Changes In Groundwater Table Research Articles

Moisture availability and groundwater recharge paced by orbital forcing over the past 750,000 years in the southwestern USA

Quaternary climate changes are driven in part by variations in the distribution and strength of insolation due to orbital parameters. Continental climate variability is well documented for the most recent glacial-interglacial cycles, yet few records extend further back in time. Such records are critically needed to comprehensively assess the entire spectrum of natural climate variability against the backdrop of anthropogenic warming. Here, we apply uranium isotope geochronology to calcite deposits to date groundwater-table changes in Devils Hole cave, Nevada. The deposits record multi-meter groundwater-table fluctuations over the last 750,000 years, reflecting the long-term evolution of hydroclimate in this presently arid region. During periods between glacial or interglacial extremes, the water table responded sensitively to variations in 65°N summer insolation, likely caused by the increasing extent of North American ice sheets during cold period, which steered moisture-laden trajectories towards the southwestern USA. These orbitally-driven hydroclimatic changes are superimposed on a tectonically-driven long-term decline in the regional groundwater table observed prior to 438,000 ± 14,000 years ago.

Dynamic change and driving response of shallow groundwater level based on random forest in southwest Songnen Plain

Study regionThe southwest Songnen Plain in China.Study focus:This study investigated the multi-time groundwater dynamic types and the trends of groundwater level. Random forest model was applied to analyse on driving factors of groundwater level.New hydrological insights:This study selected the following data from four periods in 2005, 2010, 2015, and 2020: groundwater depth, precipitation, evaporation, groundwater extraction, dry field area, and paddy field area. Pearson correlation analysis, Kriging interpolation, Mann-Kendall test, and other methods were employed to determine the groundwater dynamic types and changes in level within the four periods. Pearson correlation analysis shows that the dynamic types of groundwater table change greatly in the four periods, and the correlation between groundwater depth and precipitation decreases first and then increases. Kriging interpolation showed that the groundwater depth was lower in the west, and the depth around irrigation areas was higher. The Mann-Kendall test showed that the groundwater depth changed abruptly over time during the irrigation period and rainy season; the average groundwater depth increased by 1.36 m from 2005 to 2010, but only by 0.09 m from 2010 to 2020. The random forest model of groundwater level dynamics and its driving factors was established, and it was found that the size of the paddy field had the greatest influence on groundwater dynamics, affecting 32.15%.

Responses of deep soil moisture to direct rainfall and groundwater in the red soil critical zone: A four-stage pattern

Deep soil moisture (DSM) plays a critical role in vadose zone hydrological processes. However, the recharge processes and their underlying mechanisms remain unclear due to the scarcity of comprehensive deep hydrological data, especially in regions characterized by active interactions between surface and subsurface hydrological processes. To address this knowledge gap, we performed an event-based study using a four-year hydrological monitoring dataset, encompassing rainfall, profile soil moisture, and groundwater along a hillslope at the Red Soil Critical Zone Observatory (CZO) in China. A total of 226 rainfall events were classified into four distinct types (I-IV) using a k-means clustering algorithm. The changes in DSM and groundwater table (GWT) were dependent on the rainfall type. The GWT (0.57–0.97) played a greater role in contributing to the DSM than the direct rainfall (0.48–0.64) based on a structural equation model, which was more pronounced from upper slope to foot slope. DSM recharge pattern can be described by a four-stage conceptual model, as indicated by the relationship between DSM and GWT: a slow and linear increase (stage 1), a small plateau (stage 2), a rapid linear rise (stage 3), and finally a great plateau (stage 4). DSM at the small and great plateau were closely related with the field capacity and saturation, respectively. Rainfall type, vadose zone features, and soil physical properties jointly shaped DSM recharge patterns. These findings can offer valuable insights into understanding the deep hydrological processes of the red soil region as well as regions with similar hydroclimatic conditions.

