Challenges for determining frequency of high flow spells for varying thresholds in environmental flows programmes

ABSTRACT

High flow spells (or “pulses”) are important flow components providing ecological triggers and connectivity in rivers. While the ecological importance of flow spells is well-recognized, the link between ecosystem processes and statistical methods used to define flow spells occurrence has received little attention. Commonly, a spell is defined as an event that exceeds a threshold for a minimum number of consecutive days; however, such arbitrary metrics may be ecologically irrelevant. For example, the ecological value of a sustained high flow spell may be unaffected by a brief period in which flows fall just below the nominated threshold. The inclusion of an independence criterion has the potential to better characterize the ecological relevance of spell metrics, but it introduces the additional problem of how best to define “independence”. Existing techniques present inconsistencies in the number of spells identified as the thresholds vary, and this becomes more apparent when characterizing streamflow behaviour over shorter planning periods. This paper presents a new spell metric that resolves the identified inconsistencies and ensures that the number of high flow spells of varying duration varies in a monotonic manner with the threshold. We retain the usual conceptual basis of high flow spells, but adopt an independence criterion that facilitates their characterization for operational purposes, which is more relevant to ecological functions. The simplicity of the approach allows easy incorporation in decision support tools where identifying high flow spells plays a critical role in making important decisions.

KEYWORDS:

• Environmental flow
• fresh
• pulse
• flow threshold
• spells analysis

Acknowledgments

The authors would like to thank Wayne Koster and Jane Roberts for their expert opinions identifying relevant flow spells for different ecological endpoints. This study was funded by the Australian Research Council (ARC Linkage project LP130100174) and a number of partner agencies.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. Using publicly available flow data and environmental flows account volumes at McCoy's Bridge (gauge 405232), near the lower end of the system. Environmental water contributing to a high-flow event was defined as amounts debited to the environmental account on any day when total volume was greater than 940 Mld^-1 (the highest baseflow recommendation for this part of the river; GBCMA 2014. Goulburn River: seasonal watering proposal 2014–2015. Goulburn-Broken Catchment Management Authority, Shepparton.), beyond that necessary to take total discharge in the system to 940 Mld^-1, i.e. only environmental water used beyond the high baseflow threshold. We appreciate the irony of calculating this statistic using the type of arbitrarily defined metric that this paper sets out to replace.

Additional information

Funding

Australian Research Council [ARC Linkage project LP130100174].