Uploaded on 2017-02-13 by Heike Kaiser
California's climate and hydrology are unlike any other in the nation, with variability and uncertainty the main characteristics. 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. About half of the precipitation will evaporate, be used by vegetation, sink into the subsurface, 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. Besides variability, there are other challenges for California's water supply. 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. 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. 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. But groundwater is very much location dependent: some communities have no groundwater and rely solely on surface water while other communities may have only groundwater; other communities rely on a mix of imported water and groundwater, and even some rely solely on imported water. So in order to make this work, California has built an extensive water storage and conveyance network that is likely the most extensive anywhere on earth. It can store the water from the winter when it falls, and deliver it to the drier parts of the state in the summer. Through the development of this infrastructure, man has rearranged California's natural assets to meet societal needs, making the state unrecognizable from its pre-settlement history in the process and transforming it into one of the world's leading agricultural producers, the most populated state in the country, and the eighth largest economy in the world. Today, large urban centers exist where there are scant local resources to support their residents. There is cropland where once was a swampy marsh, man-made lakes where there once was desert, and even desert where there once was cropland. Some rivers have been completely dried up, some rivers flow through mountains into other river beds, and some rivers even flow backwards at times. However, this intensive development has not been without its consequences: Dams have blocked access to habitat for native species and altered the natural flow of water, wetlands have been drained, and invasive species have moved in, altering habitat. Populations of native fish and wildlife species have plummeted; some have gone extinct, and many more are threatened. Most of the state's waterways are impaired by pollutants from agricultural, urban, and legacy mining sources. Most of California's water infrastructure projects were designed and constructed at a time when delivering cheap water to feed economic development was a goal, and ecosystem and species concerns were rarely considered. Changing societal values have meant increased restrictions and regulations to protect endangered species have reduced the amount that can be withdrawn from our waterways and fueled political wars that have stretched on for decades as demands for water from agriculture, cities and industry must be constantly balanced against the need for maintaining water quality and protecting fisheries and wildlife.