Low Water Level Period Research Articles

Untangling the coupling effect of water quality and quantity on lake algal blooms in Lake Hulun from a dual perspective of remote sensing and sediment cores

Algal blooms and sediment diatoms are crucial indicators of lake water ecology, influenced by water quantity and quality. However, the coupled effects of water quality and quantity changes on algal blooms are still unclear, especially for lakes in cold and arid regions. This study assessed the long-term variations in algal blooms in Hulun Lake using a novel approach combining remote sensing and sediment core samples for diatom analysis. Two mutation points from the structural change test were identified in approximately 2000 and 2010 for algal bloom area (MBE) and sediment diatom richness, indicating asynchronous algal blooms. A structural equation model (SEM) demonstrated that water level (WL) changes were the dominant influencing factor, co-driving the variations in algal blooms and sediment diatoms in conjunction with total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD). The results revealed nonlinear relationships between the lake WL, TN, Chla, and COD. The water level of 543 m emerged as a critical threshold affecting the relationship between water quality and quantity. Distinct differences in this relationship were observed when water levels were above or below this threshold. These variations became particularly pronounced during periods of high and low water levels. The results provide novel insights into the dynamics of algal blooms and can further support lake ecosystem conservation and management.

Climate-driven decline in water level causes earlier onset of hypoxia in a subtropical reservoir

Hypoxia, especially in the bottom water, is occurring in deep and stratified reservoirs worldwide, threatening aquatic biodiversity, ecosystem functions and services. However, little is known about the timing of onset and ending of hypoxia, especially in subtropical reservoirs. Based on five-year (from April 2015 to January 2020) sampling of a subtropical monomictic deep reservoir (Tingxi Reservoir) in southeast China, we found the evidence of about 40 days earlier onset of hypolimnion hypoxia during low water level periods in dry years compared to wetter high water level years. We explored the effects of stratification and mixing conditions on hypoxia, cyanobacterial biomass, and nutrient dynamics; and revealed the physical and biochemical conditions that drove hypoxia. The results indicated that 1) The decline in water level increased the intensity of thermal stratification, resulting in 40 days earlier onset of hypolimnion hypoxia in dry years than in wet years; 2) The decline in water level expanded the extent of hypoxia by promoting nutrient accumulation and phytoplankton biomass growth; 3) Warmer climate and less precipitation (drought) significantly promoted the risk of hypoxic expansion and endogenous phosphorus release in subtropical reservoirs. We suggest that more attention needs to be paid to the early onset of hypoxia and its consequences on water quality in subtropical stratified reservoirs during low water level periods in a changing climate.

Spatiotemporal distribution patterns of iron, manganese, and arsenic within the river infiltration zone and the potential geochemical activity at key interfaces

The river infiltration zone is a mixed area of surface water and groundwater under the riverbed and along the riverbank, which is greatly affected by river infiltration. Currently, there is a lack of understanding regarding the migration and transformation processes of Fe, Mn, and As, along with their correlation with organic carbon in the river infiltration zone affected by riverbed scouring and siltation processes. In this study, based on techniques such as pore water sampling and the Tessier sequential extraction, the spatiotemporal variations in Fe, Mn, and As concentrations and possible geochemical processes at the riverbed sediment–water interface and along the river infiltration flow path were revealed. The results indicated that the potential geochemical activity of Fe, Mn, and As and the distribution of organic carbon in the riverbed sediment responded to riverbed scouring–siltation processes. Compared to the high water level period, the potential geochemical activity of Fe, Mn, As, and organic carbon in the riverbed sediment generally exhibited an upward trend during the low water level period, indicating more intense Fe and Mn biogeochemical reactions. During river infiltration, the labile organic carbon (LOC) was preferentially utilized, and with increasing depth, the sedimentary organic carbon (SOC) continuously transformed into LOC and dissolved organic carbon (DOC). Affected by riverbed scouring processes, the transition time from oxidizing to reducing conditions in the pore water increased, and the Fe, Mn, and As reduction zones moved deeper and farther from the riverbank, resulting in a significant expansion of the groundwater pollution area. The reductive dissolution of iron and manganese oxides/hydroxides and the oxidation of organic matter–bound Fe, Mn, and As were the main biogeochemical processes contributing to the release of Fe, Mn, and As in the river infiltration zone. These research results have crucial theoretical significance and practical application value for the sustainable utilization of groundwater resources and the remediation of groundwater pollution.

