The first question is what you mean by "mostly monthly based? Which of the following (or something else)

1. observations are made once a day each day in the month

2. observations are cumulative for each month

3. observations are temporal averages (but how exactly)

The variogram is based on "point data" whereas #2 and #3 are not point data, i.e. the variogram models

Donald Myers

Thanks for the reply.

The observations are made once a months. They are groundwater level data.

There are several stations in the catchment and each one has one value for each month.

You would only want to change the unit to "months" if each observation represented a "month", you should use "days"