Numerical Example to Estimate Groundwater Level Changes using Time-lapse Microgravity Data

The study using time-lapse microgravity (TLM) data can be helpfull to understand near surface variation caused by hidrological variation. Shallow groundwater variation can be highly correlated with density variation as time-lapse microgravity source, because the changes are very close to the surface. For several days (or weeks or months), we can observe density variation in the near surface caused by the changes of water saturated zones. The time-lapse of density variation for several days can be simplified as groundwater table changes. The purpose of this work is to study time-lapse microgravity application for groundwater level estimation. Using numerical example, we utilize lateral variation of time-lapse microgravity signal and also surface topography. In order to do the estimation groundwater level for second TLM survey, we need the groundwater level data from first survey (and vice versa). After numerical example, we applied the algorithm in the study area of (5 × 5) km square in the Blora regency. The shallow groundwater level (0 to 14 meters below the surface) are observed after the second (2016) microgravity survey. From this work, we constrained the model using second groundwater level data. After iterative calculation of using TLM anomaly (value in the range of -80 to +80 microGal) with surface topography variation (in the range 90 to 230 meters), we get the estimation the first (2014) groundwater level variation in the range of -7 to 7 meters from the second (2016) groundwater level data.

Numerical Simulation of Karst Collapse in Qingling Town, Wuhan

With background information on the geological conditions of Fenghuo village, Qingling town, Wuhan, based on Terzaghi consolidation theory and GMS software, a hydrodynamic-mechanics coupling model was established. On the basis of simulating the groundwater flow field by GMS software, a mechanical calculation program was developed, and the coupling simulation of hydrodynamic mechanics was achieved. The results showed that the stress state, vertical deformation, and damage of soil are closely related to the underground water flow. The soil pressure value and effective stress value are relatively low from July to August every year. It is the result of buoyancy produced by groundwater. Soil settlement generally occurs in the dry season. When the load is not considered, the deformation value of soil settlement is generally less than 6 mm. In the wet season from July to August every year, the soil will float up, and the floating value is less than 4 mm. The groundwater table changes frequently, and the destruction around the cave body eventually leads to karst collapse.

Simulation of water drainage and nitrate leaching at an irrigated maize (Zea mays L.) oasis cropland with a shallow groundwater table

Nitrogen pollution is a serious threat to water resources worldwide, and it poses a particular threat to groundwater in desert oases where the water table is shallow. Leaching loss of nitrogen fertilizer means economic loss and threatens the water quality of phreatic aquifer with a thinner vadose zone thickness, especially in the Hailiutu River catchment, China, where phreatic aquifer is a unique source of local domestic water. The objective of this study was to quantify N leaching for irrigated maize with different fertilizer and irrigation management practices, as well as fluctuating groundwater tables, and to recommend the best management practices for maize. A field experiment was conducted in such a locale for two consecutive years. Maize was grown where nitrogen was introduced at increasing rates (from 38 to 438 kg N ha−1, at intervals of 100 kg N ha−1). The 338 kg N ha−1 represented the conventional fertilization practice (N338), and served as a reference. Unlike the conventional 70 mm irrigation depth applied at each water application, water conserving irrigation was performed by applying only 20 mm at each irrigation — six times in 2017 and four in 2018. The HYDRUS-1D model was first calibrated against measured soil moisture and nitrate in soil water across all soil layers, then independently validated and finally repeatedly run to simulate NO3N leaching under the following simulation conditions: a) increasing N fertilization rate, but fixed 20 mm individual irrigation depth; b) increasing individual irrigation depth (from 10 to 70 mm), but only considering the reference N fertilization rate (N338). In both cases, five different shifts in water table depth were assumed, + 0.10, 0, − 0.10, − 0.20, or − 0.30 m of groundwater table change, respectively. The simulation results suggest that conventional fertilization practices result in nitrogen losses of over 100 kg ha−1, and 238 kg N ha−1 N fertilization, accompanying by a depth of 20 mm for each irrigation is the best combination of management practices. The NO3−N leaching loss was more influenced by the depth of irrigation than the fertilizer application rate. A shallower water table resulted in the greater loss of NO3N by leaching. Such results can help develop improved irrigation and nitrogen fertilization management for desert oasis cropping systems.

