Final answer:
The statement that California uses a grass reference for determining ETo is true. California's reference crop for ETo differs from the USBR's alfalfa reference crop, reflecting differences in local agricultural practices and water management needs.
Step-by-step explanation:
The statement is true; California does indeed use a grass reference for determining evapotranspiration (ETo). The reference used by California involves a hypothetical grass reference crop that is 4 to 7 inches tall, with an albedo of about 0.23, and closely resembles an extensive surface of green, well-watered grass that is actively growing. This reference is different from the alfalfa reference that the United States Bureau of Reclamation (USBR) uses. The grass reference is known as the reference evapotranspiration for a standardized crop and is extensively used for agricultural and irrigation planning in California.