Changes In Hydrological Connectivity Research Articles

Soil macropores induced by plant root as a driver for vertical hydrological connectivity in Yellow River Delta

Abstract The protection and management of the wetland should consider the changes in hydrological connectivity caused by the structural modifications of the soil macropores. The main purpose of our work is to clarify and quantify the influence of the soil macropores volume on vertical soil hydrodynamic process mechanically and statistically by taking the form of a case-study in Yellow River Delta (YRD), and further reveal the vertical hydrological connectivity in this area. Based on X-ray computed tomography (CT) and constant head permeability test, the results showed a highly spatial heterogeneity of the soil structure in the YRD, hydraulic parameter (Ks) was negatively correlated with bulk density (BD) and positively with soil macropore volume, soil aeration (SA), and maximum water capacity (MWC). Using Hydrus 1-D software and the Green-Ampt model, we estimated the characteristics of hydrodynamic process in the soil without macropores, then evaluated the effect of the soil macropore on soil hydrological connectivity by comparing the experimental results with the simulation results. We found that increasing soil microporosity improved the convenience of water movement, which would enhance the hydrological connectivity of the region. The results will further help to reveal the eco-hydrological process at vertical scale in soil and provide a theoretical guide for wetland conservation and restoration.

Deciphering the interplay between tectonic and climatic forces on hydrologic connectivity in the evolving landscapes

The intricate interplay between climate and tectonics profoundly shapes landscapes over time frames surpassing 10 million years. Active tectonic processes and climatic shifts unsettle established drainage systems, instigating fragmentation or amalgamation of watersheds. These activities yield substantial transformation in surface hydrologic connectivity, thereby underlining the profound influence of these tectonic and climatic forces on the evolution of both landscape and hydrology. Such transformations within the hydrological landscape have direct implications for the evolution of aquatic species. As connections among aquatic habitats undergo reconfiguration, they incite shifts in species distribution and adaptive responses. These findings underscore the role of tectonics and climate in not only sculpting the physical landscape but also steering the course of biological evolution within these dynamically changing aquatic ecosystems relying on hydrologic connections. Despite the significance of these interactions, scholarly literature seldom examines alterations in hydrologic connectivity over tectonic, or orogen-scale, timescales. This study aims to bridge this gap, exploring changes in hydrologic connectivity over extended periods by simulating a continental rift system akin to the Rio Grande Rift, USA, subject to various tectonoclimatic scenarios. Multiple rift basins hosting large lakes, brought into existence by active tectonic extension, are further molded by tectonic extension and post-rift climatic changes. The study focuses on phenomena such as interbasin river breakthroughs and knickpoint generation, assembling a time-series of connectivity metrics based on stream network characteristics such as flow rate, flow distance, and captured drainage areas. We anticipate that the insights gleaned from this study will enhance our comprehension of the enduring impact of tectonic and climate processes on hydrologic connectivity and the subsequent evolution of aquatic species.

The connectivity threshold of the wetlands based on water environment response, and a case study of China’s Baiyangdian wetland

The quality of the water environment of wetlands is closely related to the changes in hydrologic connectivity, so wetland responses to changing connectivity exhibit a range of optimal connectivity values. Identifying these ranges of values is of great significance for wetland ecological restoration. Current studies mostly determine the hydrologic connectivity ranges based on a single target, even though connectivity simultaneously affects multiple water environment targets. Hence, the optimal hydrologic connectivity should be determined by accounting for multiple targets. With the Baiyangdian Wetland as the study area, we developed a method to model how the water environment responded to changes in hydrologic connectivity from three perspectives: hydrodynamics, water quality, and aquatic habitat. To do so, we used the BP neural network model and a non-dominated sorting genetic algorithm to calculate the hydrologic connectivity values in a context with multiple targets. We found that under the optimal hydrodynamic target, the integral index of connectivity (IIC) threshold was [0.23, 0.61] and the probability connectivity (PC) threshold was [0.32, 0.68]; under the optimal water quality target, the IIC threshold was [0.42, 0.70] and the PC threshold was [0.48, 0.82]; under the optimal aquatic habitat target, the IIC threshold was [0.38, 0.69] and the PC threshold was [0.35, 0.71]; and under the comprehensive optimization target, the IIC threshold was [0.48, 0.64] and the PC threshold was [0.51, 0.73]. These ranges of values will help wetland managers determine the optimal ecological water releases to meet the ecosystem quality targets.

