Yes, the watersheds but also the prevalence of mountaintop aquifers that don't go beyond their part of the range. These can be extremely fragmented. You'll have ample water for the taking in one area and none a few kilometers away. AFAIK, you can still acquire revocable water rights to some of this without much fuss since it doesn't interact with any official watershed. It just isn't located anywhere remotely convenient or useful.

Sometimes the valleys have good wells but that isn't guaranteed due to the geology of Nevada. Lots of brine, sulfur, hydrocarbons, hot springs, etc. You never know what you are going to get and the fresh water eventually mixes into this underground.