Why do Pokorny's roots for water have an "a" in front?

I try to learn the basics of PIE and work now with the PIE root word(s) for "water" as an example.

Dictionaries like the American Heritage dictionary of Indo-European roots and websites like Wiktionary all seem to give consistently a root of wed- for water. E.g. here:
http://web.archive.org/web/20071217082955/http://www.bartleby.com/61/roots/IE551.html

But if I lookup the corresponding entry in Pokorny's, the entry au-9 on pages 78-81, most forms for this root all have an "a" in front, as can be seen in this online copy of the entry:
http://starling.rinet.ru/cgi-bin/response.cgi?single=1&basename=/data/ie/pokorny&text_number=+136&root=config

E.g. under b), the forms aued-, aud- and ud- are given.

Outside of Pokorny's I could not find another reference that starts PIE roots for water with an "a" in front like this.

What does that mean? Is the Pokorny entry outdated? Or is this simply a question of transliteration? Or does Pokorny just give more exact information than other works?