I can’t find good references to directly support the common claim that 80% of California water is used by agriculture. But I could find numbers on 2010 consumption. As shown below, irrigation accounts for 60.7% of water usage (this includes more than agriculture). It seems 80% figure comes from subtracting out water reserved for environmental uses.
mgpd = million gallons per day
Total = 38 billion gallons per day = 13,870 billion gallons per year
Irrigation (crop, pasture, recreational lands such as parks, golf courses) 60.7% (23,056 mgpd)
Thermoelectric power generation (equipment cooling) 17.4% (6,601 mgpd)
Public supply (domestic, commercial, industrial, pools, parks, firefighting) 16.6% (6,307 mgpd)
From California Wikipedia article
California GDP (2012) = $1.959 trillion
California agriculture production = $46.7 billion (2013) (~2.4% of California economic output).
California population (2014) = 38.8 Million
Upshot of this is ~2.4% of California economic output consumes more than half of California water.
I’m having trouble finding water use by agriculture commodity, but it’s easy to find agriculture economic output by commodity.
Nursery plants $3.5B
Cattle, Calves $3.2B
Interesting blog on California water