C**E
374K Homework on Evaporation
and Precipitation**

**Hydrology Spring
2001**

** **

This assignment uses a 24 hour period of climatic and energy
balance data recorded at the Southern Great Plains Atmospheric Radiation
Measurement site in Washita Oklahoma for July 29, 1998. Download the Excel comma delimited
files: energy.csv
and climate.csv or get them from the LRC Class
server: **/class/maidment/ce374k/evap**.

1. The Energy
Balance worksheet shows in half-hourly intervals, the Net radiation (R_{n}),
Latent heat flux (lE), Sensible heat
flux (H), and Ground heat flux (G), all in W/m^{2}, and the Bowen ratio
measured by climatology apparatus at the site. Note that the signs on the tabulated energy fluxes are positive
when the flux is downward, negative when the flux is upward. Verify that the energy balance is closed at
each time interval.

2. Make a plot showing
the time distribution over the day of Net radiation (R_{n}), Latent
heat flux (lE), Sensible heat flux (H),
and Ground heat flux (G), all in W/m^{2}. Make a table showing for R_{n}, lE, H, G, and R_{n} - G:

- daily
average value in W/m
^{2} - amount
of energy received in MJ/m
^{2}-day - equivalent
amount of water evaporation in mm/day, assuming l = 2.5 MJ/kg of water, and density of water = 1000 kg/m
^{3}

3. The Climate
worksheet shows Wind Speed in m/s (u), Air Temperature in °C
(T), Relative Humidity in % (R_{h}), and Atmospheric Pressure in kPa
(p). These variables are measured at
2m above the ground. Make a plot
showing Wind Speed and Air Temperature as a function of time during the day. Make a second plot showing Relative Humidity
and Air Pressure as a function of time during the day. What can you conclude by inspection of
these plots about the diurnal variation of these quantities? What are the average values of these
quantities for the day?

4. Calculate the
value of the Saturated Vapor Pressure (e_{s}), Vapor Pressure (e), and
the Vapor Pressure Deficit (D), all in kPa for each half-hourly interval, and
make a plot of these quantities. When
does the main period of vapor pressure deficit occur during the day? Calculate the daily average values of
these quantities.

5. Calculate the
value of the specific humidity, q_{v}, and the density of moist air, r_{a}, for each half-hourly interval
and make a plot of these quantities.
Use equation (3.2.8) in Applied Hydrology to get the value of the gas
constant for moist air R_{a}.
What is the average value and the range of these quantities during the
day? When do the maxima and minima
occur?

6. Using the tabulated Latent Heat fluxes in the Energy Balance worksheet, calculate the time distribution of the evaporation rate expressed in mm/day for each half-hourly interval. Assume that evaporation upward is positive. Allow for the variation of the latent heat of vaporization with temperature in this calculation. What is the average evaporation rate for the day? How does it compare with the value you obtained in (2)? What are the maximum and minimum rates of evaporation during the day and when do they occur?

7. Using the daily
average values of R_{n} - G, and the climate variables that you’ve
computed previously in this exercise, estimate the daily average evaporation
using the combination equation (3.5.26) in Applied Hydrology, in which the
aerodynamic component of evaporation, E_{a}, is determined using the
wind function for B given by equation (3.6.1). Compare the value you’ve determined with the actual daily
evaporation measured at the site. By
what percentage do they differ? The
values of D (DeltaEs) and g (Gamma) in the combination equation (3.5.26)
can be taken from the table evapcoeff.csv, which is
attached to this homework.

8. Do Problem 3.4.3 in Applied Hydrology

This problem set is due in on Tues Feb 13.