def get_conversion_factor(self, in_nc, num_nc_files):
        """get conversion_factor"""
        data_in_nc = Dataset(in_nc)

        # convert from kg/m^2 (i.e. mm) to m
        conversion_factor = 0.001

        # ECMWF units are in m
        if data_in_nc.variables[self.runoff_vars[0]] \
                .getncattr("units") == "m":
            conversion_factor = 1

        # ftp://hydro1.sci.gsfc.nasa.gov/data/s4pa/GLDAS_V1/README.GLDAS.pdf
        if "s" in data_in_nc.variables[self.runoff_vars[0]] \
                .getncattr("units"):
            # that means kg/m^2/s in GLDAS v1 that is 3-hr avg,
            # so multiply by 3 hr (ex. 3*3600). Assumed same
            # for others (ex. 1*3600).
            # If combining files, need to take average of these,
            # so divide by number of files
            conversion_factor *= \
                self.simulation_time_step_seconds / \
                num_nc_files
        data_in_nc.close()

        return conversion_factor