University of Colorado - Civil, Environmental and Architectural

University of Colorado
Department of Civil, Environmental and Architectural Engineering
CVEN 4333 Engineering Hydrology
Spring2005 – Midterm
Date: 3/15/2005
Time: 75min
Total: 40 points
Please answer on separate sheets of paper. This is an open book/open notes test. You
may use a programmable calculator. Write the steps CLEARLY so you can get
partial credits. Problems 4(b) and 4(c) are optional, but will fetch bonus points.
Water Budget:
1. Lake Powell has a volume of about 20 million acre feet. Streamflow into Lake Powell
averages 13800 ft3/s.
(a) Calculate the average residence time of water in Lake Powell [1 acre feet = 43560 ft3].
(b) Lake Powell has an average surface area of 406 mi2. If the annual evaporation is 50
inches what fraction of inflow is lost annually to evaporation? What average flow rate is
available for release to downstream users and for power use? [neglect seepage losses; 1
mi2 = 2.79 x 107 ft2; 12 in = 1 ft.].
Atmospheric Water
2. Consider the following idealization of a thunderstorm.
Cloud top 6000 m
Diameter D = 1 km
Updraft velocity 1 m/s
Moist air input with constant specific humidity,
relative humidity 70%, Air temperature 20 °C
Sea Level
Estimate the following
a) Condensation (cloud base) level.
b) Temperature and humidity of air exiting the system at 6000 m.
c) Precipitation rate
d) What are the dry and saturated adiabatic lapse rates?
Gas constant of dry air Rd = 287 J/Kg.K
[Hints: Use graphical interpolation off the pseudo-adiabatic diagram, in which saturation
mixing ratio is equivalent to specific humidity]
3. (a) In the global warming debate – identify the positive (warming) and negative
(cooling) feedbacks to the global temperature.
(b) Name the dominant mechanism of precipitation in the tropical regions and in the midlatitudes (i.e. in our region here)? What are some unique characteristics of these two
(c) What is one of the most significant driver (i.e. year to year variability) of global
weather and climate?
(d) What is ITCZ and what is its role?
(e) How does ‘Hadley circulation’ come about?
4. Measurement of evaporation at a location yields 0.5 cm/day. The average wind speed,
temperature and relative humidity 2.5m above the surface are 1.5 m/s, 25°C and 60%,
respectively, while the surface temperature is 28°C. The net energy influx is 194 W/m2
Estimate the following:
(a) Bowen Ratio
(b) Surface Pressure
(c) Effective surface roughness parameter (i.e., zo)
4. Consider a ripe melting snow pack (i.e., Ts = 0°C).
Considering these extremes what difference would the selection of zo make to
Calculate the sublimation on a day when the air temperature is 5 °C, wind speed 2 m/s
and relative humidity of 80%. Air temperature and relative humidity were measured at a
height of 2 m above the snow surface. The roughness height zo of snow varies from 0.05
to 0.5cm.
Which of sensible heat and latent heat is more important in this
[20 points]
2. At a weather station near a lake the following measurements are made
Air pressure
Air Temperature
Dew Point
18 oC
Net radiation
60 W/m2
Wind Speed
2.3 m/s at a height of 1.5 m.
Water temperature
21.8 oC
(a) Calculate the vapor pressure, relative humidity, specific humidity and air density
(b) Assuming negligible heat flux into the lake, z0 = 0.04cm and the heat capacity of air
at constant pressure Cp = 1005 J/kg/ oK. Estimate the rate of evaporation from the lake
using whichever of the following methods you have sufficient information for
(i) Energy Balance
(ii) Aerodynamic
(iii) Combined/Penman
(iv) Priestley Taylor
Which estimate you think is the most accurate and why?