Can a landowner vote by proxy?

Yes, provided the proxy is in proper form and properly executed and acknowledged by the landowner giving it. Water Code §50759 provides that no person shall vote by proxy at a district election unless authority to vote is evidenced by an instrument in writing acknowledged and certified in the same manner as grants of real property and filed with the election board. A form of proxy is available here. Proxy forms are also available upon request at the District office. Please note that your proxy form should be signed by the landowner and notarized.