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The morphometric analysis of alluvial drainage provides insights into its dynamics, erosion capacity, susceptibility to floods and possible genetic relations to tectonic faulting. In this study, we analysed the drainage system of Samos Island, located in the northern Aegean Sea. The results indicate a vulnerability to erosion and flooding events, and these intense phenomena concentrate mostly on third-order catchments. Two dissimilar drainage network systems are shown: an older drainage network system with a main NW–SE direction, which includes fourth- and fifth-order branches, and a recent drainage network system, which includes new, smaller order branches with a main NE–SW direction. The major tectonic fault orientations are NNW–SSE. The branches of the hydrographic network and faults present different directions, which indicates that the hydrographic pattern is not affected by tectonics.

To bring to fruition the capability of nature-based solutions (NBS) in mitigating hydrometeorologicalrisks (HMRs) and facilitatetheirwidespread uptake require a consolidated knowledge-base related to their monitoring methods, efficiency, functioning and theecosystem services they provide. We attempt to fill this knowledge gap by reviewing and compiling the existing scientific literature on methods, including ground-based measurements (e.g. gauging stations, wireless sensor network) and remote sensing observations (e.g. fromtopographic LiDAR, multispectral and radar sensors) that have been used and/or can berelevant to monitor the performance of NBS against five HMRs: floods, droughts, heatwaves,landslides, and storm surges and coastal erosion. These can allow the mapping of the risksand impacts of the specific hydro-meteorological events. We found thattheselectionandapplication of monitoring methods mostly rely on the particular NBS being monitored,resource availability (e.g. time, budget, space) and type of HMRs. No standalone methodcurrently exists that can allow monitoring the performance of NBS in its broadest view.However, equipment, tools and technologies developed for other purposes, such as forground-based measurements and atmospheric observations, can be applied to accuratelymonitor the performance of NBS to mitigate HMRs. We also focused on the capabilities ofpassive and active remote sensing, pointing out their associated opportunities and difficultiesfor NBS monitoring application. We conclude that the advancement in airborne and satellite-basedremote sensing technology has signified a leap in the systematic monitoring of NBSperformance, as well as provided a robust way for the spatial and temporal comparison ofNBS intervention versus its absence. This improved performance measurement can supportthe evaluation of existing uncertainty and scepticism in selecting NBS over the artificiallybuilt concrete structures or grey approaches by addressing the questions of performanceprecariousness. Remote sensing technical developments, however, take time to shift toward astate of operational readiness for monitoring the progress of NBS in place (e.g. green NBSgrowth rate, their changes and effectiveness through time). More research is required todevelop a holistic approach, which could routinely and continually monitor theperformanceof NBS over a large scale of intervention. This performance evaluation could increase theecological and socio-economic benefits of NBS, and also create high levels of theiracceptance and confidence by overcoming potential scepticism of NBS implementations.

