You can sub them. I get more a little floral from Hersbrucker than Hallertauer but I'm basing that on just the hops I've bought. Different crops might be slightly different.
Short answer: Hersbrucker won't be exactly the same, but it's a decent substitute.
Long answer:
Hallertau originially referred to Hallertau Mittelfruh, one of the four classic noble hops. Mittelfruh turned out to be susceptible to wilt and other diseases, and was in short supply by the 1970s.
Hersbrucker was planted extensively in the Hallertau region as a substitute, and through the 1980s and early 1990s was the most common Hallertau replacement.
By the late 1990s, a couple of varieties closely related to Mittelfruh had been bred (Hallertau Gold and Hallertau Tradition being the most common) and had flavor profiles closer to Mittelfruh, so Hersbrucker had some competition. It's still a popular hop, though.
Thanks for the explanation. That helps clear things up a bit.
I've been looking at a handful of lighter, summertime recipes that call for Hallertau. I have Hersbrucker on hand so it's good to know that this would be a decent substitute.
as for this hop, if i use the water 2.5 gallons, and the weight of the malt is 1.5 lbs. then how much of the weight for the hop should i use ? thank you very much.