Quantifying the variation of Scaly-sided merganser’ population and their divergent response to changes in hydrological connectivity of rivers in a high-latitude water tower

The Scaly-sided Merganser (SSME) is a globally endangered waterbird species, primarily inhabiting rivers. In recognition of its precarious status, it was added to the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species in 2002. The SSME is highly sensitive to ecological environmental changes driven by changes in hydrological connectivity, particularly within riverine habitats. Changbai Mountain, a high-latitude water tower in the Northeast Asia, is considered one of the most optimal breeding grounds for the SSME worldwide. Its extensive water system and dense primary forest create an environment particularly well-suited for this species. Over the past 40 years, rivers within Changbai Mountain have experienced phases of hydropower development and ecological restoration. These activities have dramatically altered the hydrological connectivity of the area. However, the response of SSME to these changes in aquatic ecology, driven by shifts in hydrological connectivity, remains uncertain. We employed the AutoRegressive Integrated Moving Average (ARIMA) model to analyze changes in the population size of the SSME. Furthermore, the GeoDetector model was used to understand the impact of ecological changes on the maximum population size (MPS) and distribution of SSME in the context of alterations in hydrological connectivity. Our findings reveal that the restoration of hydrological connectivity, a method of ecological restoration, exerts a bidirectional influence on both the distribution and the MPS of SSME. Our findings can be summarized as follows. Firstly, there was a general trend of increase in both the distribution points and population size of the SSME. Secondly, the contribution of driving factors to the MPS of SSME was ranked in the following order: temperature > Connectivity Status Index (CSI) > river width > fish abundance > river depth > velocity > altitude > forest type. Thirdly, the impacts on the distribution and MPS of SSME were not due to a single factor, but rather a complex interplay of multiple factors. Finally, the construction of small hydropower stations significantly disrupted the hydrological connectivity of rivers, thereby altering the natural habitat of the SSME. The reduction in fish population, resulting from the loss of hydrological connectivity, adversely affected the MPS. Conversely, an increase in water temperature due to loss of hydrological connectivity positively impacted the MPS. However, these effects displayed heterogeneity across different rivers. Our study suggests that the construction of gravel dams may be a suitable approach in certain sections of the river, with a recommended height limit of 1.5 m. This research provides valuable insights for managers, offering a sustainable foundation for future energy planning and ecological restoration efforts.

Integrating hydrological connectivity and zooplankton composition in Arctic ponds and lakes