Responses of ancient reservoir sediments to water level fluctuations, human activity and climatic changes over the past ∼700 yr in Guizhou Province, southwestern China

Knowledge of the responses of reservoir ecosystems to anthropogenic and climatic impacts on multi-centennial timescales is relatively poor, but is crucial for the management of numerous reservoirs in China. In this study, the history of an ancient reservoir system was reconstructed for the last ∼ 700 yr based on multiple dating methods (210Pb, 137Cs, and 14C) and geochemical data (e.g., the mass accumulation rates, loss-on-ignition values, total organic carbon, total nitrogen, total phosphorus contents, and stable carbon isotope ratios of organic matter) obtained for sediment cores from Gongqing Reservoir in Guizhou Province, southwestern China. Combined with historical literature and regional climatic records, the response of the aquatic ecosystem in Gongqing Reservoir to water level fluctuations, anthropogenic activity, and climatic changes during the last ∼ 700 yr was assessed. The results suggest that dramatic changes in the water level due to dam heightening in 1950 s significantly changed the aquatic ecosystem of Gongqing Reservoir. During periods of low water levels (average water depth = 1–2 m; 1345–1957 CE), aquatic macrophytes contributed more to the sedimentary organic matter, whereas phytoplankton increasingly contributed more during periods of high water levels (∼5 m; 1957–2020 CE). During the late stage (1700–1957 CE) of low water levels, the reservoir ecosystem was significantly affected by human activities. Combined with regional geological and historical records, we consider that increased population and enhanced human activities have markedly affected vegetation conditions in western Guizhou Province since ∼ 1700 CE. After 1957 CE, anthropogenic activities were key in affecting the aquatic ecosystem of Gongqing Reservoir. Especially in the last decade, large housing developments and enhanced agricultural activities in the watershed have led to enhanced catchment erosion and transport of more nutrients into the reservoir, thus promoting the algae productivity. These results provide new insights into how reservoir ecosystems respond to water level fluctuations, anthropogenic activity and climatic changes on multi-centennial timescales.

Daily process and key characteristics of phytoplankton bloom during a low-water level period in a large subtropical reservoir bay.

Reservoirs, heavily influenced by artificial management, often harbor phytoplankton assemblages dominated by cyanobacteria or dinoflagellates, triggering significant changes in aquatic ecosystems. However, due to limited sampling frequency and insufficient attention to species composition, the bloom processes and key characteristics of phytoplankton community structure have not been systematically elucidated. During the low-water level period when blooms are most likely to occur (June to September) in a tributary bay of the Three Gorges Reservoir, daily sampling was conducted to investigate phytoplankton community composition, identify significant environmental factors, and evaluate important structure characteristics of phytoplankton community. The results showed that Microcystis aeruginosa maintained a clear dominance for almost a month in stage 1, with low Shannon and evenness but a high dominance index. Phytoplankton total density and biomass decreased drastically in stage 2, but Microcystis aeruginosa still accounted for some proportion. The highest Shannon and evenness but the lowest dominance index occurred in stage 3. Peridiniopsis niei occurred massively in stage 4, but its dominant advantages lasted only one to two days. NH4-N was responsible for the dominance of Microcystis aeruginosa, while TP and PO4-P was responsible for the dominance of Peridiniopsis niei; however, precipitation contributed to their drastic decrease or disappearance to some extent. The TN : TP ratio could be considered as an important indicator to determine whether Microcystis aeruginosa or Peridiniopsis niei dominated the phytoplankton community. Throughout the study period, physiochemical factors explained more variation in phytoplankton data than meteorological and hydrological factors. Pairwise comparisons revealed an increase in average β diversity with stage progression, with higher β diversities based on abundance data than those based on presence/absence data. Repl had a greater effect on β diversity differences based on presence/absence data, whereas RichDiff had a greater effect on β diversity differences based on species abundance data. Co-occurrence networks for stage 1 showed the most complex structure, followed by stage 4, while the network for stage 3 was relatively sparse, although the overall community division remained compact. This study provides a useful attempt to explore the status and changes in phytoplankton community structure during the bloom process through high-resolution investigation.

