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

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

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=0.0118 for Lake Mead) Does not require Energy information

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

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

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

Figure 1-19

Method Information Requirements Mass Transfer Energy Balance Combination QN

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