If you are consistently clinically dehydrated (really, really dehydrated, like you just crawled in from the desert where you were dying of thirst), it might theoretically reduce your semen volume, just the way it would reduce your blood or tears.
If you're asking because you know for a fact (because you measured) that you used to produce more volume, and now you have measured again and aren't producing as much, for that question see your doctor.
If you're asking because you are trying to have a baby (though it doesn't sound like it because you asked about semen volume instead of sperm volume) unless you have an unusually low sperm count, one ejaculation contains millions of sperm, whether they are being carried along in .8 or 7.6 ml of semen.
But gotta say, it kind of sounds like you saw a porno with a faked-up ejaculation and got concerned that you don't shoot out that much. As Auntiejessi says, don't take what's depicted in porn as accurate about anything. Porn does whatever possible to exaggerate the sex act, almost to the point of burlesque. None of this should make you think porn is a role model for sex in real life. If you grab and pound your girlfriend and swing her around like **** Horndog did in an x-rated video you saw, it's a pretty fast way to find yourself without a girlfriend.
Keep hydrated, see your doc if you think your volume has become a lot lower than previously, and otherwise, just understand that there is nothing wrong with the amount of your semen. As Jessi said, no woman cares.
Have you been told yours is low? Are you trying to have a baby?
The average semen volume is between 1.5 to 5 ml, though some studies show it's .8 to 7.6.
Semen value literally only matters if you are trying to conceive, and sperm quality and count is also important.
If you have low sperm volume, and have been told this by a doctor, they will look into why, and address the cause. That's the only way to increase it.
If this is just an insecurity because you think you lack sperm volume because of what you see in porn or something, don't even worry about it. Porn is all fake - male porn stars often have hidden tubes that shoots the fluid. If you pay attention, they don't show the head of the penis often during orgasm, because that's where the tube is.
If your partners are women, I assure you that we don't ever think about this. In all my conversations with friends about sex, not once did someone say, "Oh he was so good, except he didn't seem to have a lot of semen."
So if you're concerned, talk to your doctor. You may be just fine. :)