Water and sediment chemistry drivers of chlorophyll-a dynamics within a Ramsar declared floodplain pan wetland system.

Floodplain pans are hydrologically dynamic in nature and characterised by variables such as chlorophyll-a (chl-a), water, and sediment chemistry over their hydroperiods. The present study investigated the spatio-temporal variations in water and sediment physico-chemical, and chlorophyll-a concentration characteristics of six floodplain pans found in the Ramsar declared Makuleke wetlands, Kruger National Park, South Africa. The water and sediment physico-chemical variable values were generally elevated during the high-water period, whereas chlorophyll-a concentrations varied across pans and hydroperiod. Benthic chl-a concentration significantly varied across pans with concentrations ranging from 161 to 1036.2mg m2. The two-way ANOVA showed significant differences in benthic chl-a concentration among hydroperiods, and no significant differences were observed in pelagic chl-a across pans and hydroperiods. Generally, pelagic and benthic chl-a concentration increased as water and sediment chemistry variables increased. Furthermore, three sediment variables, i.e. pH, calcium, and magnesium, and water conductivity were found to be significant in structuring benthic chlorophyll-a dynamics in pans. However, none of the sediment and water variables had a significant effect on pelagic chl-a. Hydroperiod had a significant effect on influencing chl-a concentration, with high and low water level periods being characterised by low and high chl-a concentration, respectively. The n-MDS results showed strong overlaps in chl-a biomass among the Makuleke floodplain pans across hydroperiods. The increasing chl-a concentration in these floodplain pans due to potential bioturbation effects as a result of large mammals could potentially lead to eutrophication, which in turn could affect the system's primary productivity and aquatic biota. Therefore, it is important to establish a continuous monitoring programme on these pans to inform sound management decisions.

CO2 and CH4 dynamics in a eutrophic tropical Andean reservoir.

We studied the dynamics of methane (CH4) and carbon dioxide (CO2) in a eutrophic tropical reservoir located in the Colombian Andes. Temporal and spatial dynamics were addressed through sampling during six field campaigns conducted throughout a two-year period. We monitored fluxes at the air-water interface, dissolved gas concentrations, physical and chemical properties of the water column, microstructure profiles of turbulence, and meteorological conditions. Throughout the study period, the reservoir was a persistent source of CH4 to the atmosphere with higher emissions occurring in the near inflow region. During periods of low water levels, both the emissions and surface concentrations of CH4 were higher and more spatially heterogeneous. The measured CO2 fluxes at the air-water interface changed direction depending on the time and location, showing alternating uptake and emissions by the water surface. Mass balances of dissolved CH4 in the surface mixed layer revealed that biochemical reactions and gas evasion were the most significant processes influencing the dynamics of dissolved CH4, and provided new evidence of possible oxic methane production. Our results also suggest that surface CH4 concentrations are higher under more eutrophic conditions, which varied both spatially and temporally.

Microbial community diversity and potential functionality in response to dam construction along the Three Gorge Reservoir, China.

River and reservoir bacterial communities are the most basic part of river biomes and ecosystem structure, and play an important role in river biological processes. Yet, it remains unclear how highly regulated dam reservoirs affect both soil and sediment bacterial communities. A temporal distribution pattern of bacterial communities was investigated using Illumina MiSeq sequencing in a transition section of the Three Gorges Reservoir (TGR). In total, 106,682 features belong to the bacteria kingdom, encompassing 95 phyla, 228 classes, 514 orders, 871 families, 1959 genera, and 3,053 species. With water level regulation, Shannon diversity index, and observed species differed significantly, with no significant difference in Simpson evenness. Both in the high water level period (October) and the low water level period (June), Proteobacteria, Acidobacteri, and Chloroflexi were the most abundant phyla. Whereas, based on PCA plots and Circos plot, the microbial community structure has changed significantly. LEfSe method was used to identify the classified bacterial taxa with significant abundance differences between the low water level and high water level periods. KOs (KEGG Orthology) pathway enrichment analysis were conducted to investigate functional and related metabolic pathways in groups. To some extent, it can be inferred that water level regulation affects community growth by affecting the metabolism of the microbial community.

