Response of California Summer Hydroelectricity Generation to Spring Temperature

Regions depending on winter snowpack for hydroelectricity generation may be adversely affected if spring temperatures increase. An inverse relationship between spring temperature and summer hydroelectricity generation is complicated by changing statistical properties of the variables involved. We use simple approaches to quantify, within broad limits, the effect of a change in spring temperature on hydroelectricity generation in subsequent months over a political entity as large and geographically diverse as the state of California, incorporating variables that are highly nonstationary in the mean and in covariances with each other. Looking at data from several simple perspectives provides insight and a physically realistic explanation of the observations. California’s high-elevation hydropower reservoirs mitigate effects of dry winters; precipitation is limiting and spring temperature has no detectable effect. Following wet winters, however, warm springs can lead to earlier snowmelt and increased spillage so that water storage is limited by high-elevation reservoir capacity; during cold springs the snowpack melts more slowly, allowing it to act as a water reservoir for a longer period of time so that water can be supplied as needed. Following winters with over 70 cm of precipitation, water supply is abundant and hydropower seems limited by generation capacity. Our results demonstrate limitations of California’s high-elevation hydropower system, especially if climate warms, and our findings should also aid in the development of more complex, physically based, hydrologic models to aid water managers.