Under climate change scenarios, it is important to evaluate the changes in recent behavior of heavy precipitation events, the resulting flood risk, and the detrimental impacts of the peak flow of water on human well-being, properties, infrastructure, and the natural environment. Normally, flood risk is estimated using the stationary flood frequency analysis technique. However, a site’s hydroclimate can shift beyond the range of historical observations considering continuing global warming. Therefore, flood-like distributions capable of accounting for changes in the parameters over time should be considered. The main objective of this study is to apply non-stationary flood frequency models using the generalized extreme value (GEV) distribution to model the changes in flood risk under two scenarios: (1) without nature-based solutions (NBS) in place and; (2) with NBS i.e. wetlands, retention ponds and weir/low head dam implemented. In the GEV model, the first two moments i.e. location and scale parameters of the distribution were allowed to change as a function of time-variable covariates, estimated by maximum likelihood. The methodology is applied to OPEn-air laboRAtories for Nature baseD solUtions to Manage hydro-meteo risks, which is in Europe. The time-dependent 100-year design quantiles were estimated for both the scenarios. We obtained daily precipitation data of climate models from the EURO-CORDEX project dataset for 1951–2020 and 2022–2100 representing historical and future simulations, respectively. The hydrologic model, HEC-HMS was used to simulate discharges/flood hydrograph without and with NBS in place for these two periods: historical (1951-2020) and future (2022-2100). The results showed that the corresponding time-dependent 100-year floods were remarkably high for the without NBS scenario in both the periods. Particularly, the high emission scenario (RCP 8.5) resulted in dramatically increased flood risks in the future. The simulation without NBS also showed that flooded area is projected to increase by 25% and 40% for inundation depth between 1.5 and 3.5 m under RCP 4.5 and RCP 8.5 scenarios, respectively. For inundation depth above 3.5 m, the flooded area is anticipated to rise by 30% and 55% in both periods respectively. With the implementation of NBS, the flood risk was projected to decrease by 20% (2022–2050) and 45% (2071–2100) with a significant decrease under RCP 4.5 and RCP 8.5 scenarios. This study can help improve existing methods to adapt to the uncertainties in a changing environment, which is critical to develop climate-proof NBS and improve NBS planning, implementation, and effectiveness assessment.Keywords: Nature-based solutions; flood frequency analysis; climate change; wetlands; GEV modelAcknowledgmentsThis work has been carried out under the framework of OPERANDUM (OPEn-air laboRAtories for Nature baseD solUtions to Manage hydro-meteo risks) project, which is funded by the European Union's Horizon 2020 research and innovation programme under the Grant Agreement No: 776848.

Nature-based solutions (NBS) for hydro-meteorological risks (HMRs) reduction and management are becoming increasingly popular, but challenges such as the lack of well-recognised standard methodologies to evaluate their performance and upscale their implementation remain. We systematically evaluate the current state-of-the art on the models and tools that are utilised for the optimum allocation, design and efficiency evaluation of NBS for five HMRs (flooding, droughts, heatwaves, landslides, and storm surges and coastal erosion). We found that methods to assess the complex issue of NBS efficiency and cost-benefits analysis are still in the development stage and they have only been implemented through the methodologies developed for other purposes such as fluid dynamics models in micro and catchment scale contexts. Of the reviewed numerical models and tools MIKE-SHE, SWMM (for floods), ParFlow-TREES, ACRU, SIMGRO (for droughts), WRF, ENVI-met (for heatwaves), FUNWAVE-TVD, BROOK90 (for landslides), TELEMAC and ADCIRC (for storm surges) are more flexible to evaluate the performance and effectiveness of specific NBS such as wetlands, ponds, trees, parks, grass, green roof/walls, tree roots, vegetations, coral reefs, mangroves, sea grasses, oyster reefs, sea salt marshes, sandy beaches and dunes. We conclude that the models and tools that are capable of assessing the multiple benefits, particularly the performance and cost-effectiveness of NBS for HMR reduction and management are not readily available. Thus, our synthesis of modelling methods can facilitate their selection that can maximise opportunities and refute the current political hesitation of NBS deployment compared with grey solutions for HMR management but also for the provision of a wide range of social and economic co-benefits. However, there is still a need for bespoke modelling tools that can holistically assess the various components of NBS from an HMR reduction and management perspective. Such tools can facilitate impact assessment modelling under different NBS scenarios to build a solid evidence base for upscaling and replicating the implementation of NBS., Graphical abstract Unlabelled Image, Highlights • There are no holistic models that integrate the functions, benefits and costs of NBS. • Numerical models with some improvements are promising for NBS efficiency evaluation. • Data intense and computational difficulties are the key cons of numerical models. • Cost–benefit/multicriteria decision analysis methods are used to monetise NBS projects. • The existing numerical models are lacking modules to optimise different NBS measures.

