Hydrological connectivity in eco-hydrology: A foundational paradigm, significance, and perspectives

Hydrologic alterations designed to provide a stable water supply and to prevent flooding are commonly used in mediterranean-climate river (med-rivers) basins, and these alterations have led to habitat loss and significant declines in aquatic biodiversity. Often the health of freshwater ecosystems depends on maintaining and recovering hydrologic habitat connectivity, which includes structural components related to the physical landscape, functionality of flow dynamics, and an understanding of species habitat requirements for movement, reproduction, and survival. To advance our understanding of hydrologic habitat connectivity and benefits of habitat restoration alternatives we provide: (1) a review of recent perspectives on hydrologic connectivity, including quantitative methods; and (2) a modeling framework to quantify the effects of restoration on hydrologic habitat connectivity. We then illustrate this approach through a case study on lateral hydrologic habitat connectivity that results from channel restoration scenarios using scenarios with different historic and climate-change flows to restore fish floodplain habitat in a med-river, the San Joaquin River, California. Case study results show that in addition to the channel alterations, higher flows are required to recover significant flooded habitat area, especially given reductions in flows expected under climate change. These types of studies will help the planning for restoration of hydrologic habitat connectivity in med-rivers, a critical step for mediterranean species recovery.

Assessing hydrological connectivity for natural-artificial catchment with a new framework integrating graph theory and network analysis

ABSTRACTAssessing hydrological connectivity is crucial for maintaining the health and integrity of wetland and river‐lake ecosystems in arid regions as it plays a key role in watershed ecological balance and sustainable development. We utilized the Joint Research Center's global surface water dataset. We combined these data with connectivity indices and circuit theory to analyze the hydrological connectivity and spatiotemporal evolution of surface water in Xinjiang from 2000 to 2020. We determined that an increase in surface water area generally enhances regional hydrological connectivity. However, the fragmentation of water bodies may affect the quality of the potential connectivity by increasing the number of connecting pathways. The expansion and fragmentation of water patches alter their role in hydrological connectivity, with patches near large catchment areas often serving as critical connection points. Additionally, we found that the combined impacts of climate change and human activities led to an increase in the number of ecological corridors in Xinjiang from 427 in 2000 to 527 in 2020, with surrounding ecological pinch points and barrier areas showing increasing trends. This study provides new evidence for the spatiotemporal evolution of hydrological connectivity in Xinjiang from 2000 to 2020 and identifies priority areas for protecting and enhancing hydrological network connectivity. Our results provide a scientific basis for optimizing the conservation framework of river‐lake networks in Xinjiang, maintaining the integrity of aquatic ecosystems, and accelerating the environmental restoration of river‐lake networks.

Hydrologic connectivity and implications for ecosystem processes - Lessons from naked watersheds

• Micro-CT scanning of three distinct permafrost soils. • Pore connectivity and size distribution quantified in permafrost. • Macropore connectivity affects microbial community diversity. Soil pore structure plays a critical role in shaping soil microbial communities, which directly influence biogeochemical cycling. A notable impact of soil pore structure on microbial communities is the inverse relationship between microbial diversity and hydrological pore connectivity, where increased hydrological pore connectivity reduces microbial diversity. Although well-studied in temperate systems, the importance of hydrological pore connectivity on soil microbial community diversity in permafrost soils is largely unknown. Although once thought to be devoid of microbial activity, more recent advances demonstrate permafrost is an active ecosystem albeit less than most unfrozen soil. Thus, these principles that govern unfrozen soils could remain impactful in permafrost. In this study, our objective was to quantify permafrost pore structure and determine if the inverse relationship between soil hydrological pore connectivity and microbial diversity persists in permafrost. To address these objectives, we analyzed eight permafrost cores from three distinct sites in Alaska. To quantify soil pore characteristics, we scanned intact permafrost using X-ray computed tomography. The Euler characteristic number was used to measure pore connectivity and serve as a proxy for potential hydrological connectivity, as direct measurement of hydrological connectivity was not possible. DNA and RNA were extracted from the scanned permafrost and analyzed via amplicon sequencing of the 16S region to quantify the total and active microbial community diversity. We found that permafrost soil shares characteristics with temperate soils despite limits in our analytical resolution (i.e., at an instrument scanning resolution of 20 µm, only macro-scale features (>75 µm) could be quantified). For example, we found that pores in the range of 75–1000 µm are the dominant pore size class and a positive relationship between total porosity and pore connectivity. Additionally, we identified pore connectivity as a potential driver of microbial diversity and provided evidence that conditions before the formation of permafrost exert a strong legacy effect on currently observed permafrost microbial diversity. These insights help to explain how soil physical structure acts to influence microbial communities in this extreme environment.