Beach rebuilding period buffers Indiana beach erosion in Lake Michigan

Recent high-water levels in the Laurentian Great Lakes caused widespread shoreline damage to beaches, homes, and coastal infrastructure. To quantify these shoreline changes and place them in historical context, shoreline imagery was analyzed to determine the magnitude, rates, and spatial variability of shoreline changes along the Indiana shoreline of Lake Michigan. The analysis shows that over the recent period of water level increase from 2013 to 2020, during which the water level rose nearly 2 m, the median shoreline recession along the Indiana coastline was 58 m and the median recession slope was 35:1 (shoreline recession : water level change). This magnitude of shoreline change is comparable to that experienced in other water level increase periods, 1965–1973 and 1990–1998. This recent shoreline recession was highly variable spatially, but not strongly correlated with shoreline type or position relative to littoral barriers. In spite of the widespread recent shoreline changes, the 2020 shoreline was generally more lakeward than previous high-water shorelines examined, with the exception of historical erosion zones downdrift of littoral barriers. Analysis of water level records and available wave hindcasts for the coastline suggest that while large, the erosion associated with the recent water level increase was tempered by a prolonged beach rebuilding period of low water levels and waves that resulted in an accreted shoreline that largely buffered the erosion associated with the water level increase. These results suggest that recent Indiana erosion could have been substantially worse had it not been preceded by the beach rebuilding period.

Characterizing the transitory groundwater-surface water interaction and its environmental consequence of a riverside karst pool

Exchange between groundwater (GW) and surface water (SW) is a common occurrence in karst water systems through sinking stream disappearance or groundwater emergence. However, the transitory GW-SW interaction caused by river backflowing into a spring is poorly observed and understood. In this study, we present an approach for characterizing the influence of GW-SW interaction in a karst spring by integrating high-resolution hydrology, carefully selected hydrochemistry monitoring and precise microbe measurements. The spring-fed pool water conditions can be distinguished as high, medium, and low-water level periods in a hydrological year. The high-water level accounts for <1 % in a year, while it is associated with the hydrological regimes of backflooding states. The inflow of river backflow was found to be 4.4 times that of the natural discharge of spring water during a rainfall event. The duration of river intrusion into the spring or karst conduit could be assessed by jointly interpreting hydrography and physicochemical signatures, while the lasting environmental consequences should be evaluated together with biotic factors such as Escherichia coli. The GW-SW interaction induced by river backflow has led to the retention of river water in a pool, spring, and karst conduit for 132, 94, and 56 h, respectively. Despite turbidity returning to normal levels after 56 h, E.coli continued to persist for an extended duration. Our study reveals that despite the transient nature of GW-SW induced by river backflow on the hydrograph, they present a lasting risk of contamination from heavy metals, organic matter, and microorganisms. This extended influence can persist within a karst aquifer lacking a hyporheic zone. This research contributes to the quantification of processes involved in transitory GW-SW interaction in a karst spring, and it highlights the underestimation of GW-SW interactions in karst water systems, which might negatively impact water resources management.

Millennial hydrological variability in the continental northern Neotropics during Marine Isotope Stages (MISs) 3–2 (59–15 cal ka BP) inferred from sediments of Lake Petén Itzá, Guatemala

