Evaporation calculation

Manual section:1


vaporize [--traceback] config_file


vaporize calculates evapotranspiration with the Penman-Monteith method. It works either with GeoTIFF files or with time series files. In either case, it reads files with temperature, humidity, solar radiation, pressure and wind speed, and produces a file with evapotranspiration. The details of its operation are specified in the configuration file specified on the command line.

The methodology used is that of Allen et al. (1998). Details can be found in evaporation — Calculation of evaporation and transpiration and in the code itself, which has comments indicating which equations it uses.


To install vaporize, see Installation.

How to run it

First, you need to create a configuration file with a text editor such as vim, emacs, notepad, or whatever. Create such a file and name it, for example, /var/tmp/vaporize.conf, or, on Windows, something like C:\Users\user\vaporize.conf, with the following contents (the contents don’t matter at this stage, just copy and paste them from below):

loglevel = INFO

Then, open a command prompt and give it this command:


vaporize /var/tmp/vaporize.conf


C:\Program Files\Pthelma\vaporize.exe C:\Users\user\vaporize.conf

(the details may differ; for example, in 64-bit Windows, it may be C:\Program Files (x86) instead of C:\Program Files.)

If you have done everything correctly, it should output an error message complaining that something in its configuration file isn’t right.

Configuration file example

Take a look at the following example configuration file and read the explanatory comments that follow it:

loglevel = INFO
logfile = C:\Somewhere\vaporize.log
base_dir = C:\Somewhere
albedo = 0.23
nighttime_solar_radiation_ratio = 0.8
elevation = 8
step_length = 60
unit_converter_pressure = x / 10.0
unit_converter_solar_radiation = x * 3600 / 1e6

With the above configuration file, vaporize will log information in the file specified by logfile. It will calculate hourly evaporation (step_length) at the specified elevation with the specified albedo and nighttime_solar_radiation_ratio (these three parameters can be GeoTIFF files instead of numbers). For some variables, the input files are in different units than the default ones (hPa instead of kPa for pressure, W/m² instead of MJ/m²/h for solar radiation) and need to be converted (unit_converter).

If the base_dir contains tif files, the calculation is performed once for each one of the sets of files; for example, if inside base_dir there are files file:temperature-2014-10-12-18-00+0200.tif, file:humidity-2014-10-12-18-00+0200.tif, and so on (including variables named wind_speed, pressure, and solar_radiation), there will be a resulting file file:evaporation-2014-10-12-18-00+0200.tif; if there are files for other dates, there will be a result for them as well. The calculation is performed only if the resulting file does not already exist, or if at least one of the input files has a later modification time. If there are any evaporation-….tif files without corresponding input files, they will be deleted.

If the base_dir contains hts files, the calculation is performed for these time series. For example, if inside base_dir there are files file:temperature.hts, humidity.hts, and so on, there will be a resulting file file:evaporation.hts, overwriting any previously existing such file.


The configuration file has the format of INI files, but without sections.



Optional. Can have the values ERROR, WARNING, INFO, DEBUG. The default is WARNING.


Optional. The full pathname of a log file. If unspecified, log messages will go to the standard error.


The directory in which vaporize will look for input files and write output files. If unspecified, it is the directory from which vaporize was started.


An integer indicating the number of minutes in the time step. In this version, vaporize can only handle hourly (60) or daily (1440) time steps.


Meters of the location above sea level; this can be either a number or a GeoTIFF file with a digital elevation model.


(Hourly step only.)

In order to estimate the outgoing radiation, the ratio of incoming solar radiation to clear sky solar radiation is used as a representation of cloud cover. This, however, does not work during the night, in which case nighttime_solar_radiation_ratio is used as a rough approximation of that ratio. It should be a number between 0.4 and 0.8; see Allen et al. (1998), top of page 75. It can be a number or a GeoTIFF file.


A number between 0 and 1 or a GeoTIFF file with such numbers. It can also be a list of twelve space-separated numbers and/or GeoTIFF files, where the first is for January, the second for February, and so on. For example:

albedo = albedo-jan.tif albedo-feb.tif albedo-mar.tif albedo-apr.tif
         albedo-may.tif albedo-jun.tif albedo-jul.tif albedo-aug.tif
         albedo-sep.tif 0.23           albedo-nov.tif albedo-dec.tif

Note that in the configuration file long lines can be wrapped by indenting the additional lines. Also note that GeoTIFF files can be mixed with numbers; in the above example, GeoTIFF files are specified for all months except for October, which has a single value of 0.23.

If a single number or GeoTIFF file is specified, it is used for all the year.


The meteorological values that are supplied with the input files of the file set sections are supposed to be in the following units:

Parameter Unit
humidity %
wind speed m/s
pressure kPa
solar radiation MJ/m²/step
sunshine duration h

If they are in different units, unit_converter_temperature, unit_converter_humidity, and so on, are Python expressions that convert the given units to the above units; in these expressions, the symbol x refers to the given value. For example, if you have temperature in ℉, specify:

unit_converter_temperature = (x - 32.0) * 5.0 / 9.0

Use 32.0 rather than 32, and so on, in order to ensure that the calculations will be performed in floating point.

You can also use this to convert wind speed to a different height. Wind speed at 2 m from the ground is required. If you have wind speed at a different height, convert it using Eq. 47, p. 56, of Allen et al. (1998). For example, if you have wind speed at 10 m, specify this:

unit_converter_wind_speed = x * 4.87 / math.log(67.8 * 10 - 5.42)

Optional. vaporize assumes that the input files are named variable-date.tif or variable.hts, where variable one of temperature, temperature_max, temperature_min, humidity, humidity_max, humidity_min, wind_speed, pressure, solar_radiation, and sunshine_duration, and, similarly, for the output file variable is evaporation. With these parameters these names can be changed; for example:

humidity_prefix = hum

In that case, the humidity files are going to have a name similar to hum-2014-10-12-18-00+0200.tif (for hourly) or hum-2014-10-12.tif (for daily).

vaporize will use the pressure if it is available in the input files, otherwise it will calculate it from the elevation.


R. G. Allen, L. S. Pereira, D. Raes, and M. Smith, Crop evapotranspiration - Guidelines for computing crop water requirements, FAO Irrigation and drainage paper no. 56, 1998.