Understanding nitrogen transport processes (NTP) is essential for effective watershed nitrogen (N) pollution management, and hydrological connectivity (HC) is an important way for studying these processes. However, the current researches primarily focused on conceptual and structural connectivity, limiting the deeper exploration of NTP. In this review, 136 papers were grouped into three categories: i) influencing factors of HC; ii) influencing factors of NTP and research methods; and iii) the relationship between HC and NTP. The reviewed contributions within each category were 36%, 33%, and 31% papers, respectively. The results showed that rainfall events, land use, and biogeochemical processes were the main factors affecting NTP. HC was mainly influenced by the river networks, human activities, and landscape patterns. The key methods used to study NTP include the stable isotope tracing method, MixSIAR, SWAT, and INCA-N. However, current research on the coupling of HC and NTP is insufficient to study changes in hydrological dynamics, hindering accurate identification of complex changes in NTP. To promote the accurate identification of NTP through the application of HC, we recommend that future research should: i) developing methods for characterizing hydrological functional connectivity (HFC) to enhance the understanding of hydrological changes processes; ii) incorporating HC indicators into NTP models to improve the understanding of NTP; iii) developing a prediction model that combines NTP models with machine learning (ML) to predict future characteristics of NTP changes. Overall, this review helps watershed managers make better decisions about when, where, and how to intervene effectively.

Historical and projected changes in hydrological and sediment connectivity under climate change in a tropical catchment of Mexico

Designing effective restoration strategies is a priority in recovering salt marsh plants. Hydrological connectivity is a main driver underpinning the success of the plant recovery process and can regulate life‐history process‐based restoration strategies. However, the relationship between these is unclear. Plant recovery needs to go through a whole life‐history process, from seed to adult. Common restoration strategies are seed addition (SA) or seedling transplantation (ST), which start from seed germination and seedling growth stage. Besides these two strategies, another strategy starting from seed retention stage, microtopographic adjustment (MA), was designed to study the relationship with hydrological connectivity. A framework was also constructed to assess a gradient of hydrological connectivity between marsh plain and sea. We conducted several field experiments to test their relationships. The composite measurement of hydrological connectivity with five geomorphic variables can well represent the variation of environmental factors. Soil moisture, inundation frequency and sediment deposition were positively correlated, while soil salinity and hardness were negatively correlated with hydrological connectivity. The success of different restoration strategies varied with hydrological connectivity. MA showed a monotone decreasing trend, while SA and ST showed a unimodal trend as hydrological connectivity increased. Importantly, each strategy occupies a non‐overlapping optimum range along the hydrological connectivity gradient. There is low hydrological connectivity for MA (0–0.28), middle hydrological connectivity for SA (0.28–0.55) and high hydrological connectivity for ST (0.55–1). Synthesis and applications. Our findings expand the quantification of the hydrological environment beyond elevation, distance or other single index to include a range of elements of hydrological connectivity, thus illustrating the underlying mechanisms of hydrological connectivity which regulate restoration strategies based on different life stages. The results provide a reliable framework to assess hydrological connectivity and offer guidance to select the optimum restoration strategy under different hydrological connectivities or to regulate the hydrological connectivity variables (topography on marsh plain and morphology of tidal creeks) to relief stresses. These findings will be beneficial to ecological restoration and coastal management.

