Impacts of changing snowfall on seasonal complementarity of hydroelectric and solar power

Complementarity of variable renewable energy sources at multiple temporal scales is important in order to ensure reliability of a decarbonizing energy system. In this study, we investigate the hypothesis that a decrease in the fraction of precipitation falling as snow (SWE/P) would increase monthly complementarity of hydro and solar power generation in the western U.S. With a focus on 123 dams responsible for 93% of generation, we found that these resources are seasonally complementary at about half of dams, as indicated by the sign of correlation coefficients (ρ). As hypothesized, average SWE/P at individual dams was generally positively correlated with ρ, but the dependence of ρ on SWE/P was non-linear and SWE/P only explained a modest portion of the variance in complementarity. At each dam, the dependence of annual ρ on interannual variations in SWE/P between 2002–2020 was assessed; these relationships were positive at 72% of dams but not statistically significant at the level of individual dams. Finally, at the system scale ρ was significantly related to SWE/P, with a stronger relationship observed than the dependence of total hydropower generation on SWE/P. Notably, the system-scale relationship between ρ and SWE/P changed dramatically in the latter part of the temporal domain (2012–2020), with a much steeper slope and greater fraction of variance explained by SWE/P. These results illustrate the historical relationship between SWE/P, monthly complementarity of hydro and solar power, complexities of these relationships due to snow and watershed hydrology and reservoir management, and a change in the observed relationship between SWE/P and hydropower generation timing. To the extent that hydro and solar power generation complementarity is responsive to SWE/P, expected declines in SWE/P may indicate greater seasonal complementarity but reduced hydropower available for load-balancing when solar power generation is highest.