Which of the following statements is true about California's water supply?

California's water supply is indeed heavily influenced by surface water sources. The state relies on a complex system of rivers, reservoirs, and aqueducts to transport water from regions where it is plentiful, such as the Sierra Nevada mountains, to areas with high water demand, including cities and agricultural lands. The Sierra Nevada snowpack is a critical source of surface water, and as this snow melts in the spring, it feeds into rivers and reservoirs that provide water throughout the year.

In addition to surface water, groundwater plays a significant role in the overall water supply, especially during droughts. However, it is not the only source. While ocean water desalinization is a growing method to supplement water supplies, it currently represents a smaller fraction of California's total water production compared to surface water. Unlike what is stated in another option, agriculture has a substantial impact on water usage, consuming a significant portion of California's water resources due to the state's extensive agricultural activities.

Thus, the statement about surface water sources being heavily influential accurately reflects the complexities and dependencies of California's water supply system.