204 - Extending Monthly Flow Back to 1872 for the Watersheds in the Central Valley of California and Assessing Historical Change and Variability Using Seasonal Decomposition and Regression

California’s hydrology exhibits large intra-annual variations; long-term hydrologic sequences from 1921 to the present have generally been used for the water resource planning activities of the state. The objective of this work is to extend the hydrologic data record to 1872 to capture hydrologic variability over a longer time horizon. We use data records from historical documents to extend monthly flow series of 10 major watersheds in California Central Valley and assess long-term flow changes in these watersheds. Several important documents published around 1880s to 1920s are examined (and data records digitized) to provide additional historical temperature, precipitation, and streamflow measurements. A combination of monthly decomposition method and regression model is used to (1) compile monthly time series of the 10 study watersheds and (2) assess historical variability in regional climate and hydrology. The extended monthly flow starting from water year 1872 confirms the decadal variability of hydrology (e.g., a wet period from 1890s to 1910s, followed by an extreme dry period of 1920s to 1930s) and climate change effects (notably, the reduction of April to July flow especially from the 1970s). This approach allows us to separately examine the effect of temperature and precipitation on the variability and long-term changes in the annual and monthly flow of the 10 watersheds. The methodology and results of this work highlight the importance of obtaining and examining historical documents for long-term hydrologic studies and demonstrate the effectiveness of empirical and statistical, time-series analytic methods.