Operational methodology for determining environmental flow
Abstract
As part of Subproject 6C of the National Laboratory for Water Science and Water Security Project (RRF-2.3.1-21-2022-00008), we are currently reviewing the methodology for determining ecological water demands related to surface waters. This study presents our findings to date.
The lawful application of ecological water demand in the field of operational water management has become a fundamental task. At the same time, in the context of mitigating and adapting to the impacts of climate change, it represents a continuously revisable responsibility for water resource managers. It is crucial to adopt a consensus-based methodology so that ecological water demand is not viewed merely as a limiting factor within water resources management, but rather as an essential water use that contributes to achieving or maintaining good water status, the survival of natural ecosystems, and, through this, the availability of sufficient and high-quality water resources for sustainable and equitable water uses.
Following a review of the international literature, we provide recommendations on the use of key terms. We propose to adopt the concept of environmental flow requirements, which enables the reconciliation of the ecological water demands – describing the needs of aquatic ecosystems – with the water demands of society within the framework of integrated water resources management.
As a first step in defining environmental flow requirements, we recommend a simple, standardizable hydrological methodology. The key innovative element of this methodology is that the minimum ecological flow value is determined on a monthly basis, depending on the size of the river basin. Similarly, the value of the natural water resource with 80% exceedance probability must also be calculated monthly. The difference between these two values represents the utilizable water resources, i.e., the maximum water quantity available for use by society – effectively defining an upper limit. This would ensure that the actual runoff available to ecosystems – shaped by water uses follows the natural flow dynamics of a given year, while also enabling more efficient utilization of water resources.
In line with the concept of environmental flow requirements, the ecological water demand values determined using this hydrological method provide a first approximation of the real water conditions necessary for the survival of natural ecosystems. In order to conserve nature, the specific ecological water demand values calculated in this way can be reviewed considering the hydromorphological parameters of the watercourse and the needs of biological elements. If necessary – for example, in data-deficient catchments, or in the presence of protected natural values and/or increased societal water demand – the ecological water demand may be modified based on a holistic assessment methodology, reflecting compromises agreed upon by societal stakeholders
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