Abstract Hydrological models often exhibit equifinality, where different parameter sets produce similar hydrometric outputs but alter internal catchment processes and partitioning. Incorporating new information amongst calibration targets helps narrow the range of viable models. While stable water isotopes have gained traction in rainfall‐runoff model calibration, their application in integrated surface‐subsurface hydrological models (ISSHM) remains limited. We test how combining hydrometric (streamflow Q, groundwater levels GWL) and isotopic (stream δ18O) targets affects parameter sensitivity, guides calibration, and informs flow path inference. Using the ISSHM HydroGeoSphere model for the sub‐arctic Pallas catchment, we applied a workflow combining global sensitivity analysis, uncertainty and automated parameter estimation with PEST++ across multiple target combinations. Sensitivity analysis showed δ18O alone produced patterns similar to those from combined targets (Q + GWL + δ18O), suggesting isotopes can independently constrain key system parameters. All calibration setups performed comparably for hydrometric outputs but diverged in internal storages and partitioning. Combining isotopic and hydrometric targets led to greater recharge via rapid deep percolation and yielded the narrowest parameter uncertainty. δ18O‐only calibration yielded the widest uncertainty bounds yet reproduced similar core internal dynamics, underscoring its standalone diagnostic power. Hydrometric‐only calibration inferred minimal recharge, increased soil storage, and produced ‐potentially misleading‐ intermediate uncertainty. These findings demonstrate that δ18O reveals internal catchment dynamics undetected by hydrometric data alone in GW‐SW modeling. Although stable water isotope information (in the form of δ18O) cannot fully resolve equifinality, its integration with hydrometric targets improves parameter identifiability and model realism. We therefore advocate wider adoption of δ18O in ISSHM calibration.

Peatlands are the most carbon-dense terrestrial ecosystem, and recent studies have shown that the northern peatlands have been (and still are) expanding into new areas. However, depending on the vegetation and hydrological regime in the newly initiated areas, the climate forcing may vary. If these new areas developed as wet fen-type peatlands with high methane emissions, they would initially have a warming effect on the climate. On the other hand, if development began as dry bog-type peatlands, these new peatland areas would likely act as a strong carbon sink from early on. However, although some research has concentrated on the expansion of the new northern peatland areas, there remains a significant lack of studies on the successional development of the newly initiated peatland frontiers. In this research, we combine paleoecological, remote-sensing, and hydrological modeling methods to study the expansion and successional pathway dynamics in a subarctic fen margin in Finnish Lapland and discuss possible implications for the carbon balance of these marginal peatland areas. Our results show that (1) the studied peatland margins started to develop ca. 2000 years ago and have continued to expand thereafter and (2) this expansion has occurred in nonlinear fashion. In addition, wet fen-type vegetation persisted in the studied margin for the majority of the developmental history, and only dryer conditions after the Little Ice Age instigated the fen-to-bog transition. However, a notable part of the fen margins in the Lompolonvuoma and Lompoloj & auml;nkk & auml; basins has remained wet fen-type vegetation, and the persistence of this vegetation type was likely caused by the hydrological conditions in the peatland and surrounding catchment. Our findings show a large variation in the peatland expansion and succession dynamics, even within a single peatland basin. Although changes in climate conditions initiated the fen-to-bog process in some margins, some vegetation remained in the wet fen stage, showing resilience to allogenic forcings. Thus, when estimating the peatland carbon stocks and predicting the future trajectories for peatland development, this heterogeneity should be taken into account to avoid errors caused by the oversimplification of peatland lateral expansion dynamics.

Abstract. Peatlands are the most dense terrestrial carbon storage and recent studies have shown that the northern peatlands have continued to expand to new areas to this day. However, depending on the vegetation and hydrological regime in the newly initiated areas, the climate forcing may vary. If these new areas develop as wet, fen-type peatlands with high methane emissions they would initially have a warming effect on the climate. On the other hand, if the development starts as dry bog-type peatlands, these new peatland areas would likely act as a strong carbon sink from early on. However, although some research has concentrated on the expansion of the new northern peatland areas, there remains a significant lack of studies on the successional development of the newly initiated peatland frontiers. In this research, we combined palaeoecological, remote sensing and hydrological modeling methods to study the expansion and successional pathway dynamics in a subarctic fen margin in Finnish Lapland and discussed possible implications for carbon balance of these margin peatland areas. Our results showed that the studied peatland margins had started to develop ca. 2000 years ago and have continued to expand thereafter, and this expansion has occurred in non-linear fashion. In addition, the wet fen-type vegetation persisted in the studied margin for majority of the development history and only the dryer conditions after the Little Ice Age instigated the fen-to-bog transition. However, a notable part of the fen margins in the Lompolovuoma and Lompolojänkkä basins has remained as a wet fen-types, and their persistence was likely caused by the hydrological conditions in the peatland and in the surrounding catchment. Our findings show a large variation in the peatland expansion and succession dynamics even within a single peatland basin. Although changes in climate conditions had initiated the fen-to-bog process in some margins, some had remained in the wet, fen stage showing resilience to allogenic forcings. Thus, when estimating the peatland carbon stocks, and predicting the future trajectories for peatland development, this heterogeneity should be taken into account to avoid errors caused by over-simplification of peatland lateral expansion dynamics.

