1. Hydrologic Losses - Evaporation
Learning Objectives
Be able to calculate Evaporation from a lake or reservoir using the following methods
Water budget (p56)
Mass transfer (p56-57)
Energy budget (p57-58)
Combined method (p59-61)
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. Mass Transfer Method (p56)
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 – depends on Ts
ea – vapor pressure of the air – depends on temperature and relative humidity
Empirical constants a, b (e.g. b= for Lake Mead)
Does not require Energy information

4. Evaporation Energy Balance
cal cm-2 day-1
cal/g T in C
cm/day

5. Energy Balance Method (Bowen Ratio) (p57-58)
Primary inputs
Available Energy
Temperature gradient
Vapor pressure gradient
Does not require wind information

6. Combined Method (Penman) (p59-61)
Equation 1-19 – Air drying power
Equation 1-18
Does not require surface information – uses only meteorological information

7. Figure 1-19

8. Method Information Requirements
Mass Transfer
Energy Balance
Combination QN

9. Summary Energy exchanges and energy balance
Turbulent diffusion into the atmosphere
Adjustment and balance QN + -
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