The escalating impacts of global climate change significantly affect regional hydrological systems, particularly in northern areas such as Estonia. This study investigates the hydrological sensitivity of Estonian catchments to climatic variability, focusing on the interplay between surface water and groundwater. Using data from 42 river catchments, it employs various statistical methods in hydrology, emphasizing the autocorrelation function, cross-correlation function, baseflow index, and flow duration curve. The analysis spans the years 2012-2022, integrating hydrological, spatial, and water quality parameters. The research identifies four distinct hydrological behavior clusters: plateau, sandstone upland, carbonate upland, and lowland. Key findings include diverse catchment sensitivities to groundwater recharge, the role of baseflow in streamflow stabilization, the memory effect in catchment responses, and insights from the flow duration curve on flow variability and extremes. The LightGBM model, predicting focus parameters, highlights the critical influence of air temperature and snowpack on streamflow characteristics. This study underscores the diverse hydrological sensitivities of Estonian catchments to hydroclimatic changes, emphasizing the impor-tance of considering catchment-specific characteristics in water resource management and policy-making. Con- tributing to the broader understanding of hydrological processes, it provides valuable insights for future research and environmental planning in the face of climate variability and change.

Neighboring states sharing transboundary aquifers should carry out joint assessment of the common groundwater resources to fulfill the EU Water Framework Directive's and Water Convention's aims. Therefore, the establishment of a representative cross-border groundwater monitoring network is essential. The transboundary catchments of Estonia (EE) and Latvia (LV) are sparsely populated and feature a relatively scarce monitoring network. Springs are natural groundwater outflows that may represent a significantly greater catchment area than monitoring wells, and their monitoring is more cost-effective. But a thorough evaluation is required to select the most representative springs for particular groundwater bodies/transboundary aquifer systems. In this study, 59 springs were investigated in the EE-LV transboundary area for 37 hydrochemical parameters. Additionally, we assessed 32 monitoring wells to define the aquifer system end-members. In total 409 groundwater samples were analyzed. The sampled springs were pre-classified to one of the three aquifer systems: Quaternary (Q), Upper-Devonian (D3) and Middle/Upper-Devonian (D2). Significant differences among the pre-classified groups in terms of spring elevation, Q thickness and discharge were detected. Multivariate and machine learning techniques implementing barium as a tracer, were applied to link the studied springs to their main contributing aquifer systems. This study shows that the application of diverse hydrochemical and statistical methods help to evaluate the sources of spring water in an area with relatively homogenous groundwater chemistry. The developed conceptual models provide new generalized interpretation of transboundary aquifers, needed to improve groundwater monitoring networks in data-scarce areas using springs.

<p>As groundwater does not follow human-drawn boundaries such as country borders, groundwater pollution in one country can adversely affect groundwater quality and availability in a neighboring country. It is vital to develop a conceptual understanding of shared groundwater resources not only to ensure their protection, but also to avoid future conflicts, especially in a changing climate. Both the Water Convention and EU Water Framework Directive emphasize the need for joint assessment and management of transboundary groundwater resources (commonly referred to as “transboundary aquifers” or “groundwater bodies''), and it is crucial to establish a representative transboundary groundwater monitoring network to gather the necessary data. Often, the coverage of monitoring points in the existing groundwater monitoring networks is scarce in the peripheral areas and the installation of new wells would be economically unreasonable. Springs are natural groundwater outflows that can fill gaps in monitoring networks. Monitoring springs can be cost-effective, make water sampling easier, moreover, their water can provide information on a significantly larger catchment area than monitoring wells. The spring site cannot be selected like monitoring wells, so selecting the best springs requires a thorough preliminary assessment. A good conceptual understanding of the recharge area is a prerequisite for the selection of suitable monitoring springs. In this study, 46 springs in the transboundary area of Estonia and Latvia (NE Europe) were screened in 2021-2022 for a variety of geochemical parameters (field parameters, major ions, biogenic and trace elements, water stable isotopes). Some springs were sampled more than once to assess seasonal variability. Then springs were clustered based on their geochemical characteristics using multivariate statistics. This study is the first step in the procedure established on how to select representative springs for transboundary aquifer monitoring.</p><p>This study is financed by the Interreg Estonia-Latvia cooperation program project “WaterAct”, the EEA and Norway Grants Fund for Regional Cooperation project “EU-WATERRES”, and by performance-based funding of University of Latvia Nr.AAP2016/B041 within the “Climate change and sustainable use of natural resources” program.</p>

The European Union (EU) Water Framework Directive is of paramount importance for water management. According to the legal text, coordination with other directives like the Floods Directive is imperative and motivated by potential synergy effects. In this paper, the degree to which such coordination is achieved is evaluated for five Nordic and Baltic countries. The evaluation is based on legal documents, management plans, as well as on organizational structure in the five countries. The results show that Estonia and Lithuania have succeeded very well with regards to coordination, whereas Norway, Finland, and especially Sweden need to improve more.