Abstract Arctic landscapes are characterised by their numerous ponds and lakes. With increased climate warming and consequent changes in hydrological connectivity, the ecology of many of these waterbodies is expected to change. We sampled 13 ponds and 22 lakes for zooplankton and various limnological and physical environmental variables in the vicinity of Cambridge Bay, Nunavut (69.1169° N, 105.0597° W), with the aim of testing how well the degree of hydrological connectivity explained patterns in species composition and abundance and contributed to crustacean production. Ponds were hydrologically isolated while the lakes were arranged in four lake chains and ranged from the headwaters to highly inter‐connected lakes receiving water from one to 768 upstream lakes. In all sites combined, 77 zooplankton species were found, including 56 rotifers, six copepods, 11 cladocerans, two fairy shrimp species, a mysid, and a tadpole shrimp. We show that the zooplankton communities differed between hydrologically isolated (ponds) and connected (lakes) systems, with 17 species unique to ponds and 20 to lakes. Furthermore, the communities were more similar within lake chains and hence in lakes closer to each other. In ponds, distances between waterbodies had no impact on community similarity. Zooplankton abundance was higher in lakes (255 ind/L) than in ponds (65 ind/L) due to the higher number of rotifers that accounted for nearly 80% of the zooplankton abundance in lakes. In ponds, rotifers, cladocerans, and copepods were equally abundant. In terms of biomass, cladocerans represented 65% of total biomass in all waterbodies except for a chain of deep lakes that had abundant fish communities. In these lakes, cladoceran abundance and biomass were low and probably limited by fish predation. Zooplankton production was higher in ponds (4.6 mgC m−3 day−1) than in lakes (1.7 mgC m−3 day−1), and within lakes was lowest in the chain composed of large lakes (1.3 mgC m−3 day−1). The high production in ponds was closely linked to high zooplankton biomass and further explained by high gross primary production supported by relatively high nutrient concentrations in these shallow systems. Our study emphasises that hydrological connectivity is key to shaping zooplankton communities, although fish presence also appears to affect them. It shows that isolated ponds (that currently account for c83% of all waterbodies on southern Victoria Island) are hotspots of aquatic biological productivity that probably play an essential role in Arctic freshwater landscapes. Finally, our study provides critical baseline information on the current composition of Arctic zooplankton communities important for estimating climate change impacts.

Evaluation of river connectivity using a composite index method and its impact on nutrients dynamics in large rivers

The connectivity of large rivers is impaired by human activities, which could cause severe consequences to hydrodynamics, water quality. To clarify the changes in hydrological connectivity of large rivers and their effects on nutrients, a composite index method of river connectivity in longitudinal, lateral and vertical dimensions was developed in this study. Taking the middle reaches of the Yangtze River as an example, the hydrological connectivity of typical hydrological years and periods in the region was analyzed, and the responses of critical nutrient indexes to varied river connectivity was assessed. The results showed that significant temporal and spatial variations in river connectivity were observed, with better river connectivity in the wet year than the dry year, and worse connectivity in headwaters, tributaries, or downstream of gates and dams than other river reaches. This was mainly due to the degree of river fragmentation (DOF) and the degree of regulation (DOR) with a relative importance of 81.28% and 56.07%, respectively, implying that gate and dam interception was the main cause of impaired river connectivity in the study area. Statistical methods indicated that the impaired connectivity in the middle reaches of the Yangtze River implied a stronger retention of nutrients by dams, and that retention effect was greater during the flood period than the non-flood period. The stronger negative correlation between connectivity and TP (correlation coefficient = −0.58) than TN (correlation coefficient = −0.46) during the flood period revealed that phosphorus was more influenced by river connectivity. The results of this study provide important information on water resources and water quality for the management of large rivers.

Identifying changes in the hydrological connectivity and their drivers in the Liaohe Delta wetland

Hydrological connectivity is of great significance to the formation and development of wetlands. Identifying the causes of reduced hydrological connectivity is also critical for restoring wetland systems. Despite its importance, research to identify connectivity or blockage and quantify the impact of different drivers on wetland hydrological connectivity remains a challenge due to the lack of appropriate data at large scales. This paper quantitatively analyzed the hydrological connectivity in the Liaohe Delta wetland using multispectral Landsat series datasets from 1986 to 2020 in the Google Earth Engine (GEE) by combining water area, connectivity function value (CFV), average monthly inundation frequency. The paper also defined identification criteria for connectivity or blockage and quantified the drivers of changes in hydrological connectivity by combining the identified results with wetland classification data. The results showed that (1) over the past 35 years, water area has slightly decreased and has shown a seasonal difference, with summer, autumn, and spring in decreasing order. The spatial distribution of water has expanded from land to sea. (2) CFV decreased and connectivity blockage occurred. The maximum hydrological connectivity distance has undergone fluctuations, exhibiting a decrease in its threshold value from 62.37 km between 1986 and 2011, to 55.2 km between 2011 and 2013. Subsequently, the threshold increased back to 62.37 km between 2013 and 2014, before declining to 55.17 km between 2014 and 2020. Heavy inundation and total inundation were mainly concentrated in nascent estuary areas and shallow sea areas. (3) Connectivity blockage has been a major trend in connectivity change over the past 35 years, with the most severe blockage occurring from 2006 to 2015. The factors influencing connectivity blockage were more natural than human factors before 2006, after which human factors gradually became the dominant driving factor. Over the past 35 years, hydrological connectivity has decreased and connectivity has been blocked. Human activities have become the main factor in the blockage of connectivity in the Liaohe Delta wetland since 2006. This study provides valuable insights for the conservation and restoration of hydrological connectivity in the Liaohe Delta wetland.

