On the use of multi-objective optimization for multi-site calibration of extensive green roofs.

Conceptual hydrological models are practical tools for estimating the performance of green roofs. Such models require calibration to obtain parameter values, which limits their use when measured data are not available. One approach that has been thought to be useful is to transfer parameters from a gauged roof calibrated locally (single-site calibration) to a similar ungauged roof in a different location. This study tested this approach by transferring calibrated parameters of a conceptual hydrological model between sixteen extensive green roofs located in four Norwegian cities. The approach was compared with a multi-site calibration scheme that explores trade-offs of model performances between the sites. The results showed that single site calibration could yield optimal parameters for one site and perform poorly in other sites. In contrast, obtaining a common parameter set that yields satisfactory results (Kling Gupta Efficiency >0.5) for different sites, and roof properties could be achieved by multi-site calibration.

Latent heat exchange in the boreal and arctic biomes.

In this study latent heat flux (λE) measurements made at 65 boreal and arctic eddy-covariance (EC) sites were analyses by using the Penman-Monteith equation. Sites were stratified into nine different ecosystem types: harvested and burnt forest areas, pine forests, spruce or fir forests, Douglas-fir forests, broadleaf deciduous forests, larch forests, wetlands, tundra and natural grasslands. The Penman-Monteith equation was calibrated with variable surface resistances against half-hourly eddy-covariance data and clear differences between ecosystem types were observed. Based on the modeled behavior of surface and aerodynamic resistances, surface resistance tightly control λE in most mature forests, while it had less importance in ecosystems having shorter vegetation like young or recently harvested forests, grasslands, wetlands and tundra. The parameters of the Penman-Monteith equation were clearly different for winter and summer conditions, indicating that phenological effects on surface resistance are important. We also compared the simulated λE of different ecosystem types under meteorological conditions at one site. Values of λE varied between 15% and 38% of the net radiation in the simulations with mean ecosystem parameters. In general, the simulations suggest that λE is higher from forested ecosystems than from grasslands, wetlands or tundra-type ecosystems. Forests showed usually a tighter stomatal control of λE as indicated by a pronounced sensitivity of surface resistance to atmospheric vapor pressure deficit. Nevertheless, the surface resistance of forests was lower than for open vegetation types including wetlands. Tundra and wetlands had higher surface resistances, which were less sensitive to vapor pressure deficits. The results indicate that the variation in surface resistance within and between different vegetation types might play a significant role in energy exchange between terrestrial ecosystems and atmosphere. These results suggest the need to take into account vegetation type and phenology in energy exchange modeling.

Practice makes the model: A critical review of stormwater green infrastructure modelling practice.

Green infrastructures (GIs) have in recent decades emerged as sustainable technologies for urban stormwater management, and numerous studies have been conducted to develop and improve hydrological models for GIs. This review aims to assess current practice in GI hydrological modelling, encompassing the selection of model structure, equations, model parametrization and testing, uncertainty analysis, sensitivity analysis, the selection of objective functions for model calibration, and the interpretation of modelling results. During a quantitative and qualitative analysis, based on a paper analysis methodology applied across a sample of 270 published studies, we found that the authors of GI modelling studies generally fail to justify their modelling choices and their alignments between modelling objectives and methods. Some practices, such as uncertainty analysis, were also found to be limited, despite their necessity being widely acknowledged by the scientific community and their application in other fields. In order to improve current GI modelling practice, the authors suggest the following: i) a framework, called STAMP, designed to promote the standardisation of the documentation of GI modelling studies, and ii) improvements in modelling tools for facilitating good practices, iii) the sharing of data for better model testing, iv) the evaluation of the suitability of hydrological equations for GI application, v) the publication of clear statements regarding model limitations and negative results.

Assessment of flow ramping in water bodies impacted by hydropower operation in Norway - Is hydropower with environmental restrictions more sustainable?

Hydropower production is a key electricity generation technology in many parts of the world which can play a significant role in the transition towards a green and clean energy system. Hydropower can mobilize flexible energy on demand (hydropeaking) to balance out intermittent electricity from wind and photovoltaics. Adoption of hydropower as a peaking power source could lead to increased frequency of flow ramping in rivers downstream hydropower tailraces, which is one of the main stressors for riverine biota in alpine rivers. Both planned and accidental shutdowns of hydropower turbines need ecological mitigation. Our survey revealed that >3000 km of rivers downstream ca 800 hydropower plants in Norway may be ecologically impacted by non-natural flow fluctuations, and few have appropriate mitigation thresholds. A considerable eco-friendly peaking service may come from the Norwegian hydropower portfolio of over 19 GW installed capacity, with outlets into fjords, reservoirs or other large water bodies which normally dampen the ecological impacts of flow ramping. Intensive flow ramping occurs with irregular intervals from most types of hydropower. Although the highest frequency of stops were revealed in hydropower turbines not impacting river flow from storage hydropower, a significant number of turbine flow stops lasting over half a day in most types and categories of diversion hydropower. We suggest that further emerging ecosystem-based mitigations need to be adapted in hydropower licenses. This includes operational thresholds for both up and down ramping, constructional measures like by-pass valves, retention basins and increased base-flow or flow cap to ensure sustainability for hydropower operations. Our data reveal some of the most intensive hydropeaking operations from hydropower impacting longer rivers. Hence, our data underpins the potential for restoring downstream modified flow by ecosystem based measures related to both up and down ramping events in many regulated rivers.

