Towards more realistic runoff projections by removing limits on simulated soil moisture deficit

Keirnan Fowler, Gemma Coxon, Jim E Freer, Wouter J M Knoben, Murray C Peel, Thorsten Wagener, Andrew W Western, Ross A Woods, Lu Zhang

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

Rainfall-runoff models based on conceptual “buckets” are frequently used in climate change impact studies to provide runoff projections. When these buckets approach empty, the simulated evapotranspiration approaches zero, which places an implicit limit on the soil moisture deficit that can accrue within the model. Such models may cease to properly track the moisture deficit accumulating in reality as dry conditions continue, leading to overestimation of subsequent runoff and possible long-term bias under drying climate. Here, we suggest that model realism may be improved through alternatives which remove the upper limit on simulated soil moisture deficit, such as “bottomless” buckets or deficit-based soil moisture accounting. While some existing models incorporate such measures, no study until now has systematically assessed their impact on model realism under drying climate. Here, we alter a common bucket model by changing the soil moisture storage to a deficit accounting system in such a way as to remove the upper limit on simulated soil moisture deficit. Tested on 38 Australian catchments, the altered model is better able to track the decline in soil moisture at the end of seasonal dry periods, which leads to superior performance over varied historic climate, including the 13-year “Millennium” drought. However, groundwater and GRACE data reveal long-term trends that are not matched in simulations, indicating that further changes may be required. Nonetheless, the results suggest that a broader adoption of bottomless buckets and/or deficit accounting within conceptual rainfall runoff models may improve the realism of runoff projections under drying climate.
Original languageEnglish
Article number126505
JournalJournal of Hydrology
Volume600
Early online date29 May 2021
DOIs
Publication statusE-pub ahead of print - 29 May 2021

Bibliographical note

Funding Information:
This study was conducted with the support of the Australian Research Council and the Government of Victoria via Linkage Projects LP170100598 and LP180100796. Authors from the University of Bristol acknowledge the support of the UK's Natural Environment Research Council grant MaRIUS: Managing the Risks, Impacts and Uncertainties of droughts and water Scarcity (NE/L010399/1) and the Engineering and Physical Sciences Research Council grant EP/L016214/1 (WISE CDT). Streamflow, precipitation, and potential evapotranspiration data used in this study are publicly available as part of the CAMELS-AUS project (Fowler et al. 2021) at https://doi.pangaea.de/10.1594/PANGAEA.921850. Groundwater data were from www.vvg.org.au. The authors thankfully acknowledge the two anonymous reviewers whose feedback greatly improved the article.

Funding Information:
This study was conducted with the support of the Australian Research Council and the Government of Victoria via Linkage Projects LP170100598 and LP180100796. Authors from the University of Bristol acknowledge the support of the UK’s Natural Environment Research Council grant MaRIUS: Managing the Risks, Impacts and Uncertainties of droughts and water Scarcity (NE/L010399/1) and the Engineering and Physical Sciences Research Council grant EP/L016214/1 (WISE CDT). Streamflow, precipitation, and potential evapotranspiration data used in this study are publicly available as part of the CAMELS-AUS project ( Fowler et al., 2021 ) at https://doi.pangaea.de/10.1594/PANGAEA.921850. Groundwater data were from www.vvg.org.au. The authors thankfully acknowledge the two anonymous reviewers whose feedback greatly improved the article.

Publisher Copyright:
© 2021 The Authors

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