Water Science and the Environment

Water Science and the Environment
Latent heat, evapotranspiration; processing large data sets
Problem 1.
Use latent heat amount (LE) data measured using eddy covariance instruments to compute evapotranspiration (ET). The LE and ET are related via the latent heat of vaporization ():
ET = LE/
where:
ET is in mm/s (millimeters of water per second)
LE is in W/m2 (Watts per square meter)
 = 2500000 J/kg (Joules per kilogram of water)
Required
Optionally: do steps 1-3 below for extra credit (10 points). Otherwise, go to step 4 below (no
extra credit).
(1) Download the data (use the procedure below)
The data source is http://ameriflux.ornl.gov/
Choose the following:
Site name: “Santa Rita Mesquite Savanna / US-SRM” [This is south of the Tucson airport.]
Type of available data product: L2
Date available: from 01/01/2008 to 12/31/2010
Meteorological variables (marked by red letters): LE (latent heat flux), PREC (precipitation),
SWC (soil water content) and TA (air temperature).
Click on “Create dataset” and fill out the form that appears.
Download the data set created by the server. It is a .csv file, readable by MS Excel.
(2) Some data points are missing (they are marked as “-9999”). Remove them from the dataset.
(3) Compute a 48-point (daily) moving average to smooth out the data (to remove noise).
Alternatively (no extra credit): download already processed daily data here:
http://quebec.hwr.arizona.edu/classes/hwr201/Santa-Rita-data.xlsx
(4) Convert LE data to ET data using the formula given above. Convert from the units of mm/s to
mm/day (multiply by 60*60*24).
(5) On four separate graphs plot the following variables as a function of time: your computed ET,
PREC, your computed average SWC and TA. [Note: time should be on the horizontal axis and the
variable of interest on the vertical axis.]
(6) Write a short explanation of the data. How does ET change over the three-year period? Are
there any regularities? Seasonal trends? Explain the maxima and minima in ET values using the
values of PREC, SWC and TA, as well as the common sense. Which of them is most strongly correlated with ET? Add any observations of your own.