@Xiaoyu yu , The second method you describe (branch) is more robust. In that method the river runoff model will also use a spunup file. River water spins up very quickly so we don't always use the branch method.
You can query available spatial resolutions by changing directory to cime/scripts and running:
./query_config --grids
hcru_hcru should appear as a possible grid.
So you would need to specify --res hcru_hcru in your create_newcase command, e.g.,
./create_newcase --case cesm220_hcru_GSWP3V1_2000 --res hcru_hcru --compset I2000Clm50BgcCrop --run-unsupported
However, an appropriate surface dataset does not appear to be available for hcru_hcru for that release of the model (the domain file does exist), so you would need to create one using the mksurfdata_map tool:
cd components/clm/tools/mksurfdata_map
./mksurfdata.pl -res 360x720cru -years 2000
and then in user_nl_clm, point to the dataset you created using:
fsurdat = 'your_dataset'