Hi @Boorhin, I just ran into the same issue. The region argument has to be of type slice, in your case slice(t) instead of just t works:

python import xarray as xr from datetime import datetime,timedelta import numpy as np dt= datetime.now() times= np.arange(dt,dt+timedelta(days=6), timedelta(hours=1)) nodesx,nodesy,layers=np.arange(10,50), np.arange(10,50)+15, np.arange(10) ds=xr.Dataset() ds.coords['time']=('time', times) ds.coords['node_x']=('node', nodesx) ds.coords['node_y']=('node', nodesy) ds.coords['layer']=('layer', layers) outfile='my_zarr' varnames=['potato','banana', 'apple'] for var in varnames: ds[var]=(('time', 'layer', 'node'), np.zeros((len(times), len(layers),len(nodesx)))) ds.to_zarr(outfile, mode='a') for t in range(len(times)): for var in varnames: ds[var].isel(time=slice(t)).values += np.random.random((len(layers),len(nodesx))) ds.isel(time=slice(t)).to_zarr(outfile, region={"time": slice(t)})

This leads however to another issue: ```python

---

ValueError Traceback (most recent call last) <ipython-input-52-bb3d2c1adc12> in <module> 18 for var in varnames: 19 ds[var].isel(time=slice(t)).values += np.random.random((len(layers),len(nodesx))) ---> 20 ds.isel(time=slice(t)).to_zarr(outfile, region={"time": slice(t)})

~/.local/lib/python3.8/site-packages/xarray/core/dataset.py in to_zarr(self, store, chunk_store, mode, synchronizer, group, encoding, compute, consolidated, append_dim, region, safe_chunks) 2029 encoding = {} 2030 -> 2031 return to_zarr( 2032 self, 2033 store=store,

~/.local/lib/python3.8/site-packages/xarray/backends/api.py in to_zarr(dataset, store, chunk_store, mode, synchronizer, group, encoding, compute, consolidated, append_dim, region, safe_chunks) 1359 1360 if region is not None: -> 1361 _validate_region(dataset, region) 1362 if append_dim is not None and append_dim in region: 1363 raise ValueError(

~/.local/lib/python3.8/site-packages/xarray/backends/api.py in _validate_region(ds, region) 1272 ] 1273 if non_matching_vars: -> 1274 raise ValueError( 1275 f"when setting region explicitly in to_zarr(), all " 1276 f"variables in the dataset to write must have at least "

ValueError: when setting region explicitly in to_zarr(), all variables in the dataset to write must have at least one dimension in common with the region's dimensions ['time'], but that is not the case for some variables here. To drop these variables from this dataset before exporting to zarr, write: .drop(['node_x', 'node_y', 'layer']) ```

Here, the solution is however provided with the error message. Following the instructions, the snippet below finally works (as far as I can tell):

```python import xarray as xr from datetime import datetime,timedelta import numpy as np dt= datetime.now() times= np.arange(dt,dt+timedelta(days=6), timedelta(hours=1)) nodesx,nodesy,layers=np.arange(10,50), np.arange(10,50)+15, np.arange(10) ds=xr.Dataset() ds.coords['time']=('time', times)

ds.coords['node_x']=('node', nodesx)

ds.coords['node_y']=('node', nodesy)

ds.coords['layer']=('layer', layers)

outfile='my_zarr' varnames=['potato','banana', 'apple'] for var in varnames: ds[var]=(('time', 'layer', 'node'), np.zeros((len(times), len(layers),len(nodesx)))) ds.to_zarr(outfile, mode='a') for t in range(len(times)): for var in varnames: ds[var].isel(time=slice(t)).values += np.random.random((len(layers),len(nodesx))) ds.isel(time=slice(t)).to_zarr(outfile, region={"time": slice(t)}) ```

Maybe one would like to generalise region in api.py to allow for single indices or throw a hint in case an a type different to a slice is provided.

Cheers