Intra‐event concentration–discharge relationships affected by hydrological connectivity in a karst catchment

AbstractConcentration‐discharge (C‐Q) relationship in streamflow provides insights into hydrological transport at the catchment scale. Changes in hydrological connectivity during runoff events often dominate flood and solute export in the karst catchment. However, only few studies have explored intra‐event C‐Q relationships and how they are affected by hydrological connectivity in karst catchment. In this study, we explored the intra‐event C‐Q relationships in underground channel flows by integrating solute concentration, discharge and modelled flow age in a 1.25 km2 karst catchment in southwest China. We apply piecewise functions to characterize the C‐Q relationships on rising and falling limbs of hydrograph. Geogenic solutes exhibited dilution C‐Q patterns during the runoff event, which could be fitted by two power‐law models with different coefficients on rising and falling limbs. Affected by the strong hydrological connectivity between surface and subsurface, hillslope and depression, a steeper C‐Q slope on the rising limb indicated an exhaustible, proximal source, that is, groundwater. In contrast, the C‐Q patterns of the soil enriched solutes changed from enrichment to dilution during the runoff event. The enrichment pattern occurring at the early of rising limb was caused by strong hydrological connectivity between the surface and subsurface, which indicated a distal and plentiful sources of soil water. Whilst the strong hydrological connectivity between hillslope and depression caused a dilution pattern at the latter of rising limb. On the falling limb of hydrograph, a dilution pattern implied that the small fractures could be another source zone of soil‐enriched solutes in addition to soil layer in the depression. The C‐Q behaviours of soil‐enriched solutes can be fitted by a combination of a parabola model and a power law model for rising and falling limbs, respectively. The current study highlights the variations in intra‐event C‐Q relationships of different solutes affected by hydrological connectivity in the karst catchment. Which is crucial for assession of hydrochemical processes and fertilization management in this area.

Impact of changes in river network structure on hydrological connectivity of watersheds

River network connectivity is a key factor affecting watershed ecosystems and an important criterion for evaluating the health of river. The river network structure of the basin has changed dramatically in recent years due to human activity, but the response of connectivity to changes in physical structure is still poorly understood. In this paper, a dynamic index (connectivity index) based on daily-scale flow and probability density function is proposed to characterize the connectivity capacity of river networks. The Haihe River basin in China is selected as the study area, and a graphic model is constructed to assess the evolutionary structure of the river network. The 54 sections were classified into Low human impact (LHI) and High human impact (HHI), and the trends and causes of connectivity changes of the sections with the evolution of the network were analyzed. The results reveal that the river network degree and clustering coefficient keep increasing, the path length decreases and becomes more dense downstream, and the river network gradually evolves into a “small-world” network. The “small-world” characteristics improve the potential efficiency and specific functionality of the river network, but make the physical structure mismatch with the hydraulic gradient, and the network flow disperses severely, leading to a significant decrease in connectivity (HHI connectivity index is lower than LHI 52.1%). Moreover, the structural changes enhance the spatial and temporal differences in hydrological connectivity, which further restricts the function of the river system. “Small-world” and connectivity indicators (duration, start time, and intermittency) show strong correlations. Therefore, physical structure is an important factor influencing connectivity capacity of river networks. The planning and management of the watershed should consider the changes in hydrological connectivity caused by the structural modifications of the river network.