European water policy requires to carry out nine tests for characterisation of groundwater bodies status, including the development of joint transboundary groundwater management principles. Gauja/Koiva and Salaca/Salatsi Rivers have a joint, Estonian/Latvian transboundary water cycle, including the groundwater recharge and discharge cycling. Despite the fact that groundwater is the only drinking water source in Gauja/Koiva and Salaca/Salatsi River basins and ensures the existence of many groundwater dependent ecosystems, the overall awareness of integrated cross-border management practice is still poorly understood and poorly linked in implementation of concrete groundwater protection actions. Taking into account the above-mentioned aspects, Latvian and Estonian groundwater and groundwater dependent ecosystems specialists from research, nature protection and groundwater resources management institutions have joined forces in the project funded by the Interreg Estonia-Latvia program: "Joint actions for more efficient management of common groundwater resources in Estonia and Latvia "(WaterAct). The ongoing Est-Lat project “WaterAct” (2020–2022) of joint transboundary groundwater management project organised into the three activity blocks: (1) The capacity building of the joint groundwater transboundary management through exchange of knowledge and best management practices between project partners and key experts in other European Union countries; (2) Assessment of groundwater resources in transboundary River basins to improve groundwater management in accordance with valid international directives; (3) Dissemination and outreach activities to increase the overall awareness of ecosystems friendly groundwater management and protection of key actors working with groundwater assessments and locals. Firstly, the joint cross-boundary principles of identification and status assessment of shared groundwater bodies will be developed. Adaptation of existing knowledge, cross-boundary harmonization and development of needed methodologies will be used. Secondly, the joint cross-boundary assessment principles will be implemented into groundwater resources management. The status assessment of shared groundwater bodies will be carried out in close cooperation between project partners to create materials necessary for the development of last River Basin Management Plans (2022-2027), required by the Water Framework Directive and Groundwater Directive. Thirdly, the dissemination of project results will be carried out (1) by compilation of Guidelines of Groundwater Dependent Ecosystems for different levels and fields of decision making and experts, (2) by compilation of Spring Water Monitoring Guide for Volunteers and starting volunteer monitoring. For volunteer monitoring, a special web-based map application will be developed (allikad.info). The project of “Joint actions for more efficient management of common groundwater resources” (WaterAct, Est-Lat155) funded by ERDF Interreg Estonia-Latvia cooperation programme.

This chapter presents the delineation approach and results of status assessment of Estonian wetlands for implementation of the objectives of the Water Framework Directive (WFD). Determination of WFD important wetlands based on selection and visualisation of wetlands soils from Estonian electronic soil map (data of Estonian Land Board), and corresponding drainage basin delineation, both for wetlands and water bodies by using the ArcMap10.2.2 software. There are 47 WFD important wetlands associated with the flowing water bodies covering more than 27,800 ha, and 19 wetlands associated with the standing water bodies covering 42,000 ha. The number of WFD important floodplain wetlands is 46 covering 15,000 ha, 24 spring mires on 390 ha and 107 coastal wetlands on 265 ha. Due to the heterogeneous landscape structure of Estonian wetlands inside the “zero” wetland contour line, the variability of both water quantity and quality of wetlands is relatively large. Therefore, without field studies of WFD important wetlands, it is impossible to perform a functional classification of the determined WFD important wetlands, i.e., their effect on nutrient retention or leaching, or effect on hydrological regime of connected with them water bodies.

Dynamic interactions between ground- and surface water are widely known, but the role of groundwater in terrestrial and aquatic ecosystems is often poorly understood and documented due to the spatiotemporal complexity. Many countries have not yet completed the assessment of groundwater dependent ecosystems (GDEs). GDEs are valuable ecosystems that depend on groundwater input and can not be considered and assessed separately. Changes in the quantity and chemical composition of groundwater recharge may result in significant and permanent damage on GDE flora and fauna. Aquifers are dynamic systems which are not subject to administrative boundaries and borders, therefore should be managed in close cooperation between neighbouring countries. According to the European Union’s Water Framework Directive 2000/60/EC, a groundwater body is considered to be in “poor status” if environmentally negative pressure on groundwater causes significant damage to related GDEs. The identification of GDEs in Estonia is currently underway. A theoretical approach on how to identify, assess, and monitor the groundwater dependent terrestrial ecosystems (GDTEs) has been developed. Similar climatic and hydrogeological conditions allow to adapt the methodology to Latvia and develop it jointly further. The first step in this joint methodology is to (i) find indicators and (ii) define criteria for (i) the evaluation of quantitative and qualitative effects of groundwater bodies on GDTEs and (ii) assessment of ecosystems. Subsequently, the quantitative and qualitative effects on GDTEs using assessment schemes must be identified. In this chapter, we are presenting a methodology for GDTE identification and assessment which could be used in similar situation in other countries.

