Groundwater is also an important part of the state’s water supplies, comprising about 40% of water used in an average year, and 60% or more in a drought year. So in order to make this work, California has built an extensive water storage and conveyance network that can store the water from the winter when it falls, and deliver it to the drier parts of the state in the summer. Additionally, most of the precipitation falls in the mountains in the middle to northern half of the state, far from major urban and agricultural centers. Most of this precipitation will occur between November and April, yet most water demand is in the hot, dry months of summer and early fall. Source: The Delta Plan, Figure 3-1īesides variability, there are other challenges for California’s water supply. About half of the precipitation will evaporate, be used by vegetation, or sink into the subsurface, salt sinks, or flow to the ocean the remaining half, known as ‘dedicated water’ is what is available for use in cities, on farms, for the environment, or to be put in storage. In an average year, the total amount of precipitation is about 200 million acre-feet however, the actual precipitation can vary anywhere from 100 million acre-feet to 300 million acre-feet, depending on whether it is a wet year, a dry year, or something in between. So why did we do all this? California’s complicated hydrology and population dynamicsĬalifornia’s climate and hydrology are unlike any other in the nation, with variability and uncertainty the main characteristics. California, arguably, is the most hydrologically-altered landmass on the planet. Some rivers have been completely dried up, some rivers flow through mountains into other river’s beds, and some rivers even flow backwards at times. There is cropland where once was a swampy marsh, manmade lakes where there once was desert, and even desert where there once was cropland. Today, large urban centers exist where there are scant local resources to support their residents.
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