Chute considérable  

Omnath, Locus de mana  

601.2g The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can't be paid.

If you choose to, you may sacrifice Omnath before spending mana. In which case, as the spell resolves, it uses last known information about Omnath to determine its power and toughness just before it left the battlefield.

608.2g If an effect requires information from the game (such as the number of creatures on the battlefield), the answer is determined only once, when the effect is applied. If the effect requires information from a specific object, including the source of the ability itself or a target that's become illegal, the effect uses the current information of that object if it's in the public zone it was expected to be in; if it's no longer in that zone, or if the effect has moved it from a public zone to a hidden zone, the effect uses the object's last known information. See rule 112.7a. If an ability states that an object does something, it's the object as it exists—or as it most recently existed—that does it, not the ability.

If you had green mana in your mana pool at that point, then Omnath's power and toughness would be affected accordingly.."

Q. If I have Omnath, Locus of Mana on the battlefield, tap four forests, and cast momentous fall, is Omnath a 5/5 when it dies?

A. It can be. It can also be 1/1.

The last step of a spell is paying all of its costs. You get to pay these costs in whatever order you want, so if you choose to sacrifice Omnath before you pay any mana that mana will still be in your mana pool and will still pump Omnath.