Effectiveness of Controlled Tile Drainage in Reducing Outflow and Nitrogen at the Scale of the Drainage System

The impact of controlled drainage (CD) on the groundwater table (GWT), drainage outflow, surface runoff, and nitrogen reduction at the drainage system scale in the Wielkopolska region was analyzed in this study. Based on field research, mainly by monitoring of GWT changes in 2019–2020, the DRAINMOD model was calibrated and validated. Hydrological soil water balance simulations were carried out with 36 and 9 combinations for CD and free drainage (FD), respectively. The modelling period was March-September for 10 different dry, wet, and normal years from the period of 1961 to 2020. The next step was to use the results of drainage outflow modelling and chemical constituent analyses of drainage water samples to determine NO3-N concentrations and calculate NO3-N pollution loads. As a result of the simulations, the importance of the timing of the start of the outflow retention in the adopted model variants was determined, indicating the earliest assumed date of 1 March. The appropriate CD start date as well as the initial GWT has a significant impact on the effectiveness of CD application in reducing the volume of drainage outflow and reducing the amount of NO3-N entering open water with it. The application of CD under the conditions of the analyzed drainage facility makes it possible to retain up to 22 kg of NO3-N per hectare.

Influence of Groundwater Table Fluctuation on Liquefaction Potential Analysis Using Cyclic Stress Approach

In addition to earthquakes, groundwater table (GWT) also contributes to uncertainty in liquefaction analysis, so it needs to be considered. This research was conducted by analyzing the liquefaction potential with a cyclic stress approach using standard penetration test data and cone penetration test data with varied GWT in a case study of building design in Sleman, Yogyakarta Special Region. The liquefaction safety factor (FS L ) was calculated to obtain the liquefaction potential index (LPI) and liquefaction severity index (LSI). Based on the survey results, the GWT ranges from -6 m to -8 m. Referring to the FS L , there are several soil layers that have liquefaction potential at all GWT variations. The shallower the GWT, the greater the LPI and LSI values at the same point will be. Changes in GWT from -8 m to -6 m cause changes in the LPI and LSI classifications at several points. Based on LPI classification, the location belongs to high to very high category at -8 m and -7 m GWT and very high category at -6 m GWT. Based on LSI classification, the location belongs to low category at -8 m GWT, and low to moderate category at -7 m and -6 m GWT.

Recognition of Landslide Triggering Mechanisms and Dynamics Using GNSS, UAV Photogrammetry and In Situ Monitoring Data

Detecting the mechanism of landslides and evaluating their dynamics is challenging, especially concerning composite landslides. For this purpose, several investigation and monitoring techniques should be implemented to obtain reliable information on landslide characteristics (e.g., geological and hydrogeological conditions and type of landslide processes), kinematics (displacement rate), and potential triggering mechanisms (e.g., change in groundwater table and precipitation). The Urbas landslide in northwest Slovenia has been studied for decades through geological, geotechnical, geodetic, and remote sensing investigations. However, due to the complexity of the landslide and the short duration of continuous monitoring, no assessment of its dynamics has been made. To meet this need, this study analysed continuous and periodic monitoring of landslide displacements using data from the global navigation satellite system (GNSS), a wire extensometer, unmanned aerial vehicle (UAV) photogrammetry, and hydrometeorological sensing (groundwater table, precipitation). The results of this study show that the dynamics of the Urbas landslide differ along the landslide area, depending on local geological and hydrogeological conditions. Consequently, certain parts of the landslide are at different evolutionary states and respond differently to the same external triggers.

A multilayer soil approach for seepage process analysis in earthen levees

The analysis of seepage through earthen levees is fundamental to investigate the levee safety condition, since that the seepage phenomenon can often lead to piping causing the collapse of the levee (Saghaee et al., 2012). This work proposes a new approach for identifying the location of the saturation line through the earthen levee by using a multilayer soil approach that assumes the levee body and the levee foundation as two different layers, characterized by using a different hydraulic conductivity parameter. The main purpose is to investigate the difference in the predicted saturation line identified through the proposed method and the one based on the single layer analytical solution of the seepage flow suggested by Marchi (1961). On comparison of the saturation lines provided by two different approaches, the ratio between the river water head and the groundwater thickness or depth increases; allowing the determination of a ratio threshold above which the Marchi equation should no longer be applied. The proposed multilayer (levee and foundation) soil approach is also used to assess the seepage vulnerability of the methodology proposed by Barbetta et al. (2017). The results show that the differences between the seepage vulnerabilities evaluated using the multilayer approach and the single layer method are quite low. Although notably, significant modifications of the seepage probability are observed when the distance between the ground level and the groundwater table changes.