Rewetted extracted peatlands are sensitive ecosystems and they can act as greenhouse gas (GHG) sinks or sources due to changes in hydrology, vegetation, and weather conditions. However, studies on GHG emissions from extracted peatlands after rewetting are limited. Methane (CH4) and nitrous oxide (N2O) emission fluxes were determined using the opaque closed chamber method along water level gradients from littoral zones to the open water body of constructed shallow lakes with different vegetation zones in a nutrient-rich rewetted extracted peatland in Sweden. Vegetation communities and their position relative to water level, together with short-term water level fluctuations, such as inundation events and seasonal droughts, and temperature had a significant impact on CH4 emissions fluxes. During "normal" and "dry" conditions and high soil temperatures, CH4 emissions were highest from Carex spp.-Typha latifolia L. communities. During inundation events with water levels > 30 cm, sites with flooded Graminoids-Scirpus spp.-Carex spp. emitted most CH4. Methane emissions from the water body of the constructed shallow lakes were low during all water level conditions and over the temperature ranges observed. Nitrous oxide emissions contributed little to the emission fluxes from the soil-plant-water systems to the atmosphere, and they were only detectable from the sites with Graminoids. In terms of management, the construction of shallow lakes showed great potential for lowering GHG emission fluxes from nutrient rich peatlands after peat extraction, even though the vegetated shore emitted some N2O and CH4.

Report, according to the EU Floods Directive (2007/60/EC) with insurance purposes was completed in Estonia, 2016. The output of this work fulfilled the Directive targets to produce the flood maps of inland water bodies, with return periods of 2, 5, 10, 25, 50, 100, 200, 500 and 1000 years. The flood maps were created in ArcMap10.2.2 and ArcHydro environments, based on data of absolute maximum water levels (WLs) of Estonian state hydrological network. Created 834 flood maps based on data of 152 gauging stations: flood maps of 37 fluvial water systems and 8 standing water bodies. The flood heights with corresponding return periods were obtained from probability analysis of WLs data. Obtained results illustrate that the higher risk for the floods is expected at intersections of branching streams in Low-Estonia and upland margins of High Estonia. Before completion of the report, the existing flood maps covered only a small fraction of Estonia, i.e., territory of 17 cities only. Developed by us the flood map creation tool, is simple, allows modelling and visualising both the flood heights and corresponding overflows of watercourses over the whole country. However, the outcome depends on the availability of hydrological data and the quality of digital elevation models.

Indices of connectivity are critical means for moving from qualitative to (semi-)quantitative evaluations of material (e.g., water, sediment and nutrients) transfer across the building blocks of a terrestrial system. In geomorphology, compared to closely related disciplines like ecology and hydrology, the development of indices has only recently started and as such presents opportunities and challenges that merit attention. In this paper, we review existing indices of sediment connectivity and suggest potential avenues of development for meeting current basic and applied research needs. Specifically, we focus on terrestrial geomorphic systems dominated by procesSes that are driven by hydro-meteorological forcing, neglecting seismically triggered events, karstic systems and environments controlled by eolian processes. We begin by setting a conceptual framework that combines external forcings (drivers) and system (intrinsic) structural and functional properties relevant to sediment connectivity. This framework guides our review of response variables suitable for sediment connectivity indices. In particular, we consider three sample applications concerned with sediment connectivity in: (i) soil studies at the plot scale, (ii) bedload transport at the reach scale, and (iii) sediment budgets at the catchment scale. In relation to the set of response variables identified, we consider data availability and issues of data acquisition for use in indices of sediment connectivity. We classify currently available indices in raster based, object or network based, and indices based on effective catchment area. Virtually all existing indices address the degree of static, structural connectivity only, with limited attention for process-based, functional connectivity counterparts. Most recent developments in indices of sediment connectivity deal, to some extent, with different styles of anthropogenic and hydro-meteorological forcings and with the temporal variability of sediment connectivity, by incorporating additional variables and parameters in existing indices. We believe that, in order to use structural connectivity as explanatory or predictive tool, indices need to be interpretable in relation to geomorphic processes, material properties, and forcing styles and magnitude-frequency spectra. Improvements in this direction can be made through studies shaped to constrain structural-functional correlations across a range of hydro-meteorological scenarios, for example employing field based techniques such as particle tracking and sediment provenance analysis, as well as numerical simulations. We further consider existing indices in relation to spatial and temporal scales. The latter have immediate implications on the distinction and application between indices and models of sediment connectivity. In this context, we suggest that sediment connectivity over millennial or longer time scales should be dealt with models, as opposed to indices.

