The San Diego County Water Authority warned Wednesday that it could cost county water customers nearly $200 million over a decade if two rural districts detach from the authority.

In an email to county political leaders, Water Authority Board Chair Mel Katz said a proposed exit fee of $4.8 million a year for five years “isn’t close to covering the actual costs that will be shifted to residents elsewhere in the county.”

The Fallbrook Public Utility District and the Rainbow Municipal Water District are seeking to join the Eastern Municipal Water District in Riverside County in hopes of securing lower-cost water for farmers.