Variation in riverine dissolved organic matter (DOM) optical quality during snowmelt- and rainfall-driven events in a forested wetland watershed

The spring snowmelt period is an important time for organic matter mobilization and export in peatland and wetland dominated systems. The hydrologic flowpaths that transport melting snow and soil water to surface waters can vary over relatively short time periods (several days to weeks) depending on the timing and rate of melt and the occurrence of freeze–thaw cycles and rain-on-snow events. Dynamic flowpaths can alter dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) optical properties in surface waters due to differential melt and mobilization of snow, ice, and soil waters. In 2018 and 2020, we measured the DOC concentration and DOM optical properties in a woody wetland-dominated watershed in northern Minnesota during spring snowmelt (2018 & 2020) and early growing season (2018 only) events. We examined patterns in riverine DOC, DOM, and hydrochemical tracers across event hydrographs to infer changes in hydrologic connectivity across this seasonal transition. Measurements of riverine DOM optical properties, calcium concentration, and water isotopic composition indicated that DOM shifted from a primarily groundwater source and more fresh character during the dormant season to a more shallow wetland soil source and recalcitrant character during the early growing season. Seasonal variations in DOM optical properties and highly complex hysteresis patterns of DOM indices (e.g., Fluorescence Index, Humification Index, β/α, E250:E365, and the fluorescence peak ratio T:C) during individual events may reflect spatial heterogeneity in soil thawing throughout the watershed and temporal variations in water table elevation, affecting flowpath depths and connectivity to DOM pools within the soil profile. This study provides an important seasonal and event-scale baseline against which to evaluate potential changes in riverine DOM quantity and quality associated with predicted changes in climate and snow cover in organic matter-rich ecosystems.

Seasonal response of Suaeda salsa to hydrological connectivity in intertidal salt marshes through changing trait networks

Hydrological connectivity could greatly influence wetland plant traits and spatial patterns. However, it remains poorly understood how the whole plant or community responds to the changes in hydrological connectivity through plant traits at different stages of plant growth. Here we constructed the hydrological connectivity index (HCIs) and plant trait network (PTNs) in three seasons to identify the relationships between HCIs and PTNs in the intertidal salt marshes. The results showed that the relationships between plant traits were enhanced from spring to fall and PTNs were becoming tighter and more complex, and the characteristics of “tight inside and loose outside” of PTNs gradually eased. In the same season, PTNs became tighter and more complex from low to high tidal flats. Results of evaluating plant traits by their node parameters showed that the branch number of the total PTN in spring got the highest score, followed by the branch number node of the PTN in the high tidal flat in fall and the density node of the PTN in the high tidal flat in spring. Generally, a nonlinear relationship between HCIs and PTN parameters was observed; the middle tidal flat with moderate hydrological connectivity was the most suitable place for Suaeda salsa to grow with branch number as a central regulating trait of Suaeda salsa, while Suaeda salsa would develop adaptive strategies by forming different trait modules in the areas with low or high hydrological connectivity. Understanding the relationships between PTN and hydrological connectivity can guide us in choosing an appropriate site to begin plant rehabilitation and make wetland restoration more efficient.

Spatiotemporal Evolution of Wetland Eco-Hydrological Connectivity in the Poyang Lake Area Based on Long Time-Series Remote Sensing Images

