Experimental daily ensemble streamflow forecasting system using physical
model output in a Bayesian hierarchical framework
Abstract
River basin floods due to summer monsoon (June-September) rainfall are
the major causes of infrastructure damage and loss of human lives in
India. Thus, skillful forecasts of daily streamflows are crucial for
flood mitigation. We develop an experimental forecasting system that
combines a deterministic physical model forecast in a Bayesian
Hierarchical Framework to generate an ensemble daily streamflow
forecast. The physical hydrologic model based on the land surface model
– Variable Infiltration Capacity (VIC) - developed in an experimental
mode to model and forecast hydrologic systems over India is used.
Rainfall forecast from the Indian Meteorological Department (IMD) at
several lead times (1-day, 2-day, 3-day, 4-day, and 5-day) is used to
drive the VIC model to provide a single deterministic forecast trace. A
Bayesian Hierarchical Model (BHM) framework is developed to post-process
the VIC model forecast and generate skillful daily ensemble streamflow
forecast. We demonstrate the BHM framework to daily summer (July-August)
streamflow forecast at five stations in the Narmada River Basin in
Central India for the period 2003-2014 and, provide preliminary
assessment for the period 2015-2018. In this framework, the daily
streamflow at each station is modeled as Gamma distribution with time
varying parameters, which are modeled as a linear function of potential
covariates that include VIC model deterministic streamflow forecast and
observed spatially-averaged precipitation from the previous days. With
suitable priors on the parameters, posterior distributions of the
parameters and predictive posterior distributions of the daily
streamflows – and thus ensembles –are obtained. The skill of the
probabilistic forecast is assessed a suite of metrics (correlation
coefficient, and BIAS), rank histograms, and skill scores such as CRPSS.
The model skills are also assessed for various flow thresholds. The BHM
framework provides a novel, flexible and powerful approach to combine
forecasts from multiple models (including qualitative) and provide a
combined skill ensemble forecast. This will be of immense help to enable
effective disaster management and mitigation strategies.