I found the following:

1. “The one big benefit of hemp? Its environmental footprint is relatively small. It requires few pesticides and no herbicides. It’s an excellent rotation crop, often used to suppress weeds and loosen soil before the planting of winter cereals. On the other hand, it requires a relatively large amount of water, and its need for deep, humus-rich, nutrient-dense soil limits growing locales.” https://modernfarmer.com/2013/10/legal-industrial-hemp-wont-matter/


2. When you add processing into the equation, cotton uses more than four times as much water as hemp. Polyester is difficult to compare, because it's not an agricultural product. But some studies suggest it's the least water-intensive of the bunch, using just one-thousandth as much water as cotton. (In fact, water is a byproduct of polyester processing.)

So where does all this leave us? Without a clear winner, unfortunately. There's an argument to make for polyester, but the nonrenewability of synthetic textiles raises serious concerns. Overall, hemp appears to be slightly easier on the environment than cotton, considering it's superior on water and land requirements, and only slightly worse for energy use.
http://www.slate.com/articles/health_and_science/the_green_lantern/2011/04/high_on_environmentalism.html