The areal distribution of precipitation and flooding during the January 1982 storm in the Santa Cruz Mountains and vicinity was influenced by the orographic effect of the Santa Cruz Mountains. Precipitation depths ranged from 12.2 inches in 1 day on the west side to 1.33 inches on the east side. Unit peak discharge ranged from 626 cubic feet per second per square mile on the west side to 83 cubic feet per second per square mile on the east side. The median recurrence interval of peak flow on the west side is 22 years, whereas on the east side it is 6.8 years. Development of a relation between rainfall at Ben Lomond and runoff on the San Lorenzo River at Big Trees shows that the significant climatic factors that describe the flood hydrology of the basin are (1) precipitation for the 3 consecutive days including the day of peak flows and (2) precipitation for a 60-day duration prior to the peak. A study of historical floods and precipitation characterisitics in the San Lorenzo River basin suggests that major floods in the area are the product of (1) greater-than-normal antecedent precipitation for up to 60 days prior to the flood and (2) subsequent intense frontal-type storms immediately prior to the peak. (USGS)