Abstract. Lake Petén Itzá (Guatemala) possesses one of the longest lacustrine sediment records in the northern Neotropics, which enabled study of paleoclimate variability in the region during the last ∼400 000 years. We used geochemical (Ti, Ca/(Ti+Fe) and Mn/Fe) and mineralogical (carbonates, gypsum, quartz, clay) data from sediment core PI-2 to infer past changes in runoff, lake evaporation, organic matter sources and redox conditions in the water column, caused by hydrological changes in the northern Neotropics during Marine Isotope Stages (MISs) 3–2. From 59 to 39 cal ka BP climate conditions were relatively wet, and the lake was marked by higher primary productivity and anoxic bottom waters. This wet environment was interrupted for two periods of possible low water level at 52 and 46 cal ka BP, when our data suggest higher evaporation, high terrestrial organic matter input and persistent oxic conditions. Between 39 and 23 cal ka BP, evaporation and input of terrestrial organic matter increased considerably, lake level declined, and lake bottom waters generally became oxic. These conditions reversed during the Last Glacial Maximum (23.5–18.0 cal ka BP), when runoff and lake productivity increased, and rising lake level caused bottom waters to again become anoxic. Comparison of our hydrologic proxy data with sea surface temperature anomalies between the eastern Pacific and the Caribbean suggests that changes in the intensity of the Caribbean Low-Level Jet (CLLJ) may have influenced long-term changes in runoff during MISs 3–2. Higher intensity of the CLLJ during the onset of MIS 3 and the LGM might have led to greater runoff into the lake, whereas the MIS 3–2 transition experienced a weaker CLLJ and consequently less runoff. A refined, high-resolution age–depth model for the PI-2 sediment core enabled us to identify millennial-scale Greenland interstadials (GIs) 14–2, Greenland stadials (GSs) 14–2 and Heinrich stadials (HSs) 5–1. In general, HSs and GSs were characterized by drier conditions. In contrast to GSs and HSs, GIs were characterized by greater runoff and overall wetter conditions, with the most pronounced GI peaks between 40 and 30 cal ka BP. Whereas GSs 9, 8, 7 and 6 began with abrupt increases in evaporation and ended with gradual increases in humidity, GSs 11 and 10 showed reversed patterns. The Lake Petén Itzá paleohydrology record, along with other regional paleoclimate records, led us to conclude that shifts in the position of the Intertropical Convergence Zone (ITCZ) altered moisture delivery to the lake on millennial timescales. During GSs and HSs, high evaporation from Petén Itzá (dry climate conditions) was associated with a more southerly position of the ITCZ, whereas wetter GIs prevailed during a more northerly ITCZ position. Although abrupt millennial-scale shifts in ITCZ and hydroclimate between GSs/HSs and GIs can be linked to instabilities in the Atlantic Meridional Overturning Circulation (AMOC), longer-term changes were additionally influenced by changes in atmospheric convection linked to modulations of the CLLJ in response to ΔSST between the equatorial Pacific and tropical Atlantic.

Reassessment of the declines in the largest freshwater lake in China (Poyang Lake): uneven trends, risks and underlying causes

Water level decline in the largest freshwater lake in China (Poyang Lake) has raised grave concerns and continuous debates regarding the proposal of a water control project. Previous hydrologic investigations on the water level decline in Poyang Lake were conducted mostly during recession season and typical dry years and hence lacked the comprehensive view of associated risks and possible spatial heterogeneity of trend during low water level periods. The present study reassessed the long-term trend and regime shift of low water level variations and their associated risks based on hydrological data across multiple stations in Poyang Lake during 1952–2021. The underlying causes of the trends of water level decline were further investigated. Results showed uneven trends and risks of water level variations in different seasons and across different lake regions. The water level of all five hydrological stations in Poyang Lake significantly dropped in the recession season, and the risks of water level decline evidently increased since 2003, which could be largely attributed to the water level drop in the Yangtze River. In terms of the dry season, clear spatial differences of long-term trend of water level were found, with water level in the central and southern lake regions significantly dropping, which was probably caused by dramatic bathymetry undercutting in the central and northern lake regions. In addition, the impacts of topographic changes became significant when water level of Hukou fell below 13.8 and 11.8m for the northern and southern lake regions, respectively. By contrast, water level in the northern lake region showed increasing trends in the dry season. In addition, only the occurrence time of water level under moderate risk significantly advanced for all stations except for Hukou. The present study could provide a full picture of low water level trends, associated risks across different lake regions and underlying causes in Poyang Lake, thus providing insights into adaptive water resources management.

