You would expect the name Dijkgraaf to occur in the regions where there is a need for the occupation of dijkgraaf (see the Meaning of dijkgraaf) and that is also how it turned out to be (see maps below), except one which appears to have originated outside of the Netherlands, see Suriname Dijkgraaf Genealogy.
So far we know off five different Dijkgraaf family trees, plus we have a file of ones we haven't matched up to a tree yet, see Unlinked Dijkgraaf Genealogy. They are as follows;
You can see the distribution of the names in the Netherlands in the below maps
1947 Dijkgraaf province distribution map from Database of Surnames in The Netherlands
This map made by www.familienaam.nl (site gone) and is based on 1993 telephone subscribers data.
Place the cursor over the map to see the names of municipalities (not all done yet)
2007 Dijkgraaf distribution map from Database of Surnames in The Netherlands