Hydrology BC ENV 3025

HYDROLOGY - Homework # 3 - due 2/9/2012

1) (12 points) Precipitation statistics. On the National Climate Data Center webpage find the weather station that is closest to your place of birth (or another station of interest) that has at least 50 years of record of preciptation. Look for one station (best by state) and then select the 'total monthly precipitation' dataset (TPCP). I recommend the comma delimited file format. You then have to enter your e-mail address (might have to have an .edu extension, so use your CU/BC address & you may have to login through an .edu domain in order to not being charged for the data) and you'll receive a link to the data set by e-mail.

a) Download the data and import them into EXCEL. The last column has annual values - but you might have to sum up the monthly precipitation values. Determine minimum, maximum, average and standard deviation of the annual precipitation data using the equivalent EXCEL functions (MIN, MAX,AVERAGE,STDEV). Note that precipitation is measured in hundreds of inch, i.e. 1000 = 10 inches.

b) Make a histogram of the precipitation data (see example below), print it out and hand it in.

c) Determine the amount of annual precipitation that is exceeded every 20 years and that the precipitation falls short of every 100 years.

For help with determining probabilities under the Normal distribution, see this tutorial.
If you have a Mac, download this extension to Excel, it gives you similar options as the PC version of MS Excel
Example discussed in class: New York/LGA data and the histogram (Fig)

2) (6 points) Reynolds number

Calculate the Reynolds number for the NYC water tunnels, assuming an inner diameter of 24ft. There are two of these tunnels in operation, determine the flow velocity from the amount of water used in the city. The city uses ~ 1.1 billion gal/day. Is the flow turbulent or laminar?

3.) (10 points) Potential evaporation Measurement of changes in volume of water in an evaporation pan is a standard technique for estimating potential evapotranspiration. United States Class A evaporation pans are cylindrical with the following dimensions: depth = 10.0 inches, and diameter = 47.5 inches. An evaporation pan can be considered a hydrological system with an inflow, outflow, and storage volume. Evaporation from pans is not the same as evaporation from natural surfaces for a range of reasons. For example, water temperatures in shallow pans will be much more variable than temperatures in a nearby lake. Evaporation measured in pans is adjusted by a factor called a pan coefficient to convert to an estimate of potential evapotranspiration.

a) Calculate the cross sectional area (m²) of a class A evaporation pan and the volume. Initially the pan contains 10 US gallons of water. Calculate the depth of water in the pan. Assuming a water density of 997.07 kg m^-3 (25^oC), calculate the mass (kg) of water in the pan. After 24 hours in an open field (no precipitation), the pan is checked and the volume of water left in the pan is determined to be 9.25 gallons. Calculate the evaporation rate (mm hr^-1) from the pan.

b) Calculate the flux of latent heat from the water in the pan to the atmosphere (W m^-2).