A simple system, like drip irrigation, could save 50%. An ebb and flow hydroponic system could save 95%. These technologies aren't new, they're just not cost effective until water becomes less cheap.
I think in 20 or 50 years we'll look back at the current sprinklers that just spray water into the air as ridiculously wasteful.
Okay, that's an slight exaggeration. But only slight.
Israel uses 1016 million m^3 of water [1] or about 800,000 acre feet on agriculture per year. While studies have shown that California as a whole could save 5.6-6.6 million acre feet per year while maintaining productivity and the same acreage farmed [2]. It is a bit more difficult to restrict the number to the central valley, but the central valley accounts for more than half of California's total agriculture [3], so we can just use the statewide number.
The entire state's annual agricultural water usage is less than 45 million acre feet [2]. Even if Israel had all of this water they would max out at under 60 years.
1. https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in... 2. http://www.nrdc.org/water/files/ca-water-supply-solutions-ag..., p.2 3. http://www.nass.usda.gov/Statistics_by_State/California/Publ... p.22-38
These guys are in for a really hard and nasty shock.
I was thinking exactly the same thing after reading the OP and the first few comments of this HN thread. Israel and other countries use drip irrigation very effectively.
But if climate change happens on a large enough scale and with enough effect, we will have more problems than can be solved by drip irrigation alone.