53:119 Hydrology
Problem #11
Evaporation Methods

The following meteorological conditions (average for 10 September 1950) were recorded at Lake Hefner, OK:

Air Temperature (°C)22.3
Relative humidity (%)68
Wind Speed (m/s)2.16
Lake Surface Temperature (°C)23.7
Pressure (mb)973
Net Radiation (W/m2)92

Estimate the open-water evaporation rates (in mm/day) using:

  1. A mass-transfer method (see below).
  2. Combination approach (Penman).

The mass-transfer equation for Lake Hefner (Bras, p. 207) is:

Qe = 3.75 U (esfc - ea)

where Qe is the latent heat flux (W/m2), U is the wind speed (m/s), and es is the vapor pressure (mb) at the lake surface (assumed to be saturated at the lake temperature), and ea is the vapor pressure (mb) at the reference height of the meteorological station.

Last changed on 09/07/12 by aab.