Shiang-Jen Wu

Real-time correction of water stage forecast using combination of forecasted errors by time series models and Kalman filter method

This study modifies a real-time correction method for water stage forecasts (named the RTEC_TS&KF model) using the time series method developed by Wu et al. (Stoch Environ Res Risk Assess 26:519-531, 2012) (named the RTEC_TS model), by incorporating the Kalman filter (KF) model. The RTEC_TS&KF model adjusts the corrected water stage forecasts resulting from the RTEC_TS model by taking into account the uncertainties in the model structure/inputs as well as the measurement bias. In detail, the water stage forecasts are corrected by separately adding the forecasted errors by the times series model and KF method into the stage forecasts. As compared to the results from the RTEC_TS model using the forecasted and observed water stages for Typhoons Morakot (2009), Saola (2012) and Soulik (2013), the RTEC_TS&KF model not only effectively lessens the uncertainties in regard to the water stage forecasts, but also consistently presents high correction performance of water level forecasts for various rainstorm events. This reveals that the RTEC_TS&KF model is superior to the RTEC_TS model in the correction of water stage forecasts. In the future, the RTEC_TS&KF model will be applied in the real-time corrections of other hydrological variates, such as the outflow of a reservoir, in the case of observation being provided on time.

Modeling the effect of uncertainties in rainfall characteristics on flash flood warning based on rainfall thresholds

This study proposes a risk assessment framework for quantifying the reliability of the rainfall threshold used in flash flood warning, which should be influenced by the uncertainties in the rainfall characteristics, including rainfall duration, depth, and storm pattern. This risk assessment framework incorporates the correlated multivariate Monte Carlo simulation method, the Sobek 1D–2D hydrodynamic model, and a logistic regression equation to establish a quantile relationship of the rainfall threshold for quantifying reliability of the rainfall threshold. The Shuhu Creek catchment locates in East Taiwan, and its historical hourly rainfall records on eight typhoon events are used as the study area and data. The results from the proposed framework indicate that the variation in the rainfall threshold declines with the duration; 12-h duration associated with a stable coefficient of variance of the rainfall threshold appears to be appropriate for the flash flood warning in the Shuhu Creek catchment. Moreover, the issued rainfall thresholds in the Shuhu Creek catchment by Water Resources Agency in Taiwan have a low exceedance probability. This infers that inundation might occur as the observed rainfall depth approaches the threshold, so that it is necessary to lower the rainfall threshold in accordance with higher exceedance probability in order to achieve the goal of early flood warning.