Abstract
Sustainable water basin management requires characterization of flow regime in river networks impacted by anthropogenic pressures. Flow regime in ungauged catchments under current, future, or natural conditions can be assessed with hydrological models. Developing hydrological models is, however, resource demanding such that decision makers might revert to models that have been developed for other purposes and are made available to them (‘off-the-shelf’ models). In this study, the impact of epistemic uncertainty of flow regime indicators on flow-ecological assessment was assessed at selected stations with drainage areas ranging from about 400 to almost 90,000km2 in four South European basins (Adige, Ebro, Evrotas and Sava). For each basin, at least two models were employed. Models differed in structure, data input, spatio-temporal resolution, and calibration strategy, reflecting the variety of conditions and purposes for which they were initially developed. The uncertainty of modelled flow regime was assessed by comparing the modelled hydrologic indicators of magnitude, timing, duration, frequency and rate of change to those obtained from observed flow. The results showed that modelled flow magnitude indicators at medium and high flows were generally reliable, whereas indicators for flow timing, duration, and rate of change were affected by large uncertainties, with correlation coefficients mostly below 0.50. These findings mirror uncertainty in flow regime indicators assessed with other methods, including from measured streamflow. The large indicator uncertainty may significantly affect assessment of ecological status in freshwater systems, particularly in ungauged catchments. Finally, flow-ecological assessments proved very sensitive to reference flow regime (i.e., without anthropogenic pressures). Model simulations could not adequately capture flow regime in the reference sites comprised in this study. The lack of reliable reference conditions may seriously hamper flow-ecological assessments. This study shows the pressing need for improving assessment of natural flow regime at pan-European scale.
Highlights
The ecological status of water bodies can be assessed on the basis of flow regime, morphology, water quality, and biological elements (EC, 2000; Pistocchi et al, 2016)
Following Murphy et al (2013), IHAs set was used as an example as it provides the most comprehensive suite of indicators, considered representative of all flow regime characteristics (Table 3; results for all indicators of Table 3 are shown in the Supplementary Information (SI))
Differences in modelled and observed flow magnitude are mirrored in the uncertainty of magnitude indicators (Fig. 7)
Summary
The ecological status of water bodies can be assessed on the basis of flow regime, morphology, water quality, and biological elements (EC, 2000; Pistocchi et al, 2016). Flow-ecological frameworks developed to provide guidelines for sustainable water basin management are typically based on characterization of flow regime and its alterations (Poff et al, 2010; Laizé et al, 2014). Under these frameworks, the assessment of ecological status is inferred by the deviation of flow regime from what are indicated as ‘natural flow’ conditions. While any IHAs can be important locally depending on the environmental pressure and the biota responses, some IHAs have been considered consistently in several studies (Murphy et al, 2013; Archfield et al, 2014; Laizé et al, 2014)
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