Hydrological connectivity is important for maintaining the stability and function of wetland ecosystems. Small-scale hydrological connectivity restricts large-scale hydrological cycle processes. However, long-term evolutionary studies and quantitative evaluation of the hydrological connectivity of wetlands in the Poyang Lake area have not been sufficiently conducted. In this study, we collected 21 Landsat remote sensing images and extracted land use data from 1989 to 2020, introducing a morphological spatial pattern analysis model to assess the wetland hydrological connectivity. A comprehensive method for evaluating the hydrological connectivity of wetlands was established and applied to the Poyang Lake area. The results showed that, over the course of 31 years, the wetland landscape in the Poyang Lake area changed dramatically, and the wetland area has generally shown a decreasing and then increasing trend, among which the core wetland plays a dominant role in the hydrological connectivity of the Poyang Lake area. In addition, the hydrological connectivity decreases as the core wetland area decreases. From 1989 to 2005, the landscape in the Poyang Lake area focused mainly on the transition from wetland to non-wetland. From 2005 to 2020, the conversion of wetland landscape types shows a clear reversal compared to the previous period, showing a predominant shift from non-wetland to wetland landscapes. The eco-hydrological connectivity of the wetlands in the Poyang Lake area from 1989 to 2020 first decreased, and then increased after 2005. In the early stage of the study (1989−2005), we found that the connectivity of 0.3444 in 2005 was the lowest value in the study period. A resolution of 30 m and an edge effect width of 60 m were optimal for studying the hydrological connectivity of wetlands in the Poyang Lake area. The main drivers of the changes in hydrological connectivity were precipitation and the construction of large-scale water conservation projects, as well as changes in land use. This study provides a good basis for assessing hydrological connectivity in a meaningful way, and is expected to provide new insights for maintaining and restoring biodiversity and related ecosystem services in the Poyang Lake area.

The concept, approach, and future research of hydrological connectivity and its assessment at multiscales.

In this review, we explore the concept, approach, and future research of hydrological connectivity and its assessment at multiscales, because according to the literature, an integrated review upon hydrological connectivity is lack. Systematic studies illustrate the effects of (i) human activities (i.e., dam construction, groundwater extraction, water flow regulation and diversion, and land management) and (ii) natural factors (i.e., climate, soil characteristics, vegetation, and topography) on hydrological connectivity. Approaches (i.e., soil water content patterns, runoff patterns and processes, numerical models, and index of hydrological connectivity) applied to evaluate hydrological connectivity are examined in detail. Lastly, hydrological connectivity at multiscales is indicated. This review concludes with a discussion of potential research trends that can improve understanding of hydrological connectivity. Reported records showed that few studies were published on hydrological connectivity from 1980 to 2003, whereas the evolution of these studies is temporally promising since 2003. We cannot define a standard concept of hydrological connectivity that works in all environments. We desire to show different concepts of hydrological connectivity in different environments. The degree and nature of hydrological connectivity are not static due to the influences of human activities and changes of natural factors. The index of hydrological connectivity and numerical models are the most significant approaches to assess the changes in hydrological connectivity. This study showed that considering hydrological connectivity in social-economical-ecological-hydrological frameworks can prevent its negative effects on surface or subsurface water quantity and quality and is beneficial for sound water sources management.

Do stream water solute concentrations reflect when connectivity occurs in a small, pre-Alpine headwater catchment?

Abstract. Expansion of the hydrologically connected area during rainfall events causes previously disconnected areas to contribute to streamflow. If these newly contributing areas have a different hydrochemical composition compared to the previously connected contributing areas, this may cause a change in stream water chemistry that cannot be explained by simple mixing of rainfall and baseflow. Changes in stormflow composition are, therefore, sometimes used to identify when transiently connected areas (or water sources) contribute to stormflow. We identified the dominant sources of streamflow for a steep 20 ha pre-Alpine headwater catchment in Switzerland and investigated the temporal changes in connectivity for four rainfall events based on stream water concentrations and groundwater level data. First, we compared the isotopic and chemical composition of stormflow at the catchment outlet to the composition of rainfall, groundwater and soil water. Three-component end-member mixing analyses indicated that groundwater dominated stormflow during all events, and that soil water fractions were minimal for three of the four events. However, the large variability in soil and groundwater composition compared to the temporal changes in stormflow composition inhibited the determination of the contributions from the different groundwater sources. Second, we estimated the concentrations of different solutes in stormflow based on the mixing fractions derived from two-component hydrograph separation using a conservative tracer (δ2H) and the measured concentrations of the solutes in baseflow and rainfall. The estimated concentrations differed from the measured stormflow concentrations for many solutes and samples. The deviations increased gradually with increasing streamflow for some solutes (e.g. iron and copper), suggesting increased contributions from riparian and hillslope groundwater with higher concentrations of these solutes and thus increased hydrological connectivity. The findings of this study show that solute concentrations partly reflect the gradual changes in hydrologic connectivity, and that it is important to quantify the variability in the composition of different source areas.

Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL

The Ethiopian rift is known for its diverse landscape, ranging from arid and semi-arid savannahs to high and humid mountainous regions. Lacustrine sediments and paleo-shorelines indicate water availability fluctuated dramatically from deep fresh water lakes, to shallow highly alkaline lakes, to completely desiccated lakes. To investigate the role lakes have played through time as readily available water sources to humans, an enhanced knowledge of the pace, character and magnitude of these changes is essential. Hydro-balance models are used to calculate paleo-precipitation rates and the potential pace of lake level changes. However, previous models did not consider changes in hydrological connectivity during humid periods in the rift system, which may have led to an overestimation of paleo-precipitation rates. Here we present a comprehensive hydro-balance modelling approach that simulates multiple rift lakes from the southern Ethiopian Rift (lakes Abaya, Chamo and paleo-lake Chew Bahir) simultaneously, considering their temporal hydrological connectivity during high stands of the African Humid Period (AHP, ~15–5 ka). We further used the Surface Energy Balance Algorithm for Land (SEBAL) to calculate the evaporation of paleo-lake Chew Bahir’s catchment. We also considered the possibility of an additional rainy season during the AHP as previously suggested by numerous studies. The results suggest that an increase in precipitation of 20 to 30% throughout the southern Ethiopian Rift is necessary to fill paleo-lake Chew Bahir to its overflow level. Furthermore, it was demonstrated that paleo-lake Chew Bahir was highly dependent on the water supply from the upper lakes Abaya and Chamo and dries out within ∼40 years if the hydrological connection is cut off and the precipitation amount decreases to present day conditions. Several of such rapid lake level fluctuations, from a freshwater to a saline lake, might have occurred during the termination of the AHP, when humid conditions were less stable. Fast changes in fresh water availability requires high adaptability for humans living in the area and might have exerted severe environmental stress on humans in a sub-generational timescale.

Exploring the effect of hydrological connectivity and soil burn severity on sediment yield after wildfire and mulching

AbstractSoil erosion can potentially threaten different resources inside and outside of burned areas, and the risk of water becoming contaminated with sediment may be particularly severe. Various postfire actions, such as applying straw mulch, have been carried out in northwestern Spain in recent years with the aim of mitigating the risk of soil erosion. Nonetheless, because of the short interval between summer wildfire and autumn rains, careful selection and prioritization of the areas to be treated is crucial. Changes in hydrological connectivity could be measured and used as a criterion for selecting such areas. However, studies addressing changes in hydrological connectivity as a consequence of forest fires are scarce. In the present study, we assessed the effects of fire and postfire helimulching on the hydrological connectivity and sediment loads in a forest catchment burned by a wildfire in August 2016. Sediment yields were recorded in 20 plots (180 m2). Hydrological connectivity was computed with a version of the Borselli index and two alternative weighting factors: the C factor from the Revised Universal Soil Loss Equation and a factor based on field surveys of soil burn severity. The effect of mulching was also considered. The results indicate that the version of the Borselli index based on field measurements of soil burn severity best reflects the susceptibility to postfire sediment delivery. Moreover, this method was also suitable for evaluating the effect of mulching on soil erosion. The study findings may help forest managers to plan postfire actions.

Using stable isotopes paired with tritium analysis to assess thermokarst lake water balances in the Source Area of the Yellow River, northeastern Qinghai-Tibet Plateau, China

