Improving Flood Forecasting Using Conditional Bias Aware Assimilation Of Streamflow Observations And Dynamic Assessment Of Flow Dependent Information Content

Accurate forecasting of floods is a long-standing challenge in hydrology and water management. Data assimilation (DA) is a popular technique used to improve forecast accuracy by updating the model states in real time using the uncertainty-quantified actual and model-simulated observations. A particular challenge in DA concerns the ability to improve the prediction of hydrologic extremes, such as floods, which have particularly large impacts on society. Almost all DA methods used today are based on least squares minimization. As such, they are subject to conditional bias (CB) in the presence of observational uncertainties which often leads to under- and over-prediction of the predict and over the upper and lower tails, respectively. To address the adverse impact of CB in DA, conditional bias penalized Kalman filter (CBPKF) and conditional bias penalized ensemble Kalman filter (CBEnKF) have recently been proposed which minimize a weighted sum of the error variance and expectation of the CB squared. Whereas CBPKF and CBEnKF significantly improve the accuracy of the estimates over the tails, they deteriorate performance near the median due to the added penalty. To address the above, this work introduces CB-aware DA, which adaptively weights the CB penalty term in real time, and assesses the flow-dependent information content in observation and model prediction using the degrees of freedom for signal (DFS), which serves as a skill score for information fusion. CB-aware DA is then comparatively evaluated with ensemble Kalman filter in which the marginal information content of observations and its flow dependence are assessed given the hydrologic model used. The findings indicate that CB-aware DA with information content analysis offers an objective framework for improving DA performance for prediction of extremes and dynamically balancing the predictive skill of hydrologic models, quality and frequency of hydrologic observations, and scheduling of DA cycles for improving operational flood forecasting cost-effectively.