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

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

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

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

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

Boundary Layer Aerodynamics From Chow et al., 1988

Combined Method (Penman) (p309) Vapor pressure deficit (drying power of the air) Equation 7.3.20 in text missing exponent 2 Does not require surface information – uses only meteorological information

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

Method Information Requirements Mass Transfer Energy Balance Combination RN

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