Groundwater-surface water (GW-SW) interactions play an important function in northern catchments, but their dynamics and role in water, carbon, and nutrient balances are still poorly understood. In this case study, we (1) explored groundwater-surface water dynamics in a sub-arctic headwater watershed in Pallaslompolo, Finnish Lapland. (2) We compared fully-integrated physically-based simulations of the groundwater and surface water and their interactions with unique spatially distributed field datasets of drone-derived thermal infrared imaging (TIR) and surface peat pore stable water isotopes. Simulations and field observations point to similar GWexfiltration areas, with an agreement between slightly over 50% for stable water isotopes and modeling and more than two thirds for thermal infrared and modeling. The agreement provides evidence that a fully-integrated GW-SW model with a simple parameterization approach can successfully identify the main features of GW-SW interactions in the high-latitude catchment. All methods suggest the complex nature of GW-SW exchange that is focused in topographical lowlands. Our simulation results also highlight how specific locations serve as groundwater exfiltration zones, surface water downwelling zones, or 'mixed' zones, where both interaction types occur at different times according to the hydrological state of the system. Modeling results suggest abrupt GWSW interaction reversals - a phenomenon known to occur in peatland areas. To conclude, the three methods should be treated as complementary when describing and understanding GW-SW interactions within peatlands, but only the fully-integrated physically-based modeling can provide comprehensive estimates for both the spatial and temporal patterns and magnitudes of GW-SW interaction fluxes.

Particularly in the Nordic region, water excess and shortage (drought) are becoming more frequent phenomena that challenge the development of agriculture and crop production. Identification of appropriate water management strategies is essential (i) to ensure sustainable water resources management for crop production and the functioning of healthy ecosystems; and (ii) to improve resilience to hydrological extremes. Integrated hydrological models offer that potential through understanding and forecasting of hydrological systems under anthropogenic and climatic influences, and providing information for improved decision-making. This study aims to develop a decision support instrument based on integrated hydrological modelling to identify appropriate management solutions and improve field- and catchment-scale water management in Nordic agriculture. The study area is Tyrnävä catchment, located in the northern part of Finland near Oulu city. Initially, the available hydro-climatological and hydrogeological data of the Tyrnävä catchment are characterized in detail. Then the hydrogeological parameters of the model are identified based on existing hydrogeological, climatic and remotely sensed data and their spatial, temporal and vertical variability. Next, a regional integrated surface-subsurface hydrological model is set up using HydroGeosphere. After successful calibration and validation using observed groundwater level, river discharge and soil moisture data, the model will be used in implementing and evaluating different management strategies (e.g., different irrigation options during droughts and controlled drainage management) for the future and their influence on the surface and groundwater systems. Uncertainty arising from different sources will be quantified using the Integrated Bayesian Multi-model Uncertainty Estimation Framework with the support of a supercomputer to improve the reliability and accuracy of the decision support instrument. Additionally, stakeholders’ involvement through local workshops is ensured throughout the modelling study, from the beginning to obtain reliable and useful decision support. Finally, based on these results, informed decisions regarding the appropriate water management can be made, which is important for sustainable water resources management for crop production and the functioning of healthy ecosystems particularly in Nordic agriculture.

