Homework No. 2

1.         For Lake Nasser, Egypt, in July, the average net radiation is 185 W m^-2; the mean daily air temperature is 28.5 ºC, the relative humidity is 55 percent and the wind speed is 2.7 m s^-1 at z=2 m. At the above temperature D/g  is equal to 3.6. Assume z_o=0.0003 m and p=101300 Pa.

a.       Obtain the open water evaporation rate in millimeters per hour using the Thornthwaite-Holzmann aerodynamic or mass transfer method.

b.      Obtain the open water evaporation rate in millimeters per hour using the Penman equation. Use both the aerodynamic method and the empirical Penman equation for the evaluation of the drying power of the air, and compare your results. For these conditions, what is the Bowen Ratio?

c.       Obtain the equilibrium evaporation rate, E_e.

d.      Obtain the Bowen Ratio implied by the equilibrium evaporation rate and compare with the Bowen Ratio obtained in part b.

e.       Obtain estimates of the sensible heat flux for these conditions.

f.  Obtain estimates of the evaporation rate under conditions of minimal advection, E_pe. Use as = 1.28. Obtain the Bowen Ratio implied by this rate of evaporation under conditions of minimal advection.

g. Obtain the evapotranspiration rate if the the above conditions apply to a (well-watered) vegetated surface for which the stomatal resistance equals 200 s/m.

2.         At a climate station, the following measurements are made: air pressure = 101100 Pa, air temperature = 25 ºC, and relative humidity = 73.5%.  Calculate the corresponding vapor pressure, specific humidity, water vapor mixing ratio, and air density.

3.      A sample of moist air has a temperature of 280 ºK at a pressure of 90000 Pa, with a mixing ratio of 0.005 (i.e., 5g/kg).  Compute the following quantities for this sample:  vapor pressure, absolute humidity, specific humidity, relative humidity and air density.