pastas.stats.signatures.interannual_variation
=============================================

.. py:function:: pastas.stats.signatures.interannual_variation(series: pandas.Series, normalize: bool = False) -> float

   
   Interannual variation after :cite:t:`martens_groundwater_2013`.

   :param series: Pandas Series with DatetimeIndex and head values.
   :type series: pandas.Series
   :param normalize: normalize the time series to values between zero and one.
   :type normalize: bool, optional

   :returns: Interannual variation (s).
   :rtype: float

   .. rubric:: Notes

   The average between the range in annually averaged 3 highest monthly heads and the
   range in annually averaged 3 lowest monthly heads.

   Inter-yearly variation of high and low water table (s):

       s = ((max_HW - min_HW) + (max_LW - min_LW)) / 2

   A higher value of s indicates a more variable head, and vice versa.

   .. warning::

      In this formulating the water table is referenced to a certain datum and
      positive, not as depth below the surface!















   ..
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