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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _pruaf(self):
 """ Return percentage runoff urban adjustment factor. Methodology source: eqn. 6, Kjeldsen 2010 """

        return 1 + 0.47 * self.catchment.descriptors.urbext(self.year) \
                   * self.catchment.descriptors.bfihost / (1 - self.catchment.descriptors.bfihost)