Cal. Water Code § 37952

The district may, by ordinance, require proof of the accuracy of the waterflow measuring device from the operator and may, absent adequate proof of accuracy, order the operator, at the operator’s sole cost, to have the waterflow measuring device calibrated in a manner acceptable to the district. If the district has probable cause to believe that the extraction of groundwater from any extraction facility is in excess of the amount reported in groundwater extraction statements, or if no statements are filed covering an extraction facility, the district may investigate the extraction of water from each extraction facility.