ALBERT

Description: The 100 W-class ISCT100-v2 Hall Thruster (HT) has been characterized in terms of far-field plume properties. By means of a Faraday Cup and a Retarding Potential Analyzer, both the ion current density and the ion energy distribution function have been measured over a 180 ∘ circular arc for different operating points. Measurements are compared to far-field plume characterizations performed with higher power Hall thrusters. The ion current density profiles remain unchanged whatever the HT input power, although an asymptotic limit is observed in the core of the plume at high discharge voltages and anode mass flow rates. In like manner, the ion energy distribution functions reveal that most of the beam energy is concentrated in the core of the plume [ − 40 ∘ ; 40 ∘ ] . Moreover, the fraction of low energy ion populations increases at large angles, owing to charge exchange and elastic collisions. Distinct plume regions are identified; they remain similar to the one described for high-power HTs. An efficiency analysis is also performed in terms of current utilization, mass utilization, and voltage utilization. The anode efficiency appears to be essentially affected by a low voltage utilization, the latter originating from the large surface-to-volume ratio inherent to low-power HTs. Experimental results also show that the background pressure clearly affects the plume structure and content.

Description: The ecological integrity of freshwater ecosystems is intimately linked to natural fluctuations in the river flow regime. Anthropogenic alterations in flow regimes threaten water security and freshwater biodiversity in many regions of the world. The impacts of climate change on the hydrological cycle change local flow regimes and thus impact on the ecological systems. In catchments with little human-induced hydro-morphological changes, existing hydrological models can be used to predict changes in local flow regime in order to assess whether its rivers remain a suitable living environment for endemic species. However, hydrological models are traditionally calibrated to give a good general fit between observed and simulated hydrographs, e.g., using an optimisation with an objective function such as the Nash-Sutcliffe, or the Kling–Gupta efficiencies. Much ecological research has shown that aquatic species respond to very specific characteristics of the hydrograph, whether magnitude, frequency, duration, timing, and rate of change of flow events. Since each community in a river may be particularly sensitive to a few very specific streamflow characteristics, alternative hydrological model calibration strategies are needed, focussing on good performance for those specific characteristics. This study investigates the performance of a set of specially developed, bespoke, objective functions made of combinations of specific streamflow characteristics relevant for fish and invertebrate communities. These are compared with the more traditional objective functions on a set of 33 Irish catchments with little human regulation. A split-sample test with a rolling-window procedure is applied to reduce the influence of variations between the calibration/evaluation periods on the conclusions. These bespoke objective functions are shown to be better suited to predict the targetted streamflow characteristics in terms of performance in evaluation; however, traditional objective functions yield more consistent behavioural parameter sets, indicating a trade-off between model performance and model consistency when predicting streamflow characteristics, especially when the number of target streamflow characteristics are low.

Description: The ecological integrity of freshwater ecosystems is intimately linked to natural fluctuations in the river flow regime. In catchments with little human-induced alterations of the flow regime (e.g. abstractions and regulations), existing hydrological models can be used to predict changes in the local flow regime to assess any changes in its rivers' living environment for endemic species. However, hydrological models are traditionally calibrated to give a good general fit to observed hydrographs, e.g. using criteria such as the Nash–Sutcliffe efficiency (NSE) or the Kling–Gupta efficiency (KGE). Much ecological research has shown that aquatic species respond to a range of specific characteristics of the hydrograph, including magnitude, frequency, duration, timing, and the rate of change of flow events. This study investigates the performance of specially developed and tailored criteria formed from combinations of those specific streamflow characteristics (SFCs) found to be ecologically relevant in previous ecohydrological studies. These are compared with the more traditional Kling–Gupta criterion for 33 Irish catchments. A split-sample test with a rolling window is applied to reduce the influence on the conclusions of differences between the calibration and evaluation periods. These tailored criteria are shown to be marginally better suited to predicting the targeted streamflow characteristics; however, traditional criteria are more robust and produce more consistent behavioural parameter sets, suggesting a trade-off between model performance and model parameter consistency when predicting specific streamflow characteristics. Analysis of the fitting to each of 165 streamflow characteristics revealed a general lack of versatility for criteria with a strong focus on low-flow conditions, especially in predicting high-flow conditions. On the other hand, the Kling–Gupta efficiency applied to the square root of flow values performs as well as two sets of tailored criteria across the 165 streamflow characteristics. These findings suggest that traditional composite criteria such as the Kling–Gupta efficiency may still be preferable over tailored criteria for the prediction of streamflow characteristics, when robustness and consistency are important.