California was a wheat-growing state in the latter half of the 1800s. In 1890 only Minnesota produced more wheat than California. Soon thereafter, the state began a massive agricultural transformation away from grain and towards vegetables, fruit, and nuts.  Today, California grows pretty much everything except for the major grains, which creates a logistical challenge in measuring production. 

There are two main sources of California production data: County Agricultural Commissioner reports and the National Agricultural Statistics Service (NASS) of USDA. A third source is satellite imaging.

To understand the role of the Ag Commissioners, a little history is helpful.  Alan Olmstead and Paul Rhode argue that California's transformation to specialty crops was caused in large part by biological learning. Through trial and error, farmers learned how and where to grow horticultural crops productively. A drop in interest rates also helped spur investment in specialty crops, but other factors such as irrigation and improved rail networks came later.

Part of the trial and error process involved disease and pest management. Settlers and farmers brought a wide variety of tree and vine species into the state, many of which were infected with disease and pests. In 1881, the state legislature allowed counties to establish local boards of commissioners to handle disease and pest management. State law now stipulates that every county have an agricultural commissioner.

Over time, the responsibilities of the agricultural commissioner have evolved. However, pest and disease management remains a focal point, which I notice whenever I enter Napa County. 

Every year, each Ag Commissioner is required to "compile reports of the condition, acreage, production, and value of the agricultural products" in the county. These reports are typically published on county websites. Reports since 1980 have also been compiled into spreadsheets and published on USDA's website.  Many older reports are posted on county websites. (If you know whether these have been digitized, please let me know.)  

We have compiled the Ag Commissioners reports since 1980 and published them on our CA Crops data app. This app allows you to view and download data for the 12 largest crops in the state. 

Separately from Ag Commissioners reports, USDA NASS also conducts surveys on agricultural production in the state. The NASS data are generally considered to be authoritative, but they are typically available only at the state level and not the county level. Moreover, for some commodities the NASS data are only available for recent years.

So how closely do the two data sources align?  To answer this question, we have added a new tab to our CA Crops data app to show the data side by side. 

For the big field crops, state acreage numbers are similar across the two datasets.

For the tree nuts, NASS data are only available for recent years, and acreage numbers have diverged in the last five years. The Ag Commissioners data show much larger increases in acreage than NASS.

Yield estimates from the two sources are much closer for the tree nuts, suggesting that discrepancies may come not from differences in the average farm surveyed, but in how those survey samples are scaled up to get total acreage and production estimates. 

Satellite data may provide a way to resolve the acreage discrepancies. USDA produces the Cropland Data Layer, which estimates from satellites which crop is planted on each 30mx30m pixel in the United States (further details here).  You can use our Cropland Data Layer data app to view these data (see image below).  However, satellite data can be inaccurate in California because we grow so many crops. My graduate students and I are currently working on a project to reduce and quantify measurement errors. We plan to publish a field-level crop map for the state for the last 10-12 years.

Huge kudos to ARE PhD students Seunghyun Lee and Andrew Swanson for their work adding the Ag Commissioners vs NASS tab to our CA Crops data app.