entifier for each "site" corresponding to all the
variables in <code>site_vars</code> and <code>closure_id</code> concatenated together with
underscore separators.</p></li>
<li><p><code>closure_id</code>: a unique ID for each closure period. If <code>annual_closure =   TRUE</code> this ID will include the year. If <code>n_days</code> is used an index given the
number of blocks of <code>n_days</code> days since the earliest observation will be
included. Note that in this case, there may be gaps in the IDs.</