Author(s): L. Wei; T. Kinouchi
Linked Author(s): Wei Li
Keywords: The CASC2D model; Rainfall-runoff; Soil erosion and transport; The Kuchibuto River catchment
Abstract: A huge mass of radioactive substances were scattered when the Fukushima Daiichi Nuclear Power Plant exploded in Tohoku earthquake in Japan on March 11th 2011. Water flow is a vital driving force for radioactive substances migration. On the one hand, the fallout deposited from atmosphere onto ground by wet-deposition, on the other hand, once, radioactive substances reach the ground surface, and they will be adsorbed by soil particles. The contaminated soil particles will transport from terrestrial to aquatic systems, especially, in flood event when soil erosion is serious. So, better comprehensive of water cycles processes, typically, rainfall-runoff and contaminated sediment transport processes is very important for us to understand radioactive substances behavior and fate. The CASC2D model (TREX model) is an event-based model, and widely used in flood forecasting, soil erosion, and analysis of contaminant transport and fate. However, it is challenge for this Hortonian overland flow based model to apply in forested catchments. In this study, CASC2D model was improved by taking subsurface flow, groundwater flow, and evapotranspiration into account. Lateral and vertical subsurface flows in top soil were calculated by a simple kinematic storage model and a storage routing method, respectively. A linear reservoir method was employed to calculate groundwater flow. Evapotranspiration was estimated by FAO Penman-Monteith method. Infiltration and interception parts also are modified. Moreover, by considering of soil erosion caused by raindrop energy and water flow, we modified equations of soil erosion and transport on upland. The improved CASC2D model was used to continuously simulate runoff and sediment transport in the Kuchibuto River catchment, where large areas have been contaminated by radioactive Cesium. Storm events in 2011 and 2012 were selected for model calibration and validation, respectively. NashSutcliffe efficiency (NSE) values of simulated flow rates in 2011 and 2012 are 0.92 and 0.74; The R2 between simulated and observed TSS concentration are 0.64 and 0.52, and the RMSE of simulated TSS concentration are 0.1699 kg/m3, and 0.1127kg/m3 in 2011,2012 respectively. The error between simulated and observed accumulation of TSS exporting for the Kubutoto River to the Abukuma River from Jun 21 2011 to Dec. 31,2012 is-7%. Those results indicate simulated flow rate and sediment concentration showed a good agreement with those of observed; simulated results are acceptable, and the improved model can be used for continuous simulation in the study area. The results can be used for future study on transport and fate of radioactive substances in this catchment.
Year: 2014