Morphometric analysis can be used to investigate catchment dynamics and tectonic processes responsible for the development of drainage catchments and to support flood risk assessment. In this study, a comparative GIS-based morphometric analysis between the main southern and northern sub-catchments of the Sperchios River basin, Central Greece, was performed, using geospatial and remote sensing data. The goal was to investigate their correlation with the peculiar geotectonic activity and the frequent flash-flood events that occur in the river floodplain. All sub-catchments characteristics are linked with the geological formation types of the area, in combination with ongoing tectonic activity. The results indicate that drainage network development is significantly controlled by the region&rsquo, s overall tectonic activity. The morphometric characteristics&mdash, i.e., bifurcation ratio, drainage density, circularity ratio, elongation ratio and water concentration&ndash, time values, reflect the flood-prone character of the southern part of Sperchios River catchment in comparison to the northern part, especially during intense rainfall events. The study can provide valuable insight into identifying how morphometric characteristics are associated with increased flood hazard.

Landslides can cause severe problems to the social and economic well-being. In order to effectively mitigate landslide hazards, the development of detailed susceptibility maps is required, towards implementing targeted risk management plans. This study aims to create detailed landslide susceptibility (LS) and landslide risk (LR) maps of the Sperchios River basin by applying an expert semi-quantitative approach that integrates the Geographic Information Systems (GIS)-based multicriteria analysis and Earth Observation (EO) data. Adopting the analytic hierarchy process (AHP) for a weighted linear combination (WLC) approach, eleven evaluation parameters were selected. The results were validated using a historic landslide database, enriched with new landslide locations mapped by satellite and aerial imagery interpretation and field surveys. Moreover, the landslide risk map of the area was also developed, based on the LS delineation, considering additionally the anthropogenic exposure and overall vulnerability of the area. The results showed that the most susceptible areas are located at the west and south-west regions of the basin. The synergistic use of GIS-based analysis and EO data can provide a useful tool for the design of natural hazards prevention policy at highly susceptible to risk landslide risk areas.

Droughts are comprehensive and complex naturally occurring hazards in any climatic region around the world and often result in the loss of life and severe ecosystem damage. Drought monitoring is usually based on single-variables that may not represent the corresponding risk appropriately to its multiple causation and impact characteristics under current and future climate scenarios. In order to address this issue, the multidimensional copulas function, which is a flexible statistical tool, could be applied to develop multivariate drought indicators and solve the complicated and nonlinear associations. The aim of this paper is to develop reliable designing, monitoring and prediction indicators for the proper assessment and intervention of drought risk by nature-based solutions (NBS). Using a copula-based multivariate drought indicator (CMDI) that considers all possible variables related to meteorological, agricultural and hydrological droughts is essential for better drought risk assessment and intervention. The CMDI was developed by integrating univariate marginal cumulative distribution functions of meteorological (precipitation), agricultural (soil moisture) and hydrological (streamflow) variables into their joint cumulative distribution function. CMDI was then applied to the selected study catchment (Po Valley, Italy and Spercheios River, Greece) using hydro-meteorological data from gauging stations and ERA5 gridded data for the period 1979-2017. The result of CMDI showed moderate, severe and extreme drought frequencies in the two selected catchments. The constructed CMDI captured more severe to extreme drought occurrence than the considered single drought indicators. This proved that the CMDI could appropriately represent the complex and interrelated natural variables. The uncertainty analysis based on Monte Carlo experiments confirmed that CMDI is a more robust and reliable approach for assessing, planning and designing a nature-based intervention for drought risk. The findings of this research can provide a reliable way to develop approaches that can be used for assessing and predicting non-linearly related variables or any risk that may occur simultaneously or cumulatively over time.

