code: def urbext(self, year):
          
  
          # 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
docs: 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