The influence of extreme water levels on coastal wetland extent across the Laurentian Great Lakes

Laurentian Great Lakes coastal wetlands (GLCW) are ecological hotspots and their integrity depends upon dynamic hydrologic regimes of the Great Lakes. GLCW naturally adjust to changes in hydrologic regimes via migration, but Great Lakes water levels may be shifting faster than wetlands can manage: 2000–2015 marked an extended low water level period and was followed by record highs in 2017–2020. Our objective was to quantify how Great Lakes water levels impact GLCW linear extent (from the shoreline to open water). We calculated wetland extent and migration from 2011 to 2019 using data from 1538 vegetation transects at 342 sites across the U.S. shoreline of the Great Lakes. Mediated multiple linear regression with Bayesian hierarchical modeling investigated the relationship between water levels and wetland extent. We employed Bayesian hierarchical modeling because (1) the dataset was spatially nested, with sampling points within wetlands within Great Lakes and (2) Bayesian statistics offer flexibility for environmental modeling, such as the inclusion of mediation in models, where we can assess both direct influences of Great Lake water levels on wetland extent and indirect (i.e., mediated) influences of water levels via the presence of vegetation zones on thus wetland extent. Results showed that, overall, there was a landward migration from 2011 to 2019 (although 38 % of wetlands had lakeward migration of the wetland-upland border). Wetland length and inundation length decreased with increased water levels, as mediated by the presence of certain vegetation zones. This decrease in wetland extent is of concern because it likely relates to a decrease in wetland function and habitat. A better understanding of how GLCW migrate with shifts in water levels enables decision makers to better predict where Great Lakes coastal wetlands are at risk of being lost and thus where to prioritize management efforts.

Temperature and precipitation control the seasonal patterns of discharge and water isotopic signals of the Nyang River on the southeastern Tibetan Plateau

River hydrology on the southeastern Tibetan Plateau (SETP) is particularly sensitive to climate change, through the serious impacts of the decline in Indian summer monsoon (ISM) rainfall and the accelerated melting of snow and glaciers during the recent decades. Investigating the effects of meltwater and rainfall on the seasonality and quantity of river runoff in the region is crucial for improving regional hydrological monitoring and water management, and water stable isotopes can provide an effective method. Based on the event-based data of precipitation and river water isotopes, river discharge and meteorology monitored at a hydrometric station of the Nyang River on the SETP in 2011, we show that river discharge has a strong reliance on temperature and precipitation simultaneously through their controls on the meltwater and rainfall runoff, and river water deuterium excess (d) and δ18O potentially track the meltwater and rainfall effects separately. The seasonal signals of river discharge and isotopic data provide evidence that meltwater is the key factor of river discharge and isotopic changes during the pre-ISM season (low water level period) and ISM rainfall during the early to mid-ISM season (growing and high water level period) but key factors are not clear during the mid to post-ISM season (recession period). These inferred seasonal runoff source relationships are in close agreement with the runoff source contributions estimated using a season-specific δ18O-d mixing model approach. Specifically, the result suggests meltwater contributes ∼28 %, ∼41 % and ∼25 % of the total river runoff, and snowmelt does make a major contribution to the meltwater (compared to glacier melt) during the three individual hydrological periods. This study investigated the event-based river discharge and isotopic responses to temperature and precipitation and the relevance of seasonal meltwater and rainfall effects for a representative basin on the SETP, and we propose that long-term isotopic record of river water in this region may document catchment changes in runoff source relationships (especially snowmelt vs ISM rainfall).

Improving measures to improve the hydrological regime of the Kuban water basin on the example of the Pshish River

The article discusses ways to improve the hydrological regime of the Pshish River in the Kuban water basin. An effective way to reduce damage from floods and floods is to use simulation mathematical models of floods, taking into account joint work, so in the context of a progressive shortage of fresh water, especially in southern Russia, studies of the water resources of the Kuban River basin, especially medium and small rivers, are of particular relevance. Hydrologically, the Pshish River and the Krasnodar Reservoir remain poorly studied, so when the time of flood comes, the level of the water's edge increases significantly, since they will be active feeding boundaries for groundwater in the protected area, and at low low water periods they are drains. When the water levels in the reservoir are less than 29 m, and this is the time of the period of low water level in the river. Pshish in the area of the low-pressure earth dam, and the impact of the water level on the protected area is significantly reduced. The coastline of the Krasnodar reservoir recedes from the transverse low-pressure dam far to the northwest, exposing the previously flooded river bed. Taking into account the emerging database on the real technical condition of hydraulic structures of the flood control system of the Lower Kuban, obtained using non-destructive methods and satellite technologies. Simulation mathematical modeling makes it possible to optimize the operating mode of spillways of the flood control system of the Lower Kuban, namely for a low-pressure earthen dam for summer and winter floods, for the conditions for the occurrence of congestion and the occurrence of surge phenomena on the river. All this makes it possible to develop specific anti-flood measures before, during and after the passage of the flood. The effectiveness of engineering flood protection largely depends on the technical condition of the flood control system in the Lower Kuban. Simulation mathematical modeling allows you to determine those parts of the system that require priority investment.