Evaluation of the perennial spatio-temporal changes in the groundwater level and mineralization, and soil salinity in irrigated lands of arid zone: as an example of Syrdarya Province, Uzbekistan

Salt accumulation in irrigated lands is an intercontinental environmental issue that adversely influences the sustainable land use, agricultural land productivity and global food security. With the higher level of groundwater table (GWT) and its increased mineralization, global climate emergency in arid lands also obviously increase soil salinity (SS) in Uzbekistan. Analyses of perennial data on saline agricultural lands were performed by principally depending on conventional outdated methods. The integration of GIS approach to map the widespread of GWT, groundwater mineralization (GWM) and SS in irrigated lands of Syrdarya province in Uzbekistan were manipulated within the limited time period. Nevertheless, in particular irrigated areas where GWT is shallow and highly mineralized, insufficient scientific data and GWT maps created in accordance with GIS-based methods is identified, restricting a better establishment of tracing the SS of irrigated land over time as GWT and GWM rise. Thus, our study is to trace and delineate the dynamic and spatial changes in GWT, GWM and SS caused by climate factors over 2000–2019 by integrating traditional research methods and cross-validated GIS methods in the example of irrigated land of Syrdarya province in Uzbekistan. To reach this goal, a fieldwork was organized and relying on the in-situ data collected from around 3800 different points for soil sampling and 1500 observation wells for groundwater studies, GIS-based maps were created for four last experimental years 2016–2019 using the Inverse Distance Weighting (IDW) interpolation. By comparing the IDW maps with the in-situ data, an overall accuracy of maps per each variable was calculated and data from maps were quantified. A statistical approach was applied to examine the interrelationship between GWT, GWM, SS and climatic factors such as seasonal air temperature and seasonal precipitation. GWT and GWM are moderately correlated each other, but, these two variables are considered as the main drivers to the enlargement of saline irrigated areas in the province.

Estimation, modeling, and prediction of land subsidence using Sentinel-1 time series in Tehran-Shahriar plain: A machine learning-based investigation

Land subsidence (LS) is a major geologic hazard threatening most areas in Iran due to excessive groundwater extractions. The main objective of this study was to estimate LS applying PS-InSAR technique and then model and predict it through machine learning methods. Sentinel-1 time series images were used to retrieve LS from 2014 to 2019. Random forest regression (RFR) and artificial neural networks (ANN) were implemented to model LS of 2014–2017 using some conditioning factors including groundwater level decline, aquifer media, precipitation rate, slope, landuse type, depth to water table, distance from exploiting wells, distance from rivers and distance from faults as independent variables. The best calibrated method was used to predict LS over 2018–2019. In addition, all k-combinations (k=1: n, n is the number of factors) of conditioning factors were evaluated to identify the best combination resulting in the best output. The procedure was examined in the Southwest of Tehran and Shahriar plain, Iran. The maximum cumulative displacement values were 561.78 and 638.5 mm for Tehran and Shahriar, respectively. The accuracy assessment showed the RMSE value of 9.97 mm for LS retrieval. The experimental results of LS modeling indicated that RFR outperformed ANN, representing more robust and reliable results with the RMSE value of 1.35 mm for the both study areas. The investigation of the possible factor combinations showed that using the 4-factor combination consisting of precipitation, ground water table change and depth to water table along with distance from faults for S1 and the thickness of fine-grained soil layers (clay and silt) for S2 have presented promising results in modeling and predicting LS. Using all the conditioning factors, LS was predicted with RMSE value of 2.53 mm and 2.09 mm for S1 and S2, respectively. Using the 4-factor combination, however, the prediction RMSE increased by 0.13 mm and 0.04 mm for S1 and S2, respectively. This 5-percent difference can be ignored against decreasing the total cost of applying all factors. This study demonstrates the usability of processing Sentinel-1 data for LS retrieval and applying RFR for LS prediction.

