Estimate the `urbext2000` parameter for a given year assuming a nation-wide urbanisation curve.

        Methodology source: eqn 5.5, report FD1919/TR

        :param year: Year to provide estimate for
        :type year: float
        :return: Urban extent parameter
        :rtype: float

def urbext(self, year):
        """
        Estimate the `urbext2000` parameter for a given year assuming a nation-wide urbanisation curve.

        Methodology source: eqn 5.5, report FD1919/TR

        :param year: Year to provide estimate for
        :type year: float
        :return: Urban extent parameter
        :rtype: float
        """

        # Decimal places increased to ensure year 2000 corresponds with 1
        urban_expansion = 0.7851 + 0.2124 * atan((year - 1967.5) / 20.331792998)
        try:
            return self.catchment.descriptors.urbext2000 * urban_expansion
        except TypeError:
            # Sometimes urbext2000 is not set, assume zero
            return 0