def get_grass_fraction(self, interval='season'):
        """
        todo implement this for data from a real run !!!
        """

        if interval != 'season':
            raise ValueError('Other temporal sampling than SEASON not supported yet for JSBACH BOT files, sorry')

        ls_mask = get_T63_landseamask(self.shift_lon)

        filename = '/home/m300028/shared/dev/svn/trstools-0.0.1/lib/python/pyCMBS/framework/external/vegetation_benchmarking/VEGETATION_COVER_BENCHMARKING/example/historical_r1i1p1-LR_1850-2005_grass_crop_pasture_2001.nc'
        v = 'var12'
        grass = Data(filename, v, read=True,
                     label='MPI-ESM tree fraction ' + self.experiment, unit='-', lat_name='lat', lon_name='lon',
                     #shift_lon=shift_lon,
                     mask=ls_mask.data.data, start_time=pl.num2date(pl.datestr2num('2001-01-01')), stop_time=pl.num2date(pl.datestr2num('2001-12-31')), squeeze=True)

        return grass