Reduced groundwater use and increased grain production by optimized irrigation scheduling in winter wheat–summer maize double cropping system—A 16-year field study in North China Plain

Optimizing irrigation strategies to increase water utilization efficiency and achieve higher yield is vital for balancing groundwater use and improving food security during water shortage in the North China Plain (NCP). Based on a 16-year field experiment (2003–2018) using seven irrigation schedules from W0M0 to W4M3 (numbers are irrigation times in wheat (W) and maize (M) season, 75 mm each) in the winter wheat–summer maize double cropping system, we analyzed annual total water consumption ( ET a ) and groundwater table change in terms of net groundwater depletion, annual total grain yield, water productivity (WP), irrigation water productivity (IWP) and marginal benefit of the whole wheat–maize system. Relationship between yield or WP and irrigation or ET a were also revealed. Results showed that (1) total ET a increased as irrigation input increased, ranging from 427.3 mm (Rainfed, W0M0) to 891.0 mm (W4M3). Soil water storage contributed nearly 30% to ET a for winter wheat under water deficit conditions. Pre-sowing soil water storage played an important role in improving the annual yield and WP of both wheat and maize by promoting germination, seedling emergence and root growth; (2) the rainfed treatment (W0M0) was best for mitigating the groundwater table decline (0.1 m yr -1 ), followed by W1M1 (0.5 m yr -1 ) and W2M1 (0.8 m yr -1 ). Groundwater table decline in M2W2 almost overlapped the observed data at the station (1.1 m yr -1 ). In W3M2, the farmers’ traditional practice, the groundwater table declined by 1.4 m yr -1 , obviously over exploitation, while W4M2 and W4M3 declined by almost 2.0 m yr -1 ; (3) the relationship between total annual yield and irrigation (or ET a ) followed a quadratic curve. Total annual yield significantly increased from W0M0 to M1W1 (25%) to M2M1 (5%) and then kept stable. Average annual WP decreased as irrigation increased, from 2.4 kg m -3 (W0M0) to 1.6 kg m -3 (W4M3). Average annual IWP and marginal benefit also declined as irrigation increased. These results over 16 years indicated that the W2M1 is the most balanced irrigation regime for wheat– maize rotation to mitigate groundwater decline, maintain grain production, and improve water use efficiency in the NCP. ● A 16 - year field experiment of seven irrigation regimes for wheat-maize was conducted. ● ET a increased and water productivity decreased with increasing irrigation amount. ● W2M1 was the best optimized irrigation regime for balancing groundwater use and yield.

Evaluation of Water Content in Soils of Žitný Ostrov from the Point of View of Its Exploitation for Biosphere

The lithospheric zone between the soil surface and the first ground water horizon, respectively the ground water table, has a character of three-phase system. It consists of solid phase having fine to rough disperse granularity. This creates a structure of porous environment with characters that can be physically determined. The water occurs in pores in different forms of state and its bond with solid phase. Its energetic bond is clearly quantified by means of moisture retention curve. The gas phase fills pores with the water up to the value of the full porosity, i.e. it fills the part of pores that is not saturated with water. Therefore, this lithospheric zone is called the soil aeration zone. The volume of water occurring in the soil aeration zone corresponds to the concentration of water in the framework of hydrological cycle components. This water serves as the water resource for the vegetation cover. The data used for calculation were particular soil types in the area, depth of ground water table, hydrolimits (wilting point, point of decreased availability, field water capacity) and aeration zone thickness. The water content in the soil aeration zone between hydrolimits field water capacity (FWC) and wilting point (WP) is the critical interval of water content for vegetation cover in a given locality. Water from this interval is available for the vegetation cover. This water has no properties of free water, and plants have to have a developed root system and such suction pressure, that is able to overcome the bond between water and soil. Calculated results were verified with the help of monitored water content. Both calculated and measured values of soil water content in the aeration zone show that the water content is affected by appurtenant soil type. The human activity in a landscape directly affects the dynamics of this water resource, either from quantitative or qualitative viewpoint. This affect is demonstrated by changes of the ground water regime, i.e. changes of ground water table and amplitude of its fluctuation. The paper brings results of water content evaluation in the soil aeration zone in the Žitný ostrov area.

