Core class for the Daymet functions.

Module Contents#, coords=None, crs=4326, method='hargreaves_samani', params=None)#

Compute Potential EvapoTranspiration for both gridded and a single location.

  • clm (pandas.DataFrame or xarray.Dataset) – The dataset must include at least the following variables:

    • Minimum temperature in degree celsius

    • Maximum temperature in degree celsius

    • Solar radiation in in W/m2

    • Daylight duration in seconds

    Optionally, relative humidity and wind speed at 2-m level will be used if available.

    Table below shows the variable names that the function looks for in the input data.



    tmin (degrees C)


    tmax (degrees C)


    srad (W/m2)


    dayl (s)


    rh (-)


    u2m (m/s)


    If relative humidity and wind speed at 2-m level are not available, actual vapour pressure is assumed to be saturation vapour pressure at daily minimum temperature and 2-m wind speed is considered to be 2 m/s.

  • coords (tuple of floats, optional) – Coordinates of the daymet data location as a tuple, (x, y). This is required when clm is a DataFrame.

  • crs (str, int, or pyproj.CRS, optional) – The spatial reference of the input coordinate, defaults to epsg:4326. This is only used when clm is a DataFrame.

  • method (str, optional) – Method for computing PET. Supported methods are penman_monteith, priestley_taylor, hargreaves_samani, and None (don’t compute PET). The penman_monteith method is based on Allen et al.1 assuming that soil heat flux density is zero. The priestley_taylor method is based on Priestley and TAYLOR2 assuming that soil heat flux density is zero. The hargreaves_samani method is based on Hargreaves and Samani3. Defaults to hargreaves_samani.

  • params (dict, optional) – Model-specific parameters as a dictionary, defaults to None.


pandas.DataFrame or xarray.Dataset – The input DataFrame/Dataset with an additional variable named pet (mm/day) for DataFrame and pet for Dataset.



Richard G Allen, Luis S Pereira, Dirk Raes, Martin Smith, and others. Crop evapotranspiration-guidelines for computing crop water requirements-fao irrigation and drainage paper 56. Fao, Rome, 300(9):D05109, 1998.


Charles Henry Brian Priestley and Robert Joseph TAYLOR. On the assessment of surface heat flux and evaporation using large-scale parameters. Monthly weather review, 100(2):81–92, 1972.


George H. Hargreaves and Zohrab A. Samani. Estimating potential evapotranspiration. Journal of the Irrigation and Drainage Division, 108(3):225–230, sep 1982. URL:, doi:10.1061/jrcea4.0001390.