COMPARISON OF CLIMATIC TRENDS AND WELL RECORDS IN SOUTHERN CALIFORNIA: IMPLICATIONS FOR GEOTECHNICAL AND HYDROGEOLOGICAL PRACTICE

Comparison of climate (rainfall) records from Los Angeles with water well records beginning in 1930 from the nearby San Gabriel Valley, shows a correspondence in cycles of relative wet and dry on a decadal scale, as well as similar longer term trends. The climate record of Los Angeles spans the termination of the Little Ice Age (+ 1890 A.D.), and plots of cumulative departure from the long-term mean of the record suggest drying conditions over the last century. With respect to the present day, these plots also suggest the last significant wet period from the point of view of increasing groundwater recharge was the 1940’s. The well records noted above establish the historic high groundwater levels in the area similarly occurred in the decade. Since then, however, rainfall and groundwater level trends appear to be in decline. This raises the question of whether historic high groundwater is an appropriate criterion in liquefaction analysis in all cases, since its use presumes the probable return of historic conditions over the design life of a facility. Alternatively, the criterion might be based on trend analysis of groundwater levels where there is also corresponding climatic data. Recharge trends are also analyzed by the water-table fluctuation method.