UC Santa Cruz

The lack of adequate quantity and quality of water is a world-wide problem, which fosters concerns about the impacts of climate change. Obtaining forecasts of future water changes are important to allow early impact mitigation and adaption efforts.

This study forecasts precipitation changes, not through climate models, but by analysis of observations to derive trends of three metrics: event Intensity, event Duration, and lull Pause. From 50 long-term stations in California we obtained median trends of Intensity -0.45% per decade, Duration 0.50% per decade, and Pause 0.13% per decade.

One problem in the analysis of observations was proper techniques to handle gaps from missing data. Multiple Imputation (MI) was applied through fitting of Weibull probability distributions to the three metrics. This was tested by artificially injecting gaps into the mostly complete Sacramento record. MI partially restored deviations caused by that injection.

viii

Permutation resampling techniques were applied with MI to derive significance p-values for each trend. Significance at 95% for Intensity was from 11 of the 50 stations, Duration from 16, and Pause from 19, of which 12 were 99% significant. Trends were combined by weighting them with the reciprocal of their p-values. Significance weighted California trends are Intensity -4.61% per decade, Duration 3.49% per decade, and Pause 3.58% per decade.

Two California basins with hydrologic models were studied: Feather River in the northern Sierra Nevada mountains and central coast Soquel-Aptos. Most hydrologic components between the two basins were shown to behave differently primarily because of climate differences.

Three metric trends were computed for each basin by combining trends from nearby observations. Each metric was changed without change to other metrics or the total precipitation and input into the models. Most hydrologic impacts were modest with magnitudes less than half the corresponding precipitation changes.

Feather River Basin's critical supply to Lake Oroville and the State Water Project were benefited from a streamflow increase by 0.5%. Soquel-Aptos Basin's value for water supply was harmed by groundwater recharge decrease by -2.5% and streamflow decrease by -1.1%. Neither of these impacts seem amenable to mitigation, thus adaptation is indicated.