponent 2
Does not require surface information – uses only meteorological information

8. Priestley – Taylor Method (p309)
~70%
~30%

9. Method Information Requirements
Mass Transfer
Energy Balance
Combination RN

10. Summary Energy exchanges and energy balance
Turbulent diffusion into the atmosphere
Adjustment and balance RN + -
Conditions adjust to varying inputs. Calculations can interpret measurement, but should not be used to predict the effect of changing one variable without considering the adjustments of connected variables