The microthermal climate of California is much like
that found in the Alps where summers are short and cool and winters
are vigorous. Average temperatures in the coldest month are below
freezing at the higher elevations of the Sierra Nevada, the Modoc
Plateau, and the Klamath Mountains.
Most of California's water supply originates in these
higher elevations as winter snowpack and spring runoff. About three
fourths of the annual precipitation occurs in the mountainous northern
third of the state, whereas about 80 percent of the water demand
(mostly for agriculture) occurs in the southern two-thirds. Moving
water from where it naturally occurs to where demand has been created
has been one of the greatest challenges in the history of California.
