Analyzing the performance and interpretability of hybrid hydrological models

Hydrological hybrid models have been proposed as an option to combine the enhanced performance of deep learning methods with the interpretability of process-based models. Among the various hybrid methods available, the dynamic parameterization of conceptual models using LSTM networks has shown high potential.  In this contribution, we extend our previous related work (Acuna Espinoza et al., 2023) by asking the questions: How well can hybrid models predict untrained variables, and how well do they generalize? We address the first question by comparing the internal states of the model against external data, specifically against soil moisture data obtained from ERA5-Land for 60 basins in Great Britain. We show that the process-based layer can reproduce the soil moisture dynamics with a correlation of 0.83, which indicates a good ability of this type of model to predict untrained variables. Moreover, we compare this method against existing alternatives used to extract non-target variables from purely data-driven methods (Lees et al., 2022), and discuss the differences in philosophy, performance, and implementation. Then, we address the second question by evaluating the capacity of such models to predict extreme events. Following the procedure proposed by Frame et al (2022), we train the hybrid models in low-flow regimes and test them in high-flow situations to evaluate the generalization capacity of such models and compare them against results from purely data-driven methods. Both experiments are done using large-sample data from the CAMELS-US and CAMELS-GB dataset. With these new experiments, we contribute to answering the question of whether hybrid models give an actual advantage over purely data-driven techniques or not. References Acuna Espinoza, E., Loritz, R., Alvarez Chaves, M., Bäuerle, N., & Ehret, U.: To bucket or not to bucket? Analyzing the performance and interpretability of hybrid hydrological models with dynamic parameterization. EGUsphere, 1–22. https://doi.org/10.5194/egusphere-2023-1980, 2023. Frame, J. M. and Kratzert, F. and Klotz, D. and Gauch, M. and Shalev, G. and Gilon, O. and Qualls, L. M. and Gupta, H. V. and Nearing, G. S., :Deep learning rainfall--runoff predictions of extreme events, Hydrology and Earth System Sciences, 26 ,3377-3392, https://doi.org/10.5194/hess-26-3377-2022, 2022 Lees, T., Reece, S., Kratzert, F., Klotz, D., Gauch, M., De Bruijn, J., Kumar Sahu, R., Greve, P., Slater, L., and Dadson, S. J.: Hydrological concept formation inside long short-term memory (LSTM) networks, Hydrology and Earth System Sciences, 26, 3079–3101, https://doi.org/10.5194/hess-26-3079-2022,  2022.