The Role of the Catchment Area in Shaping Water Quality in the Lowland Springs of the Knyszyn Forest (NE Poland)

The Puszcza Knyszyńska springs are an important element of the environment, conditioning the high geo- and biodiversity in the region. These springs are layered outflows that drain the waters of the Quaternary level. More than 200 outflows have been cataloged, and 80 of them have measured/estimated yields and analyses of their water qualities. During periods of low water levels, the discharge efficiency most often fluctuated within a range of 0.5–2.0 dm3·s–1. In light of the applicable legal standards, the tested waters were of excellent quality. They belonged to the two hydrochemical types HCO3-Ca and HCO3-Ca-Mg, indicating the Quaternary aquifer’s homogeneity in the post-glacial areas. Small concentrations of most of the chemical parameters of water were found in the forest springs. Larger transformations in the chemical composition of water occurred in the outflows located in agricultural land and rural areas. A characteristic feature of the chemical composition of the water in the region’s lowland springs was an increased concentration of biogenic compounds (nitrogen, phosphorus, and carbon compounds), both organic and inorganic. The average concentration of dissolved organic carbon in the water of the lowland springs in summer exceeded 4 mg·dm−3. In some outflows, most often of a swamp nature or located in peat areas, the concentration of dissolved organic carbon even exceeded a dozen or so mg·dm−3.

Data mining technology for the identification and threshold of governing factors of landslide in the Three Gorges Reservoir area

Due to the complex geological conditions and external triggering factors, the deformation of landslide disaster often has spatial differences, especially in the Three Gorges Reservoir area. It is of great significance to explore the governing factors and their thresholds in different parts of landslide. In this research, the deformation of Shuping landslide is analyzed. First, 9 hydrological factors are selected for data mining analysis by comprehensive research, including 5 reservoir water factors and 4 rainfall factors. Then, the numerical variables are transformed into discrete variables by two-step clustering method, and the Apriori algorithm is utilized to deal with the classified variables to generate the correlation criterion meeting the minimum confidence, and the correlation criterions between triggering factors and landslide displacement are established. Finally, the threshold of governing factors are mined out by decision tree C5.0 models. The results indicate that governing factors controlling the deformation of different parts of landslide are distinct. Specifically, the rear landslide is jointly controlled by the reservoir water and rainfall. On the contrary, the reservoir level controls the deformation of other parts of Shuping landslide. Generally, the daily drop of water level is the most important factor causing the deformation of Shuping landslide. During the period of low water level (138.951 ~ 147.437 m), once the daily drop of water level exceeded 0.416 m/d, the landslide will show severe deformation. This research reveals that the study of association criterion and threshold is of great significance for landslide deformation analysis. Data mining technology can be better applied to the prediction of the reservoir landslide.

Impact of Withdrawals on Karst Watershed Water Supply

Karst systems consist of heterogeneous aquifers characterized by non-linear hydrogeological behavior. This is intrinsically linked to the coexistence of saturated versus unsaturated, open-channel versus closed-conduit flow and laminar versus turbulent flows within these aquifers. These multiple dualities together with a lack of knowledge of their internal structure lead to increasing difficulties in the management of groundwater resources related to karst aquifers. However, karst aquifers constitute strategic fresh water resources and many stakeholders carry out withdrawals sometimes continuously or centered on the period of low water level as for the irrigation. These withdrawals generate discharge decreases that can constitute sources of conflicts between upstream and downstream users of the resource. In this study, we propose a methodology to assess the impact of withdrawals on the spring discharge of a karst aquifer based on a conceptual non-linear reservoirs model. This methodology is applied to the second largest karst system in France: The Touvre karst system (La Rochefoucauld). The simulated influence of the total withdrawals on runoff deficit is larger than the total withdrawals in relationship with the non-linearity of the conceptual models. Globally, this impact is about one and a half of the total withdrawal depending of the year under consideration. Moreover, we show that it also implies a decrease from 10% to 20% of the low-flow annual discharge. The next step will be to also address the issues raised by the climate change projections.

