California’s water agency has overestimated its supply because of a failure to account for climate change in its management of the state’s hydrological resources, a state auditor found.

The Department of Water Resources significantly overestimated the state water supply in 2021 because of an error to take into account severe drought due to climate change, the California State Auditor said in a report Thursday. The agency has relied on historical climate data when developing its forecasts, instead of taking into account new methods that better account for extreme conditions, the report said.

The nation’s most populous state has suffered consecutive years of drought, upending supply from a network of dams and canals built to deliver snowmelt and storm runoff to farms and residents. Water grew so scarce that dairy farms sent cows to slaughter as they ran short of feed and water, while much of the famed Central Valley was left unplanted because of a lack of irrigation.

The water resources agency hasn’t developed a comprehensive, long-term plan for managing its supply during periods of more extreme drought, according to the report. The auditor recommended the agency make a number of changes including updating its forecasting model based on the observed impacts of climate change.