Pacific Gas and Electric Company (PG&E) forecasts and schedules seasonal runoff for it's 68 hydroelectric powerhouses (includes one pump storage facility) and an additional 19 powerhouses that belong to it's Partnership Irrigation Districts and Water Agencies. These powerhouses are located in California's Sierra Nevada and southern Cascade mountain ranges, which extend from the Kern River east of Bakersfield, north to the Pit River with headwater drainage just south of the Oregon border. A single PG&E powerhouse is located in the coast range east of Ukiah. Historically during the past 30 years, hydro generation has been derived from the following sources of runoff with an approximate averaged percentage of each source: 1)groundwater-38%, 2)snowpack-37%, and 3)rainfall-25%. The PG&E hydroelectric system was mostly designed prior to the 1970's and built to accommodate a specific mix ratio of rainfall-snowmelt produced runoff with timing and quantity of runoff along specific river reaches derived from the prior 'known' historical data period. Design and placement of seasonal storage reservoirs and diversion dams took elevation into consideration as it relates to precipitation type. The anticipated proportion or ratio of rain and snowfall, as a factor that influenced runoff quantity and timing of inflow, was important input for best determining reservoir size and location.

However, a review of PG&E's water and climate data indicates that change in runoff timing appears to be taking place with a decrease in snowmelt-produced runoff during the past 50 years as compared with the first half of the 20th century. This trend toward decreasing runoff from snowmelt appears to be the result of a decrease in the low elevation snowpack, with a corresponding increase in rain-produced runoff from the low elevation contributing drainage. The result is larger and more variable winter and early spring runoff with increased risk for reservoir filling from snowmelt alone. This paper will present some preliminary findings and discuss types of data needed, including data analysis that would be most useful for identifying and further evaluating change in runoff timing and quantity. Some of the types of commonly collected hydrometeorological data and data calculations, which seem to best describe and track timing shift of unimpaired runoff for our hydroelectric system in California are: 1) aquifer outflow rates from northeastern California's volcanic drainages, 2) the winter and spring ratio of compiled unimpaired flows, including ratio variance, 3) the ratio of low-mid elevation snowpack compared with high elevation snowpack, and 4) air temperatures. For all types of commonly collected hydrometeorological data, increased emphasis on improving data quality as it relates to the watershed in its entirety is needed. Improved data quality would likely lead to increased confidence in utilizing this data to identify climate change and to calculate possible impact on future hydroelectric generation production.