Factors affecting the seasonal succession of phytoplankton functional groups in a tropical floodplain reservoir in Vietnam

Abstract Phytoplankton communities can be classified into different groups based on physiological, morphological, and ecological functions. In this study, the responses of phytoplankton functional groups (PFGs) to physicochemical variables and hydraulic regimes were investigated and used as ecological indicators in the Tri An Reservoir (TAR), a eutrophic tropical floodplain reservoir located in southern Vietnam. Altogether, 148 phytoplankton taxa were identified and assigned to 16 PFGs; the four predominant PFG groups were M (Microcystis spp.), MP (filamentous cyanobacteria: Oscillatoria spp., and some other diatoms: Gomphonema angustatum, Navicula sp.), J (green algae: Coelastrum spp., Cosmarium spp., Pediastrum spp., Scenedesmus spp., Staurastrum spp., Tetradesmus spp., Tetraëdron spp., and Xanthidium spp.), and P (Closterium spp., Aulacoseira granulata, Fragilaria spp., Pinnularia spp., and Desmidium baileyi). The average PFG biovolume ranged from 79.6 ± 20.2 to 230.1 ± 69.1 mg/L with M being the dominant group. The trophic state index (TSI) indicated that the water condition was light-eutrophic to hyper-eutrophic. It was found that the large water level fluctuation resulted in seasonal nutrient dynamics, with higher nutrient concentrations and higher turbidity during the low water level period and vice versa. The redundancy analysis (RDA) indicated that the nutrient concentrations, water level fluctuation, and mixing regimes were critical factors in the PFG selection in the TAR. Therefore, we assumed that water level fluctuation management coupled with biological competition have the potential to control toxic cyanobacteria in the TAR. In conclusion, the PFGs are suitable for examining the effects of environmental conditions on phytoplankton dynamics in tropical floodplain reservoirs, but their sensitivity to long-term changes in water quality and eutrophication requires further investigation.

Effects of the Three Gorges Reservoir Operation on Vertical Distribution of Chlorophyll a and Environmental Factors in Tributaries

The hydrodynamics and environmental factors in the Xiangxi River (XXR) and Shennong River (SNR), which are tributaries of the Three Gorges Reservoir (TGR), were monitored from July to August (the low water level period) and in October (the impoundment period) in 2018. The vertical distribution characteristics of chlorophyll a and other indicators of the two tributaries were analyzed during the different operation periods, and the factors that affected the vertical distribution in each period were discussed. The results showed that the vertical distribution of dissolved oxygen, water temperature, pH value, and chlorophyll a of the XXR and SNR during the low water level period was relatively consistent. The indexes 0-10 m (0-5 m for chlorophyll a) from the surface of the XXR and SNR, respectively, showed significant stratification and decreased with increasing water depth; the stability index of thermal stratification (RWCS/H) was 13.71-29.07 m-1, which was stable. After the water depth reached 10 m (5 m for chlorophyll a), the indexes tended to be stable along the water depth. During the impoundment period, there was no obvious stratification for each index; the stability index of thermal stratification was 0-0.5 m-1, the stability of the water body was weak, and the vertical variation of each index was relatively stable. The comprehensive trophic state index (TLI) of the XXR and SNR were 55 and 53 during the low water level period, respectively, indicating that they were in a slightly eutrophic state, and 39 and 46 during the impoundment period, respectively, indicating a mesotrophic state. Linear regression analysis showed that chlorophyll a, dissolved oxygen, water temperature, and pH in the two tributaries were significantly correlated in the vertical direction in the low water level period, indicating that dissolved oxygen, water temperature stratification, and pH were important factors affecting the vertical distribution of chlorophyll a. During the impoundment period, a large amount of backflow from the Yangtze River, a large fluctuation in tributary water level, and the decrease in RWCS/H were the important factors that affected the small vertical change in the water body. The enhancement of vertical mixing and the decrease in Zeu/Zmix were the key factors affecting the nutritional status of the water.