Abstract Droughts are comprehensive and complex naturally occurring hazards in any climatic region around the world and often result in the loss of life and severe ecosystem damage. Drought monitoring is usually based on single-variables that may not represent the corresponding risk appropriately to its multiple causation and impact characteristics under current and future climate scenarios. In order to address this issue, the multidimensional copulas function, which is a flexible statistical tool, could be applied to develop multivariate drought indicators and solve the complicated and nonlinear associations. The aim of this paper is to develop reliable designing, monitoring and prediction indicators for the proper assessment and intervention of drought risk by nature-based solutions (NBS). Using a copula-based multivariate drought indicator (CMDI) that considers all possible variables related to meteorological, agricultural and hydrological droughts is essential for better drought risk assessment and intervention. The CMDI was developed by integrating univariate marginal cumulative distribution functions of meteorological (precipitation), agricultural (soil moisture) and hydrological (streamflow) variables into their joint cumulative distribution function. CMDI was then applied to the selected study catchment (Po Valley, Italy and Spercheios River, Greece) using hydro-meteorological data from gauging stations and ERA5 gridded data for the period 1979-2017. The result of CMDI showed moderate, severe and extreme drought frequencies in the two selected catchments. The constructed CMDI captured more severe to extreme drought occurrence than the considered single drought indicators. This proved that the CMDI could appropriately represent the complex and interrelated natural variables. The uncertainty analysis based on Monte Carlo experiments confirmed that CMDI is a more robust and reliable approach for assessing, planning and designing a nature-based intervention for drought risk. The findings of this research can provide a reliable way to develop approaches that can be used for assessing and predicting non-linearly related variables or any risk that may occur simultaneously or cumulatively over time. Keywords: Drought risk; multidimensional copulas; multivariate indicators, uncertainty analysis; frequency Acknowledgements: This work is carried out under the framework of OPERANDUM (OPEn-air laboRAtories for Nature baseD solUtions to Manage hydro-meteo risks) project, which is funded by the Horizon 2020 under the Grant Agreement No: 776848.

Nature-based solutions (NBS) are being promoted as adaptive measures against predicted increasing hydrometeorological hazards (HMHs), such as heatwaves and floods which have already caused significant loss of life andeconomic damage across the globe. However, the underpinning factors such as policy framework, end-users' interestsand participation for NBS design and operationalisation are yet to be established. We discuss theoperationalisation and implementation processes of NBS by means of a novel concept of Open-Air Laboratories(OAL) for its wider acceptance. The design and implementation of environmentally, economically, technicallyand socio-culturally sustainable NBS require inter- and transdisciplinary approaches which could be achievedby fostering co-creation processes by engaging stakeholders across various sectors and levels, inspiring more effective use of skills, diverse knowledge, manpower and resources, and connecting and harmonising the adaptationaims. The OAL serves as a benchmark for NBS upscaling, replication and exploitation in policy-makingprocess through monitoring by field measurement, evaluation by key performance indicators and buildingsolid evidence on their short- and long-term multiple benefits in different climatic, environmental and socioeconomicconditions, thereby alleviating the challenges of political resistance, financial barriers and lack ofknowledge.We conclude that holistic management of HMHs by effective use of NBS can be achieved with standard compliant data for replicating and monitoringNBS inOALs, knowledge about policy silos and interaction betweenresearch communities and end-users. Further research is needed for multi-risk analysis of HMHs andinclusion of NBS into policy frameworks, adaptable at local, regional and national scales leading to modificationin the prevalent guidelines related to HMHs. The findings of this work can be used for developing synergies betweencurrent policy frameworks, scientific research and practical implementation of NBS in Europe and beyondfor its wider acceptance.