Subarctic ecohydrological processes are changing rapidly, but detailed and integrated ecohydrological investigations are not as widespread as necessary. We introduce an integrated research catchment site (Pallas) for atmosphere, ecosystems, and ecohydrology studies in subarctic conditions in Finland that can be used for a new set of comparative catchment investigations. The Pallas site provides unique observational data and high‐intensity field measurement datasets over long periods. The infrastructure for atmosphere‐ to landscape‐scale research in ecosystem processes in a subarctic landscape has recently been complemented with detailed ecohydrological measurements. We identify three dominant processes in subarctic ecohydrology: (a) strong seasonality drives ecohydrological regimes, (b) limited dynamic storage causes rapid stream response to water inputs (snowmelt and intensive storms), and (c) hydrological state of the system regulates catchment‐scale dissolved carbon dynamics and greenhouse (GHG) fluxes. Surface water and groundwater interactions play an important role in regulating catchment‐scale carbon balances and ecosystem respiration within subarctic peatlands, particularly their spatial variability in the landscape. Based on our observations from Pallas, we highlight key research gaps in subarctic ecohydrology and propose several ways forward. We also demonstrate that the Pallas catchment meets the need for sustaining and pushing the boundaries of critical long‐term integrated ecohydrological research in high‐latitude environments.

The influence of seasonally frozen ground (SFG) on water, energy, and solute fluxes is important in cold climate regions. The hydrological role of permafrost is now being actively researched, but the influence of SFG has received less attention. Intuitively, SFG restricts (snowmelt) infiltration, thereby enhancing surface runoff and decreasing soil water replenishment and groundwater recharge. However, the reported hydrological effects of SFG remain contradictory and appear to be highly site- and event-specific. There is a clear knowledge gap concerning under what physiographical and climate conditions SFG is more likely to influence hydrological fluxes. We addressed this knowledge gap by systematically reviewing published work examining the role of SFG in hydrological partitioning. We collected data on environmental variables influencing the SFG regime across different climates, land covers, and measurement scales, along with the main conclusion about the SFG influence on the studied hydrological flux. The compiled dataset allowed us to draw conclusions that extended beyond individual site investigations. Our key findings were: (a) an obvious hydrological influence of SFG at small-scale, but a more variable hydrological response with increasing scale of measurement, and (b) indication that cold climate with deep snow and forest land cover may be related to reduced importance of SFG in hydrological partitioning. It is thus increasingly important to understand the hydrological repercussions of SFG in a warming climate, where permafrost is transitioning to seasonally frozen conditions.

Seasonally frozen ground (SFG) occurs on ~25% of the Northern Hemisphere’s land surface, and the influence of SFG on water, energy, and solute fluxes is important in cold climate regions. The hydrological role of permafrost is now being actively researched, but the influence of SFG has been receiving less attention. Intuitively, water movement in frozen ground is blocked by ice forming in soil pores that were open to water flow prior to freezing. However, it has been shown that the hydrological influence of SFG is insignificant in some cases, with soil remaining permeable to water even when frozen. There is a clear knowledge gap concerning (1) how intensively and (2) under what physiographical and climate conditions SFG influences hydrological fluxes. We conducted a systematic literature review examining the hydrological importance of SFG we found reported in 143 publications. We found a clear hydrological influence of frozen ground in small-scale laboratory measurements, but a more ambiguous effect when the spatial scale under study increased to hillslopes, catchments, or watersheds. We also found that SFG may be hydrologically less important in forested areas or in regions with deep snow cover. Our systematic review suggests that hydrological influence of SFG may become more important in a future warmer climate with less snow and intensified land use in high-latitude areas.

Fully integrated physically based hydrological modeling is an essential method for increasing hydrological understanding of groundwater‐surface water (GW‐SW) interactions in peatlands and for predicting anthropogenic impacts on these unique ecosystems. Modeling studies represent peat soil in a simplistic manner, as a homogeneous layer of uniform thickness, but field measurements consistently show pronounced spatial variability in peatlands. This study evaluated uncertainty in groundwater levels and exfiltration fluxes associated with the simplified representation of the peat soil layer. For transferability of the results, impacts of selected topographical and hydrogeological conceptual models on GW‐SW exchange fluxes were simulated in a hypothetical hillslope representing a typical aquifer‐mire transect. The results showed that peat soil layer geometry defined the simulated spatial GW‐SW exchange patterns and groundwater flow paths, whereas total groundwater exfiltration flux to the hillslope and groundwater level in the peatland were only subtly altered by different conceptual peat soil geometry models. GW‐SW interactions were further explored using different scenarios and dimensionless parameters for peat hydraulic conductivity and hillslope‐peatland system slope. The results indicated that accurate representation of physical peat soil properties and landscape topography is important when the main objective is to model spatial GW‐SW exchange. Groundwater level in virtual peatland was not greatly affected by groundwater drawdown in an adjacent aquifer, but the magnitude and spatial distribution of GW‐SW interactions was significantly altered. This means that commonly used groundwater depth observations near peat‐mineral soil interfaces and within peatlands may not be a suitable indicator for monitoring the hydrological state of groundwater‐dependent peatland ecosystems.