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A Deep Learning Post-Processing Method for Improving the Accuracy of NWM Streamflow Forecasts: Design and Validation of a Modified ETSformer


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Created: Oct 29, 2026 at 6:21 a.m. (UTC)
Last updated: Oct 30, 2026 at 2:20 p.m. (UTC)
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Abstract

Operational long-range (30-day) streamflow forecasts from NOAA’s National Water Model (NWM) carry a combination of model structural error and forecast-forcing error that grows with lead time, limiting their practical value for water-resources decision-making. This paper presents a learned post-processing framework that improves NWM long-range forecast skill without retraining the underlying physics-based model. The framework adopts the Exponential Smoothing Transformer (ETSformer) as its backbone and augments it with three components: a Conditional Frequency Channel Attention (CFCA) module that incorporates static basin attributes such as dam-regulation status, Reversible Instance Normalization (RevIN) to address non-stationary distribution shift across basins, and a zero-cost Linear Scaling step that removes residual systematic bias. We train and evaluate the framework exclusively on the NWM’s archived operational long-range forecast product, retrieved from the public BigQuery archive for Sep,2018 to Nov, 025 across 20 USGS-gauged river reaches distributed across the contiguous United States. We benchmark the framework against four LSTM variants (LSTM, SAM-LSTM, MHAM-LSTM, BOA-LSTM) and two alternative Transformer architectures (Pyraformer, iTransformer). The framework reduces unbiased root-mean-square error by approximately 77% relative to the strongest LSTM baseline and maintains its skill across the full 30-day forecast window rather than degrading with lead time. Ablation experiments show that CFCA, RevIN, and Linear Scaling each contribute, and that their gains are complementary. A leave-one-basin-out generalization experiment indicates that the model transfers to unseen watersheds with only marginal loss in accuracy. The results demonstrate that a Transformer combined with explicit non-stationarity correction and static-attribute conditioning can deliver stable long-horizon bias correction for the NWM operational forecast product.

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How to Cite

SHI, J., Yang, T., Ames, D. (2026). A Deep Learning Post-Processing Method for Improving the Accuracy of NWM Streamflow Forecasts: Design and Validation of a Modified ETSformer, HydroShare, http://www.hydroshare.org/resource/40b6200ace0345ed9987836c98a9b954

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