A spatially distributed network of thermokarst lakes undergoing significant environmental changes was sampled in 2014 and 2016 to develop a comprehensive understanding of lake water balances in lakes across a gradient of frozen ground conditions. Frozen ground ranges from seasonally frozen ground (SFG) to sporadic discontinuous permafrost (SDP) to extensive discontinuous permafrost (EDP), and is representative of complex conditions in the Source Area of the Yellow River, northeastern part of Qinghai-Tibet Plateau. Radioactive and stable water isotopes in reference lakes (non-thaw lakes), thermokarst lakes, precipitation, wetlands, ground ice and supra-permafrost groundwater are analyzed to characterize systematic variations and to assess lake water balances using stable isotope mass balance (IMB). IMB, paired with analysis of tritium decay gradients, is shown to be a valid approach for detecting short-term shifts in lake water balance, which allows evaluation of the proportion of precipitation-derived versus permafrost-derived water inputs to lakes. All lakes except EDP thaw lakes are evaporation-dominated (E/I > 0.5). Negative water balances occurred most frequently in reference lakes due to hydrological connectivity with rivers. Precipitation-derived water inputs result in positive water balances in SFG and SDP thermokarst lakes, but negative-trending water balances are found in SDP thermokarst lakes due to substantial reduction in water yield. Increasing contributions from thawing permafrost in EDP thermokarst lakes result in strong positive water balance. Permafrost degradation may also lead to the changes in hydrological connectivity between precipitation and wetlands or thermokarst lakes. Based on these findings, a conceptual model of the hydrological evolution of thermokarst lakes under the influence of permafrost degradation is proposed.

Changes in connectivity and hydrological efficiency following wildland fires in Sierra Madre Oriental, Mexico

Fire modifies soil surface, and hence soil hydrological properties change after wildland fires. High fire severity causes partial or total removal of vegetation, reduction of soil aggregate stability and increased water repellency, which are associated with high runoff and erosion. The spatial connection among these runoff sources is an important factor to consider when evaluating fire-induced changes on hillslope and catchment hydrology, as fire generates connected areas of bare soil, which may increase hydrological connectivity and hence post-fire runoff and erosion. The aim of this study was to quantify changes in hydrological connectivity and efficiency in two burned areas in central Mexico. By integrating rainfall simulation and spatial analysis, an index of connectivity (IC) and the lateral hydrological efficiency index (LHEI) were computed based on land/cover use, fire severity and topography within 287 burned sub-basins. Post-fire IC and LHEI were compared with the pre-fire scenario, and the relationship between LHEI and the proportion of burned area was assessed at the sub-basin level. Thresholds of the burned area per fire severity needed to increase LHEI were determined by a classification tree. The index of connectivity and LHEI were higher after wildland fires. The burned area was positively related with LHEI, and at least 43.3% of area burned with high severity is sufficient to produce the highest LHEI. The results are evidence of the effect of fire on hydrological connectivity and efficiency which adds to the understanding of fire–hydrology relations and can be used for integrated catchment management, ecological restoration and risk assessment.

What happens to groundwater ecosystems when you take out the groundwater?

The removal of groundwater results in the lowering of water tables, which, for groundwater organisms, translates to reduced habitat availability and changed environmental conditions in the habitat that remains. While changes in groundwater levels may be well modelled and predicted, the impacts on groundwater ecosystems remain poorly known. There are three key processes associated with groundwater drawdown in shallow alluvial aquifers that may threaten groundwater ecosystems. These processes are 1. the physical decline of water levels, from which fauna can be stranded in isolated or unsaturated sediments; 2. the loss of or change to habitat, particularly as water levels move through different geological strata and 3. changes in hydrological connectivity, that may influence water quality as a result of increasing distance or disconnection from the surface and other water sources. Results from laboratory studies show the variable capacity for stygobiotic invertebrates to move with declining water tables, dependent on both drawdown rate and sediment attributes. Once isolated in unsaturated habitats, our tests show that survival of fauna is limited beyond 48 h. Invertebrates are constrained by sediment size and unable to use those with relatively small pore spaces and may not be able to use all available saturated habitats. This talk will present a framework that identifies the key threats of groundwater drawdown to groundwater ecosystems and will highlight the current state of knowledge of each of these threatening processes. We present the results of empirical studies on the response of stygobiotic invertebrates to specific elements of the framework.

近20年白洋淀湿地水文连通性及空间形态演变

近20年白洋淀湿地水文连通性及空间形态演变