Temporal patterns control carbon balance in forest and agricultural tropical peatlands in North Selangor, Malaysia

Tropical peat swamp forests can sequester significant amount of carbon (C). However, there is dearth of understanding on the tropical soils’ C stocks and emissions because of the changes in peatland use, land use policies, and micro-climate. The objective of this study was to determine the C stocks and fluxes of two peat swamp forests and a peatland under mixed cropping in Selangor, Malaysia. Standard procedures were used to determine aboveground biomass, belowground biomass, selected peat soil physical, chemical, and biological properties, and environmental variables that are related to peat soil respirations. The mean C stocks for the peat swamp forest and mixed cropping sites were 1788.79 Mg C ha-1 and 1023.57 Mg C ha-1, respectively. The carbon dioxide emission rates of peat swamp forest and mixed cropping sites ranged from 7.20 to 73.13 tCO2 ha-1 year-1 and 26.50 to 43.43 tCO2 ha-1 year-1, respectively. These emissions are related to seasonal changes because the relative humidity, soil temperature, and ground water of the experimental sites had significant effects on soil respiration. Unlike the mixed cropping sites, the fluxes of the peat swamp forest were significantly higher in the dry season compared with the wet season. These findings suggest that peat soil respiration is controlled by relative humidity, temperature, and the changes in ground water table. Continued monitoring and conservation efforts to preserve stored C in peatlands are essential.

Diversified crop rotations enhance groundwater and economic sustainability of food production

AbstractEarth's water resources are critical for supporting livelihoods and food security but are being increasingly overexploited to support global agriculture. Diversifying cropping systems could potentially resolve unsustainable water use but trade‐offs with other aspects of sustainability and food security have not yet been assessed. We performed a detailed analysis of 31 different field crop rotations conducted during 1990–2019 in the North China Plain, to assess the potential impact of crop diversification on actual evapotranspiration (ETa), changes in regional groundwater table, grain yield, economic output, and water use efficiency (WUE) and to identify configurations that can achieve co‐benefits across multiple dimensions. We found that a combination of lowering the cropping index (i.e., harvest frequency), incorporating fallow periods, and introducing higher‐value crops into the currently dominant winter wheat–summer maize double cropping system can reduce growing season ETa by as much as 31%, mitigate groundwater decline by 19% or more, and increased economic output and economic WUE by more than 11% and 3%, respectively. We also found that multiple diversified wheat‐maize–based rotations—all with rotation lengths greater than 2 years—achieve co‐benefits across all evaluated dimensions. This study provides new empirical evidence of the opportunities for diversified crop rotations to balance the multiple objectives of food production, sustainable groundwater use, and farmer profitability. Extending this solution to other water‐stressed agricultural regions could be an effective strategy in achieving more sustainable food production system globally.

Soil Enzyme Activity as Affected by Land-Use, Salinity, and Groundwater Fluctuations in Wetland Soils of the Prairie Pothole Region

Land-use change and climatic variability are significant drivers for the loss of ecosystem services and soil quality in the prairie pothole region (PPR) wetland systems. Land-use induced changes in groundwater table and salinity may influence biogeochemical processes facilitated by extracellular enzymes (EEs) involved in soil organic matter (SOM) decomposition. The effects of changing groundwater table and salinity on β-glucosidase (BG), N-acetyl glucosaminidase (NAG), and alkaline phosphatase (AP) activities were assessed in wetland soils collected from three different adjacent riparian land-use practices in the PPR. In a microcosm study conducted over ten weeks, soils were treated with groundwater salinity (control, 6 mS cm−1, and 12 mS cm−1) and declining groundwater table depths. Extracellular enzyme activities (EEAs) differed significantly (p < 0.05) among soils from different land-uses and between groundwater table depths. The impact of groundwater salinity on soil EEAs were non-significant (p > 0.05). Soil EEAs were significantly higher in soils from pasture, suggesting that the land-use effects resulted from background SOC and TN. Soil EEAs significantly (p < 0.05) reduced under a deeper groundwater table depth, except reverse for BG in site B, indicated that the lowered groundwater table could lead to transitory drought stress for SOM decomposers.Graphical Abstract

