I am not a climatologist, I am not an expert, but I have looked into the water shortage in the Central Valley since I am a resident. In fact I am a resident that uses a well, so my house's water is tied to the ground water that farmers also depend on during droughts to make their livelihood. What follows is an edited letter that I send to a friend who suggested that now that its raining its back to the normal production of fruits for the central valley.
So a drought is characterized by two things, lack of water and an abundance of time. This is a point that many people gloss over. The severity of a drought is measured in length of time usually. Because if it doesn’t rain for a year, or it rains once in a year who cares that basically the same severity of drought? The time with lower levels of rainfall is what counts. As soon as enough water is falling from the sky the drought is 'over.' The drought is officially over even if there is no real way to catch all that water falling and use it to ease the drought stricken area.
At a company where I worked we learned that rain while providing water doesn’t actually help with crops, in fact it makes things worse often at industrial farms. So if we got a similar dose of rain in the early summer it would introduce mold into the crops that we were growing. The lack of sunlight or BTU’s (depending on the fruit) means a longer wait for ready fruit. So rain is not the preferred method of irrigation its just too inconsistent.
The second thing we learned about rain is that it doesn’t actually keep the ground wet for very long. Maybe a week, if it’s a real downpour. The only thing that rain does usefully is refill reservoirs… but wait! There’s more. In order to fill a reservoir it would have to rain so much that there would literally need to be flooding. For example right now in the central valley, we have flooding. The reservoirs are full enough for controllers to open up the gates. Now they are using the canals and rivers systems that have been dry for years to release pressure. So all that water will make its way to the ocean and that will be that. The reservoirs are not really full, but they are above the levels that they seasonally should be. There is a reason that controllers are letting water run through the waterways, but to understand why we have to understand how reservoirs actually function.
Two weather phenomenons fill reservoirs, rain and snow. Rain is the most immediate option, but its not a long term solution to the long summers that the central valley experiences. Snow on the other hand circumvents the reservoir capacity. Then when spring comes it ‘slowly’ melts ….. over TIME. So the annual droughts of summer and fall are softened and water is available for farmers and cities and everyone else. This means that you need rain and cold temps in the winter every year or no drought relief. We had a 3 week of rain up here a few years ago in December but it was unseasonably warm too. No snow! The company I worked for literally had connections to web cams in Sierras so we can see how much snow is accumulating….
The central valley’s long term problem is one of resource management. The size of industrial farming and the city of Fresno ( the fastest growing city in California) is not being matched by the size of mountain growth. It never will be. The more houses with lawns or the more farms that need to pull water, either from a canal or from a well, the harder the mountains need to produce snow.
The well problem is compounded by human intervention concerning water control. Reservoirs are great and all, but the way the water tables replenish is from rivers. When you have above ground rivers going for long enough time they also foster underground waterways, and those often help bring the water table up. Rain fall does not. Turning on rivers when you need to relieve pressure from your water stores doesn't do anything for the water table. Rivers in particular would also help ease the pumping from wells that farmers would do and that would help the water table recover slightly, but the damage is semi-permanent because when you pump out an area for long enough the voids in the water table collapse so the sponge can’t hold as much water. So the water table literally wears out! Some well owners have taken to ‘fracking’ their industrial wells to reach further away from the shaft when they start to produce slowly. Its crazy.
All of this is known to engineers in industry but no one has a cost effective solution. I put a time limit on how long California will be the cornucopia of the US. I actually predicted that because of the population growth in Fresno and suburbs and the now incoming moratorium on drilling wells that most industrial plants only have 10 years. Again, no expert but this is what i have pieced together as I actually have spent many many days studying this problem.