Changes in short term river flow regulation and hydropeaking in Nordic rivers.

Quantifying short-term changes in river flow is important in understanding the environmental impacts of hydropower generation. Energy markets can change rapidly and energy demand fluctuates at sub-daily scales, which may cause corresponding changes in regulated river flow (hydropeaking). Due to increasing use of renewable energy, in future hydropower will play a greater role as a load balancing power source. This may increase current hydropeaking levels in Nordic river systems, creating challenges in maintaining a healthy ecological status. This study examined driving forces for hydropeaking in Nordic rivers using extensive datasets from 150 sites with hourly time step river discharge data. It also investigated the influence of increased wind power production on hydropeaking. The data revealed that hydropeaking is at high levels in the Nordic rivers and have seen an increase over the last decade and especially over the past few years. These results indicate that increased building for renewable energy may increase hydropeaking in Nordic rivers.

The Water Footprint of Hydropower Production-State of the Art and Methodological Challenges.

This paper reviews published estimates of water consumption from hydropower production and the methodologies applied. Published values range from negative to more than 115 000 m3 MWh-1. Most gross water consumption rates are in the range 5.4-234 m3 MWh-1, while most net values are in the range 0.2-140 m3 MWh-1. Net values are often less than 40% of the gross values, sometimes only 1% of the gross water consumption estimates. The extremely wide range in estimates is explained by an inconsistent methodology and the very site-specific nature of hydropower projects. Scientific challenges, such as allocation from multipurpose reservoirs, and spatial assignments in river basins with several hydropower plants, affect the results dramatically and remain unresolved. As such, it is difficult to propose "typical values" for water consumption from hydropower production. This paper points out directions of research in order to prepare a consistent and improved methodology for the calculation of water consumption from hydropower projects. This should take into account the role of reservoirs in the provision of a large range of water services, as well as providing regulated power to the energy system.

Detection and apparent survival of PIT-tagged stream fish in winter.

Environmental fluctuations exert strong control on behavior, survival, and fitness of stream biota. Technical improvements increasingly allow for tracking the response of large numbers of individuals to environmental fluctuations, for instance, by remote detection of animals equipped with PIT (passive integrated transponder) tags. PIT tags were implanted into 393 juvenile and adult brown trout Salmo trutta L. and European sculpin Cottus gobio L. in a boreal stream subjected to considerable ice formation. With weekly trackings over 6 months, we quantified apparent survival and detection probability in relation to biological, environmental, and methodological factors. Individuals with a higher physical condition in autumn showed a higher apparent survival; this pattern was consistent across all species and age classes. Detection probability decreased with increasing thickness of the surface ice layer; this effect was most pronounced for juvenile trout and benthic-living sculpin, both tagged with smaller-sized tags. Detection probability was reduced in structurally complex habitats. Our study demonstrates that apparent survival and particularly detection probability may show pronounced spatiotemporal variation. In order to compare results from different sampling occasions and sites, a good knowledge of the study site and of the regulating factors is crucial.

Hydrological and thermal effects of hydropeaking on early life stages of salmonids: A modelling approach for implementing mitigation strategies.

Alterations in hydrological and thermal regimes can potentially affect salmonid early life stages development and survival. The dewatering of salmon spawning redds due to hydropeaking can lead to mortality in early life stages, with higher impact on the alevins as they have lower tolerance to dewatering than the eggs. Flow-related mitigation measures can reduce early life stage mortality. We present a set of modelling tools to assess impacts and mitigation options to minimise the risk of mortality in early life stages in hydropeaking rivers. We successfully modelled long-term hydrological and thermal alterations and consequences for development rates. We estimated the risk of early life stages mortality and assessed the cost-effectiveness of implementing three release-related mitigation options (A,B,C). The economic cost of mitigation was low and ranged between 0.7% and 2.6% of the annual hydropower production. Options reducing the flow during spawning (B and C) in addition to only release minimum flows during development (A) were considered more effective for egg and alevin survival. Options B and C were however constraint by water availability in the system for certain years, and therefore only option A was always feasible. The set of modelling tools used in this study were satisfactory and their applications can be useful especially in systems where little field data is available. Targeted measures built on well-informed modelling tools can be tested on their effectiveness to mitigate dewatering effects vs. the hydropower system capacity to release or conserve water for power production. Environmental flow releases targeting specific ecological objectives can provide better cost-effective options than conventional operational rules complying with general legislation.