Hydrological and sediment connectivity under freeze–thaw meltwater compound erosion conditions on a loessal slope

Functional connectivity related to road linear erosion at rainfall event scale in an agricultural watershed on the Loess Plateau

<p>Water is the main factor restricting and maintaining biological activities, and hydrological connectivity is closely related to many ecological processes. As a process that characterizes the transfer of energy and organisms among landscapes during the water cycle, hydrological connectivity establishes the interconnection between the material and energy flow of the landscape during the water cycle. Using bibliometric methods, hydrological connectivity related researches were searched via Web of Science and CNKI database, combining with Bibexcel, Ucient and Citespace procedures to obtain high-frequency words and keyword co-occurrence network views, we reviewed the research progress of hydrological connectivity abroad. The results showed that: 1) Regarding hydrological connectivity, the volume of publications both at home and abroad has shown an upward trend. The number of the publications showed significantly increased. 2) In terms of the frequency of keywords, many studies tend to focus on the research on hydrological connectivity of different types of ecosystem structure and function changes. 3) The analysis of the frequency of outbreak words showed that hydrological connectivity and climate change, biodiversity and ecosystem services have become research hotspots in this field. 4) According to the co-occurrence network view, we found that hydrological connectivity and ecological processes, the impact of different types of ecosystem hydrological connectivity on material transport, and the impact of changes in ecosystem structure and function on hydrological connectivity are the current research hotspots. Carrying out multi-scale hydrological connectivity mapping and multi-scale hydrological connectivity quantitative assessment and model simulation based on geographic information technology and long-term field monitoring data are the trends of future hydrological connectivity research directions.</p>

Identifying freshwater wetland suitable habitat through a hydrological-biological connectivity framework: A case study of Naoli River wetlands, China

The drivers that determine the hydrological connectivity (HC) are complex and interrelated, and disentangling this complexity will improve the administration of the river–lake interconnection system. Dongting Lake, as a typical river–lake interconnected system, is freely connected with the Yangtze River and their HC plays a major role in keeping the system healthy. Climate, hydrology, and anthropogenic activities are associated with the HC. In this study, hydrological drivers were divided into the total flow of three inlets (T-flow) and the total flow of four tributaries (F-flow). To elucidate the HC of the Dongting Lake, HC was calculated by geostatistical methods in association with Sentinel-2 remote sensing images. Then, the structural equation model (SEM) was used to quantify the impacts of hydrology (F-flow, and T-flow) and meteorology (precipitation, evaporation, and temperature) on HC. The geostatistical analysis results demonstrated that the HC showed apparent seasonal change. For East and West Dongting Lake, the dominant element was north–south hydrological connectivity (N–S HC), and the restricted was west–east hydrological connectivity (W-E HC), but the dominant element was E–W HC and the restricted was N–S HC in South Dongting Lake. The results of SEM showed that N–S HC was mainly explained by T-flow (r = 0.49, p < 0.001) and F-flow (r = 0.28, p < 0.05). T-flow, temperature (r = 0.33, p < 0.05), and F-flow explained E–W HC. The finding of this work supports the management of both the Dongting Lake floodplain and other similar river–lake floodplain systems.

Quantification of the spatio-temporal variations in hydrologic connectivity of small-scale topographic surfaces under various rainfall conditions

ABSTRACTMicrobial community assembly processes are closely related to the composition, structure and distribution of microbes. The changes in environmental conditions and species dispersal capacity induced by hydrological connectivity may significantly impact the microbial community assembly process in surface water, but the mechanisms remain unclear. To reveal how hydrological connectivity affects microbial community assembly processes, surface water samples were collected from the study watershed during periods of low, intermediate and high hydrological connectivity. An integrated 16S rRNA amplicon sequencing technology and phylogenetic null model approach were used to identify the assembly processes of the bacterial communities. The results showed an inverse relationship between hydrological connectivity and environmental heterogeneity, with the highest environmental heterogeneity observed at low connectivity levels. Bacterial alpha diversity under high hydrological connectivity gradients significantly exceeded those under low and intermediate hydrological connectivity. Beta diversity exhibited a trend towards biotic homogenisation as hydrological connectivity increased. The co‐occurrence network of bacterial communities under low hydrological connectivity was characterised by robust clustering and intricate interactions, whereas those under intermediate hydrological connectivity tended to form a more straightforward network. Furthermore, stochastic processes play a crucial role in bacterial community assembly, accounting for approximately 80% of the observed patterns. This was substantiated by piecewise structural equation modelling, which showed that environmental factors and biotic interactions exerted minimal influence on the bacterial community assembly. As hydrological connectivity increases, the assembly process shaping the bacterial community appears more stochastic. Moreover, the contributions of drift and heterogeneous selection in assembly processes were found to increase with hydrological connectivity, while the impact of dispersal limitation and homogeneous selection diminished. These insights provide a deeper understanding of the ecological mechanisms that govern microbial distribution patterns and succession in watershed surface water.