Effect of precipitation-influenced river influx on Lake Biwa water levels: time scale analysis based on an impulse response function

Lake water levels fluctuate at time scales ranging from minutes to many years. While extensive research has been devoted to seasonal and multiannual time scales, we examined water level changes at time scales of minutes to hours. By applying impulse response functions to Lake Biwa and its catchment, we demonstrated that it is possible to (1) identify the effects of natural oscillations governed by the shape of the lake, (2) determine the precipitation retention time at the surface (above the impermeable layer) of the catchment, and (3) estimate how long it takes precipitation falling in catchment areas to reach the lake. Between 2010 and 2012 we recorded water levels at high frequency in the South Basin of Lake Biwa and calculated response functions between precipitation and water levels. We categorized the delay time determined from the calculated response functions as direct precipitation on the lake, seiches, and river inflow (surface flow, return inflow). We then identified the runoff patterns of surface and return flows by considering the time scale of the delay. The method used in this study enables flood predictions in cases involving heavy rain and appropriate planning of preventive measures against flood damage. These results will be valuable for the development of management strategies to respond to climate change in lake catchment systems.