BILLINGS — The Blackfeet Tribe and state negotiators have reached a deal to allow the tribe to increase the amount of water it can draw from several rivers in northern Montana, state officials said.

In a related deal that has been signed by Gov. Brian Schweitzer but not tribal leaders, the state would pay the tribe $14.5 million to defer drawing more from one waterway, Birch Creek, for the next 25 years, said Schweitzer spokeswoman Sarah Elliott. In the interim, Four Horns Reservoir on the reservation would be expanded to create a larger water supply for both tribal and non-tribal users.

Birch Creek is used to irrigate more than 80,000 acres of non-tribal cropland.

‘‘We respect their right as a sovereign nation not to sign it,’’ Elliott said.

The broader water rights compact, a separate agreement, still must be ratified by the Legislature, Congress and tribal members.

The deal would affect the St. Mary, Two Medicine and Milk rivers, along with Badger, Birch and Cut Bank creeks. The Blackfeet would gain more water from those rivers and creeks for irrigation and drinking water projects.

Non-tribal irrigators oppose the compact as it now stands.

John Bloomquist, an attorney for the Pondera Canal and Reservoir Company, said that after the 25 years is up, local farmers and ranchers risk losing water for as much as 40,000 acres.

Bloomquist said Wednesday that his clients will ask Congress not to ratify the compact — unless the state and federal governments set up a fund to pay the tribe for the irrigators’ continued use of Birch Creek.

Susan Cottingham with the Montana Reserved Water Rights Compact Commission said the state plans to ask the Legislature for an additional $3 million to try to address those concerns.

She said the state could spend $10 million to $15 million more to enlarge Four Horns Reservoir.

‘‘By the time the state is done, they are going to be putting $30 (million) to $35 million into this settlement, basically for the benefit of the Birch Creek users,’’ Cottingham said.