<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def surface_temp(self, unit='kelvin'):
 """Returns the soil surface temperature :param unit: the unit of measure for the temperature value. May be: '*kelvin*' (default), '*celsius*' or '*fahrenheit*' :type unit: str :returns: a float :raises: ValueError when unknown temperature units are provided """

        if unit == 'kelvin':
            return self._surface_temp
        if unit == 'celsius':
            return temputils.kelvin_to_celsius(self._surface_temp)
        if unit == 'fahrenheit':
            return temputils.kelvin_to_fahrenheit(self._surface_temp)
        else:
            raise ValueError('Wrong temperature unit')