(Reuters) - A package of bills aimed at regulating drought-parched California's stressed groundwater supplies has come under fire from agricultural interests, injecting doubt into the measures' fates in the waning days of the state's legislative session this week.

The bills, which would allow the state to take over management of underground aquifers and water accessed via wells, tighten oversight of water at a time when groundwater levels are shrinking in the third year of a catastrophic drought.

"If we don't get started on fixing this problem we are going to find ourselves in a very dire situation, especially if drought persists," said Andrew Fahlund, deputy director of the California Water Foundation, which supports the two bills in the package.

Farmers in California's agricultural breadbasket rely on water from wells to irrigate their crops when the state cuts back on supplies from streams and the fragile San Joaquin-Sacramento River Delta.

But recent studies have shown groundwater levels receding throughout the Southwest, prompting concern among environmentalists and others that the use needs to be better regulated.

About a million people statewide rely on private wells for drinking water, many of which have gone dry as groundwater levels have receded.

Critics say the proposed legislation would impose overly rigid guidelines on farmers and wouldn't address the different geographic needs of water users. Opponents also say the legislation was hastily written.

"This could be the largest piece of water legislation regarding water rights that people in legislature will vote on in their career," said Justin Oldfield, lobbyist for the California Cattlemen's Association. "Should they really make the decision in such a short time period?"

Democratic state Senator Fran Pavley, who authored one of the bills, said she collaborated with farmers to draft the measure, and that some, including Community Alliance with Family Farmers, had signed on to support it.

"The legislation was crafted with extensive input from stakeholders, including water agencies, farmers, businesses, local governments, environmental organizations and individual community members," Pavley said in an email.

The legislature has until Aug. 31, the last day of this year's session, to pass the bills or let them die.

(Reporting by Joaquin Palomino in San Francisco; Editing by Sharon Bernstein and Leslie Adler)