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 evapo- transpiration (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)
(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).
(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 cor- related with ET? Add any observations of your own.