Macro and micro analysis on deformation characteristics of fine sand under different groundwater table change

By using the samples of fine sand in Shanghai’s seventh layer, creep tests were performed to investigate the deformation characteristics of fine sand under different groundwater table change. It was revealed that the creep of fine sand is closely related to groundwater table change: when the effective force caused by groundwater table change is smaller than pre-consolidation force, the deformation is mainly elastic. When effective force caused by groundwater table change is bigger than pre-consolidation force, the compression deformation is large and the rebound is small; and the deformation is mainly viscoelastic plastic when equal to pre-consolidation stress. Through the scanning electron microscope (SEM) analysis it was found that creep of fine sand is dominated by the slip of particles, not by the breaking of particles. And the fine sand particles are squeezed under the load induced by the groundwater table change, and the particles slide along the indirect contact surface of the particles, resulting in the continuous adjustment of position and relative displacement. Mercury injection test shows that the pore size distribution of fine sand has little change before and after the test.

Depression of Groundwater Table and Reduced Nitrogen Application Jointly Regulate the Bacterial Composition of nirS-Type and nirK-Type Genes in Agricultural Soil

Despite the known influence of nitrogen fertilization and groundwater conditions on soil microbial communities, the effects of their interactions on bacterial composition of denitrifier communities have been rarely quantified. Therefore, a large lysimeter experiment was conducted to examine how and to what extent groundwater table changes and reduced nitrogen application would influence the bacterial composition of nirK-type and nirS-type genes. The bacterial composition of nirK-type and nirS-type genes were compared at two levels of N input and three groundwater table levels. Our results demonstrated that depression of groundwater table, reduced nitrogen application and their interactions would lead to drastic shifts in the bacterial composition of nirS-type and nirK-type genes. Structural equation models (SEMs) indicated that depression of groundwater table and reduced nitrogen application not only directly altered the species composition of denitrifier bacterial communities, but also indirectly influenced them through regulating soil nutrient and salinity. Furthermore, the variation in soil NO3−–N and electrical conductivity caused by depression of groundwater table and reduced nitrogen application played the most important role in altering the community composition of denitrifier bacterial communities. Together, our findings provide first-hand evidence that depression of groundwater table and reduced nitrogen application jointly regulate the species composition of denitrifier bacterial communities in agricultural soil. We highlight that local environmental conditions such as groundwater table and soil attributes should be taken into account to enrich our knowledge of the impact of nitrogen fertilization on soil denitrifier bacterial communities, or even biogeochemical cycles.

Effects of Groundwater Table Decline on Vegetation Transpiration in an Arid Mining Area: A Case Study of the Yushen Mining Area, Shaanxi Province, China

We studied the effects of coal mining on the shallow groundwater regime and vegetation in the Yushen mining area (YMA) in northwestern China using a Hydrus-1D numerical model, which was calibrated and validated with field observations, and then used to simulate the effects of groundwater table (GWT) changes on transpiration rates. The actual transpiration (Ta) was influenced by both GWT depth (GWTD) and precipitation. The ratios of Ta and potential transpiration (Tp) vs. GWTD were fitted to a mono-exponential decay of GWTD with correlation coefficients as high as 0.98. Two threshold values, d(Ta/Tp)/d(H) ≤ − 2% and Ta/Tp = 0.30, were then used to calculate the unaffected and critical depths. The average unaffected and critical water depths were 3.60 and 2.20 m, respectively, which coincided fairly well with the field investigation. The sensitivity analysis showed that although reducing hydraulic conductivity (Ks) caused the Ta/Tp to decline, decreasing the inverse of the air-entry pressure (α) caused the Ta/Tp to increase, so variations in Ks and α had only a minor effect. Considering that the pre-mining GWTD is between 0 and 4.0 m for about half of the YMA, the vegetation there might be degraded or lost by a GWT decline due to mining. Also, the influence of a GWT decline on transpiration was more obvious during a drought in the YMA. Therefore, effective measures must be taken to avoid a decrease in the GWT. For example, mine water should be reinjected and the mining height reduced to protect the YMA’s shallow groundwater.