Nature-based solutions (NBS) are being promoted as adaptive measures against predicted increasing hydrometeorological hazards (HMHs), such as heatwaves and floods which have already caused significant loss of life and economic damage across the globe. However, the underpinning factors such as policy framework, end-users' interests and participation for NBS design and operationalisation are yet to be established. We discuss the operationalisation and implementation processes of NBS by means of a novel concept of Open-Air Laboratories (OAL) for its wider acceptance. The design and implementation of environmentally, economically, technically and socio-culturally sustainable NBS require inter- and transdisciplinary approaches which could be achieved by fostering co-creation processes by engaging stakeholders across various sectors and levels, inspiring more effective use of skills, diverse knowledge, manpower and resources, and connecting and harmonising the adaptation aims. The OAL serves as a benchmark for NBS upscaling, replication and exploitation in policy-making process through monitoring by field measurement, evaluation by key performance indicators and building solid evidence on their short- and long-term multiple benefits in different climatic, environmental and socio-economic conditions, thereby alleviating the challenges of political resistance, financial barriers and lack of knowledge. We conclude that holistic management of HMHs by effective use of NBS can be achieved with standard compliant data for replicating and monitoring NBS in OALs, knowledge about policy silos and interaction between research communities and end-users. Further research is needed for multi-risk analysis of HMHs and inclusion of NBS into policy frameworks, adaptable at local, regional and national scales leading to modification in the prevalent guidelines related to HMHs. The findings of this work can be used for developing synergies between current policy frameworks, scientific research and practical implementation of NBS in Europe and beyond for its wider acceptance.

The scope of this paper is to evaluate the short-term predictive capacity of the stochastic models ARIMA, Transfer Function (TF) and Artificial Neural Networks for water parameters, specifically for 1, 2 and 3 steps forward (m = 1, 2 and 3). The comparison of statistical parameters indicated that ARIMA models could be proposed as short-term prediction models. In some cases that TF models resulted in better predictions, the difference with ARIMA was minimal and since the latter are simpler in their construction, they are proposed for short-term prediction. Artificial Neural Networks didn’t show a good short-term predictive capacity in comparison with the aforementioned models.

In Scopia basin, central Greece, a hydrochemical investigation was completed. Groundwater samples from 41 sites were used to assess the natural and anthropogenic impacts in groundwater, utilizing the principal component analysis (PCA) involved with the inverse distance weighted (IDW) interpolation modeling and hierarchical cluster analysis (HCA). Best fit model to explain the spatial distribution of both hydrochemical parameters and PCA was chosen by optimizing the IDW interpolator’s parameters. Precision of the model was picked based on less root-mean-squared prediction error (RMSPE) amongst predicted and actual values measured at the same locations. Groundwater exhibit Ca–Mg–HCO3 as the dominant hydrochemical type and their greater part are mixed waters with non-dominant ion. Interpolation models demonstrate high estimations of nitrates in zones with agricultural activities and high estimations of nickel and chromium in regions with the strong presence of ultrabasic rocks. Dominant part of the groundwater samples surpasses in many cases the European Community (EC) drinking water permissible limits. Thus, they are unsuitable for human consumption. PCA illustrated four factors, which clarified 80.62% of the aggregate variance of data and HCA classified two statistically significant clusters of sampling sites. Results show natural procedures ascribed to the weathering of the minerals contained in the ultrabasic rocks and anthropogenic influences related to the use of fertilizers and wastewater leak. In light of FAO standards and Richards’s classification, the groundwaters are reasonable for irrigation purposes, featuring waters with low sodium hazard and moderate salinity hazard.

In the catchment area of Scopia, Central Greece, a lumped applied approach of water balance was accomplished, by applying the Zygos model, which delineates an essential water balance forms. The model is in view of Thornthwaite model, in its modified version. It can be adjusted automatically, with the evolutionary annealing-simplex technique for nonlinear optimization, or manually and the input data are precipitation and the potential evapotranspiration. In the present study, both programmed and manual calibration occurred. Programmed calibration took place, utilizing a specimen of measured runoff values from October 2009 to March 2011. Despite the fact that the Nash–Sutcliffe coefficient (NSC) value was high (0.87), the simulated parameters of water balance had abnormal significance for Scopia catchment area. On the contrary, manual calibration uncovered that the actual evapotranspiration constitutes 64.6% (450.1 mm) of the precipitation. Runoff and percolation represent 20.6% (143.6 mm) and 14.8% (102.9 mm) of the precipitation, respectively. The NSC (0.70) and the validation criteria exhibit an ideal adjustment of simulated to measured runoff, while the hydrological parameters appeared to have a physical significance for the study site. Zygos model connects the emergence of springs with the development of the karstification in the carbonate rocks of the region and affirms the predominance of runoff versus percolation due to the hydrolithological structure of the Scopia catchment. This is related to the presence of episodic floods in the area. The yearly precipitation values were found with an error of 0.2% and are viewed as insignificant.

