Combined Effects of Solar Activity and El Nio on Hydrologic Patterns in the Yoshino River Basin, Japan

Relatively little is known about the combined effects of solar activity and El Nio Southern Oscillation (ENSO) on both surface and subsurface hydrologic patterns, especially in Japan. Analyses of hydrologic responses to solar activity and ENSO could provide scientific guidance on regional water resource management. In this work, annual and decadal variations of hydrological processes (precipitation, river discharge, groundwater level) in Yoshino River basin and their possible interactions with solar activity and ENSO were analyzed using wavelet techniques. Results show the following: (1) The hydrological processes mainly fluctuate in the periodicities of 0.5, 1, 2-7 and 11 years. Strong cross power and high coherence between hydrological variability and sunspot number/Sea Surface Temperature (SST) were obtained in the periodicities of 2-7 and 11 years, indicating potential influence of solar and El Nio activities on hydrological patterns. (2) The solar-modulated ENSO plays a critical linkage between solar activity and hydrological processes. The ENSO transferred the solar energy to the local precipitation in the 22-year band; the influence was transferred by ENSO to streamflow in the periodicities of 7 and 11 years; and the linkage did not function in the SUN-ENSO-aquifer interaction system. (3) The precipitation, river discharge and aquifer water level show different responses to solar activity and ENSO. Solar activity and ENSO propagate both direct and indirect impacts on hydrological processes in different spatial and temporal domains.