2.1 Environmental setting and data
The California’s climate varies widely, from hot desert to subarctic, depending on latitude, elevation, and proximity to the coast. California’s coastal regions, the Sierra Nevada foothills, and much of the Central Valley have a Mediterranean climate, with warm to hot, dry summers and mild, moderately wet winters (Fig. \ref{div-677474}a, b). The influence of the ocean generally moderates temperature extremes, creating warmer winters and substantially cooler summers in coastal areas. The rainy period in most of the country as well as in Teheran is from November to May (Fig. \ref{div-677474}a).
Prevailing westerly winds from the Pacific Ocean also bring moisture. The average annual rainfall in California is about 350 mm, with the northern parts of the state generally receiving higher rain amounts than the south. The reference evapotranspiration follows a more complex pattern, mostly in relation to elevation and distance from sea (Fig. \ref{div-677474}c). Temperature and evapotranspiration are especially important in California, where water storage and distribution systems are critically dependent on winter/spring rainfall, and excess demand is typically met by groundwater withdrawal (Diffenbaugh et al., 2015). The PDSI time series derived from \citet*{Griffin2014} reconstructed drought for California.