The impact of shellfish farming on the water column nutrient concentration and the benthic community structure was investigated seasonally at a mussel farm (Mytilus galloprovincialis L.) of Maliakos Gulf (Eastern Mediterranean). In most cases concentration of nutrients and chl a at the farm site was lower than those observed at the control sites. Statistical analysis in concentration of all nutrients and chl a indicated significant interactions between sites and seasons, which in all cases were temporal. Furthermore, no significant interactions were found between sites and seasons for all sediment and benthic community parameters, except for Shannon–Wiener diversity index and species richness. The significant interactions detected in the biotic data were temporal. Analysis of similarity and similarity percentages analysis revealed a certain fluctuation of benthic macrofaunal community during the overall sampling year mainly due to temporal rather than spatial effects. Cluster analysis and multidimensional scaling ordination plot indicated the clearly separation of the farm site and the control sites in most of the seasons. K-dominance curves showed small difference in elevation and overlap indicating minimal difference in biological stress between the farm site and the control sites. All the above indicates that probably the farming of mussels was beneficial for the trophic status of Maliakos Gulf and that there was a minimal environmental stress caused by the shellfish farming for the surrounding benthic environment.

In the catchment of Xynias drained Lake, hydrologic processes simulation took place using a lumped approach with the conceptual model Zygos. The model implements a conceptual soil moisture accounting scheme extended with a groundwater tank and the input data were the monthly time series of rainfall and the potential evapotranspiration. The automatic optimization procedure of the model was implemented using the evolutionary annealing-simplex algorithm for maximum 11,000 iterations, inserting an 18-month observed runoff time series. It showed that hydrologic balance factors had non-physical significance for the study area. The model’s manual calibration for a Nash coefficient of 0.85 revealed that actual evapotranspiration constitutes 62.5 % (389.7 mm), runoff 22.7 % (141.8 mm) and infiltration 14.8 % (92.2 mm) of precipitation, showing optimal adaptation of simulated to observed runoff. The model estimated the initial reserve of soil moisture related to the presence of organic matter which increases water retention, a residue of the former lake. It confirmed zero runoff values during the summer months and connected the occurrence of springs and the outflows to other catchments (59.8 mm) with the karstification degree of the study area. The error on the annual rainfall is 4.9 % and is considered acceptable.

In Xynias drained Lake Basin's area, central Greece, a hydrogeochemical research took place including groundwater sampling from 30 sampling sites, chemical analysis, and statistical analysis. Groundwaters present Ca-Mg-HCO3 as the dominant hydrochemical type, while their majority is mixed waters with non-dominant ion. They are classified as moderately hard to hard and are characterized by oxidizing conditions. They are undersaturated with respect to gypsum, anhydrite, fluorite, siderite, and magnesite and oversaturated in respect to calcite, aragonite, and dolomite. Nitrate concentration ranges from 4.4 to 107.4 mg/L, meanwhile 13.3 % of the samples exceed the European Community (E.C.) drinking water permissible limit. The trace elements Fe, Ni, Cr, and Cd present values of 30, 80, 57, and 50 %, respectively, above the maximum permissible limit set by E.C. Accordingly, the majority of the groundwaters are considered unsuitable for drinking water needs. Sodium adsorption ratio values (0.04-3.98) and the electrical conductivity (227-1200 μS/cm) classify groundwaters as suitable for irrigation uses, presenting low risk and medium soil alkalization risk. Factor analysis shows that geogenic processes associated with the former lacustrine environment and anthropogenic influences with the use of fertilizers are the major factors that characterized the chemical composition of the groundwaters.