Analyses of 50-year (1962–2011) monthly air temperature and precipitation data indicated substantial climate change in the locations of two raised bogs (Linnusaare and Männikjärve) in central-east Estonia. During recent years the cross-year winter air temperature increased by 1.7 ºC, while the cold-season precipitation increased by 4 mm. The fluctuation amplitude of temperature and precipitation values decreased. Snow depth proved to be the most sensitive variable to winter warming, followed by groundwater levels together with mean and maximum soil frosts. Long-term groundwater levels on the domes of the bogs and in the forested/treed lagg areas were 0.3−0.4 m and 0.4−0.8 m below the soil surface, respectively. Warming caused changes in groundwater level amplitude of 3−22 cm in the bog domes and 3−14 cm in the forested lagg zones. The lowest groundwater levels in ridge-pool ecotopes at Männikjärve rose by 6−10 cm (i.e. these ecotopes became wetter); but the incidence of low groundwater levels increased in most ecotopes, indicating a more general trend towards drier conditions in the bog.

The forest landscape across the Nordic and Baltic regions hosts numerous lakes and watercourses, which must be included in forest management. In this study, national policy designs regarding protection zones for surface waters on forest land were reviewed and compared for the Nordic countries, Estonia and Latvia. The focus was how each country regulates protection zones, whether they are voluntary or mandatory, and the rationale behind adopting a low or high degree of prescriptiveness. Iceland and Denmark had a low degree of policy prescriptiveness, whereas Norway, Estonia and Latvia had a high degree of prescriptiveness. Sweden and Finland relied to a large extent on voluntary commitments. The prescribed zone widths within the region ranged from 1 m to 5 km. The results indicated that land-use distribution, forest ownership structure and historical and political legacies have influenced the varying degrees of prescriptiveness in the region.

Restoration of wetlands is a high priority world-wide. Peat extraction areas can be restored by rewetting, however affecting the environment. It could be expected to turn the drained peat-cutover area from a source to a sink of most elements. This study examined effects of such rewetting on peat, hydrology and water chemistry over 15 years at two sites in Sweden; the nutrient-poor Porla peatland and the nutrient-rich Västkärr peatland. Rewetting caused minor changes to peat chemistry, but at the Västkärr site ammonium concentrations increased in superficial peat layers while nitrate decreased. In terms of hydrology, rewetting of the Porla site decreased annual runoff and both high and low discharges. Water pH at the Porla site stayed fairly stable, but at the Västkärr site pH, after an initial 4 years dip, gradually increased to higher values than before rewetting. Water colour and organic matter content were fairly stable, but slightly lower values were found after 15 years than in initial 4–5 years. The concentrations of base cations and of inorganic N were lower after rewetting, while total P was higher. However, these impacts could change from an initial phase as the wetlands in the long-term perspective develop into mires.

Digital elevation models (DEMs) describe the landscape topography, which is both a product and a control of the activity of geomorphic processes. In the same way, the connectivity of landscape units with respect to water and sediment fluxes can be seen as both a driver and an emergent property of the spatiotemporal interaction of hydrological and geomorphic processes. As DEMs are available with increasing quality, resolution and spatial coverage, they form an important basis for the quantitative assessment of connectivity through indices.

Restoration of wetlands is of high priority in Europe. After-use of peat excavation areas, including rewetting, is one such measure and has been investigated at two sites in Sweden. Water quality changed after rewetting with fairly stable or higher pH and concentrations of base cations. Nutrient concentrations were initially high but decreased after a number of years. Oxygen contents in water were similar to ordinary small lakes, also with occasional depletion in bottom layers at stagnation periods. The colonization of vegetation was rapid at Vastkarr site but slower at Porla site, also with start of Sphagnum colonization. The bottom fauna consisted of high numbers of species and individuals the very first years after rewetting. After a few years the bottom fauna decreased to lower levels but is now slowly rising.