"""This module contains methods to compute the Standardized Groundwater
Index."""
from numpy import linspace
from scipy.stats import norm
[docs]def sgi(series):
"""Method to compute the Standardized Groundwater Index [sgi_2013]_.
Parameters
----------
series: pandas.Series
Returns
-------
sgi_series: pandas.Series
Pandas time series of the groundwater levels. Time series index
should be a pandas DatetimeIndex.
References
----------
.. [sgi_2013] Bloomfield, J. P. and Marchant, B. P.: Analysis of
groundwater drought building on the standardised precipitation index
approach, Hydrol. Earth Syst. Sci., 17, 4769–4787, 2013.
"""
series = series.copy() # Create a copy to ensure series is untouched.
# Loop over the months
for month in range(1, 13):
data = series[series.index.month == month]
n = data.size # Number of observations
pmin = 1 / (2 * n)
pmax = 1 - pmin
sgi_values = norm.ppf(linspace(pmin, pmax, n))
series.loc[data.sort_values().index] = sgi_values
return series