COVERAGE DESCRIPTION: The 'PRECIPITATION' layer represents lines of equal rainfall (isohyets) based on long-term mean annual precipitation data compiled from USGS, California Department of Water Resources,and California Division of Mines map and information sources. Source maps are based primarily on U.S. Weather Service data for approximately 800 precipitation stations. In the Los Angeles and San Francisco Bay areas the USWS data has been supplemented by county and local agency precipitation data. The data was collected over a sixty year period (1900-1960). Minimum mapping unit is 1000+ acres. The isohyetal contour intervals are variable due to the degree of variation of annual precipitation with horizontal distance.