TY  - JOUR
A1  - Pathiraja, Sahani Darschika
A1  - Moradkhani, H.
A1  - Marshall, L.
A1  - Sharma, Ashish
A1  - Geenens, G.
T1  - Data-driven model uncertainty estimation in hydrologic data assimilation
JF  - Water resources research : WRR / American Geophysical Union
N2  - The increasing availability of earth observations necessitates mathematical methods to optimally combine such data with hydrologic models. Several algorithms exist for such purposes, under the umbrella of data assimilation (DA). However, DA methods are often applied in a suboptimal fashion for complex real-world problems, due largely to several practical implementation issues. One such issue is error characterization, which is known to be critical for a successful assimilation. Mischaracterized errors lead to suboptimal forecasts, and in the worst case, to degraded estimates even compared to the no assimilation case. Model uncertainty characterization has received little attention relative to other aspects of DA science. Traditional methods rely on subjective, ad hoc tuning factors or parametric distribution assumptions that may not always be applicable. We propose a novel data-driven approach (named SDMU) to model uncertainty characterization for DA studies where (1) the system states are partially observed and (2) minimal prior knowledge of the model error processes is available, except that the errors display state dependence. It includes an approach for estimating the uncertainty in hidden model states, with the end goal of improving predictions of observed variables. The SDMU is therefore suited to DA studies where the observed variables are of primary interest. Its efficacy is demonstrated through a synthetic case study with low-dimensional chaotic dynamics and a real hydrologic experiment for one-day-ahead streamflow forecasting. In both experiments, the proposed method leads to substantial improvements in the hidden states and observed system outputs over a standard method involving perturbation with Gaussian noise.
KW  - data assimilation
KW  - model error
KW  - uncertainty quantification
KW  - particle filter
KW  - nonparametric statistics
Y1  - 2018
U6  - https://doi.org/10.1002/2018WR022627
SN  - 0043-1397
SN  - 1944-7973
VL  - 54
IS  - 2
SP  - 1252
EP  - 1280
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Pathiraja, Sahani Darschika
A1  - Anghileri, Daniela
A1  - Burlando, P.
A1  - Sharma, A.
A1  - Marshall, L.
A1  - Moradkhani, H.
T1  - Insights on the impact of systematic model errors on data assimilation performance in changing catchments
JF  - Advances in water resources
N2  - The global prevalence of rapid and extensive land use change necessitates hydrologic modelling methodologies capable of handling non-stationarity. This is particularly true in the context of Hydrologic Forecasting using Data Assimilation. Data Assimilation has been shown to dramatically improve forecast skill in hydrologic and meteorological applications, although such improvements are conditional on using bias-free observations and model simulations. A hydrologic model calibrated to a particular set of land cover conditions has the potential to produce biased simulations when the catchment is disturbed. This paper sheds new light on the impacts of bias or systematic errors in hydrologic data assimilation, in the context of forecasting in catchments with changing land surface conditions and a model calibrated to pre-change conditions. We posit that in such cases, the impact of systematic model errors on assimilation or forecast quality is dependent on the inherent prediction uncertainty that persists even in pre-change conditions. Through experiments on a range of catchments, we develop a conceptual relationship between total prediction uncertainty and the impacts of land cover changes on the hydrologic regime to demonstrate how forecast quality is affected when using state estimation Data Assimilation with no modifications to account for land cover changes. This work shows that systematic model errors as a result of changing or changed catchment conditions do not always necessitate adjustments to the modelling or assimilation methodology, for instance through re-calibration of the hydrologic model, time varying model parameters or revised offline/online bias estimation.
Y1  - 2017
U6  - https://doi.org/10.1016/j.advwatres.2017.12.006
SN  - 0309-1708
SN  - 1872-9657
VL  - 113
SP  - 202
EP  - 222
PB  - Elsevier
CY  - Oxford
ER  -