In the present study, an estimation of the future change in precipitation within the 21 century was carried out for the 12 climatic zones of Greece. The estimation is based on the results of 12 Regional Climate Models (RCM) simulations of the ENSEMBLES Program. The precipitation is projected to decrease mildly in the near future (2021-2050) and moderately in the far future (2071-2100) by 5% and 20% respectively for the area of Greece. The decrease seems to be most prominent during spring and summer, less prominent during winter. The autumn precipitation is projected to increase mildly in the near future and to decrease mildly in the far future. The projected precipitation change in Greece is expected to result in drastic changes on the ground water balance and on several sectors of human activity and environment parameters, as well.

To bring to fruition the capability of nature-based solutions (NBS) in mitigating hydro-meteorological risks (HMRs) and facilitate their widespread uptake require a consolidated knowledge-base related to their monitoring methods, efficiency, functioning and the ecosystem services they provide. We attempt to fill this knowledge gap by reviewing and compiling the existing scientific literature on methods, including ground-based measurements (e.g. gauging stations, wireless sensor network) andremote sensingobservations (e.g. from topographic LiDAR, multispectral and radar sensors) that have been used and/or can be relevant to monitor the performance of NBS against five HMRs: floods, droughts, heatwaves, landslides, and storm surges and coastal erosion. These can allow the mapping of the risks and impacts of the specific hydro-meteorological events. We found that the selection and application of monitoring methods mostly rely on the particular NBS being monitored, resource availability (e.g. time, budget, space) and type of HMRs. No standalone method currently exists that can allow monitoring the performance of NBS in its broadest view. However, equipments, tools and technologies developed for other purposes, such as for ground-based measurements andatmospheric observations, can be applied to accurately monitor the performance of NBS to mitigate HMRs. We also focused on the capabilities of passive and active remote sensing, pointing out their associated opportunities and difficulties for NBS monitoring application. We conclude that the advancement in airborne and satellite-based remote sensing technology has signified a leap in the systematic monitoring of NBS performance, as well as provided a robust way for the spatial and temporal comparison of NBS intervention versus its absence. This improved performance measurement can support the evaluation of existing uncertainty and scepticism in selecting NBS over the artificially built concrete structures or grey approaches by addressing the questions of performance precariousness. Remote sensing technical developments, however, take time to shift toward a state of operational readiness for monitoring the progress of NBS in place (e.g. green NBS growth rate, their changes and effectiveness through time). More research is required to develop a holistic approach, which could routinely and continually monitor the performance of NBS over a large scale of intervention. This performance evaluation could increase the ecological and socio-economic benefits of NBS, and also create high levels of their acceptance and confidence by overcoming potential scepticism of NBS implementations.

Anumber of Nature Based Solutions (NBS) are being used around the world in order toaddress various hydrometeorological hazards as a more environmentally friendly alternative tohard structures. Such a solution has been created in the Spercheios river basin in Central Greece,which is susceptible to heavy rainfall and river bank overflow due to flood water from upstream, inorder to mitigate flood and drought impacts under current and future climate conditions. Here afirst attempt is made touseactualmeasurements taken from various sources in the river, includingin‐situ and satellite data, in order to establish early experimental evidence of the NBS efficiency inthe area. The measurements include data from automated hydrological stations from the OpenHinetwork, satellite remote sensing data and field measurements performed along the SpercheiosRiver basin. For each measurement used, different analysis has been performed based on dataavailability and pertinence to the NBS efficiency. Preliminary results presented here show that theNBS functions as designed and provides protection against flooding in the area, and can potentiallydiminish the risk of drought. The results are in agreement with the numerical outputs alreadypresented in our previous work.