Increasing Groundwater Depth Research Articles

Patterns in Tree Cavities (Hollows) in Euphrates Poplar (Populus euphratica, Salicaceae) along the Tarim River in NW China

Populus euphratica Oliv., an indicator species for eco-environmental change in arid areas, plays a key role in maintaining the stability of fragile oasis–desert ecosystems. Owing to human interference as well as to the harshness of the natural environment, P. euphratica forests have suffered severe damage and degradation, with trunk cavities (i.e., hollows) becoming increasingly pronounced, and thus posing a great threat to the growth, health, and survival of the species. Currently, there is a gap in our understanding of cavity formation and its distribution in P. euphratica. Here, cavities in the trunks and branches of a P. euphratica in a typical transect (Arghan) along the lower Tarim River were studied based on field positioning observations combined with laboratory analysis. The results revealed a large number of hollow-bearing P. euphratica stands in the study area; indeed, trees with hollows accounted for 56% of the sampled trees, with approximately 159 trees/ha. Sixty-six percent of hollow trees exhibited large (15 cm ≤ cavity width (CW) < 30 cm) or very large (CW > 30 cm) hollows. The main types of cavities in the trees were trunk main (31.3%), trunk top (20.7%), branch end (19.5%), and branch middle (19.5%). Tree parameters, such as diameter at breast height (DBH), tree height (TH), east–west crown width (EWCW), height under branches (UBH), and crown loss (CL) were significantly different between hollow and non-hollow trees. Both cavity height and width were significantly and positively correlated with DBH and CL, as well as with average crown width (ACW) (p < 0.001) and the distance from the tree to the river. The proportion of P. euphratica trees with cavities showed an overall increasing trend with increasing groundwater depth. Our findings show that cavities in P. euphratica varied with different tree architectural characteristics. Water availability is a major environmental factor influencing the occurrence of hollowing in desert riparian forests. The results provide scientific support for the conservation and sustainable management of existing desert riparian forest ecosystems.

Soil CO2 and CH4 Dynamics and Their Relationships with Soil Nutrients, Enzyme Activity, and Root Biomass during Winter Wheat Growth under Shallow Groundwater

Soil CO2 and CH4 concentrations are crucial determinants of crop physiology and the soil environment. However, the intricate relationships among soil respiration, soil nutrients, enzyme activities, and winter wheat growth in the presence of shallow groundwater remain enigmatic. This study aimed to investigate the dynamics of soil CO2 and CH4 concentrations and their correlations with soil nutrient content, enzymatic activities, and wheat root biomass to better understand the influence of shallow groundwater on soil environmental conditions. Lysimeter experiments were conducted at five groundwater depths (20, 40, 50, 60, and 80 cm) and three fertilizer application rates (low, 75%; normal, 100%; high, 125%). Soil CO2 (soil layer > 10 cm) and CH4 concentrations significantly decreased with increasing groundwater depth. The maximum values of root parameters and shoot biomass were mainly concentrated at 50–60 cm at the high fertilization level (except root length density, which was higher at the normal fertilization level), and were 0.36–77.4% higher than other treatments. Soil CO2 concentration showed positive correlations with organic matter and total N content, enzyme activities, and root biomass. Soil CH4 concentration had significant correlations with soil organic matter, total N, and available K. Compared to the fertilization level, groundwater depth emerged as a crucial factor as it affected soil physicochemical properties, soil enzymatic activities, root respiration, and winter wheat growth in shallow groundwater.

Impact of altered groundwater depth on soil microbial diversity, network complexity and multifunctionality.

Understanding the effects of groundwater depth on soil microbiota and multiple soil functions is essential for ecological restoration and the implementation of groundwater conservation. The current impact of increased groundwater levels induced by drought on soil microbiota and multifunctionality remains ambiguous, which impedes our understanding of the sustainability of water-scarce ecosystems that heavily rely on groundwater resources. This study investigated the impacts of altered groundwater depths on soil microbiota and multifunctionality in a semi-arid region. Three groundwater depth levels were studied, with different soil quality and soil moisture at each level. The deep groundwater treatment had negative impacts on diversity, network complexity of microbiota, and the relationships among microbial phylum unites. Increasing groundwater depth also changed composition of soil microbiota, reducing the relative abundance of dominant phyla including Proteobacteria and Ascomycota. Increasing groundwater depth led to changes in microbial community characteristics, which are strongly related to alterations in soil multifunctionality. Overall, our results suggest that groundwater depth had a strongly effect on soil microbiota and functionality.

Effects of groundwater depth on groundwater recharge and soybean growth dynamics in Northeast China Plain.

To explore the groundwater recharge rate and soybean growth dynamics under different groundwater depths, we conducted a field experiment with four groundwater depth treatments (1 m, D1; 2 m, D2; 3 m, D3; 4 m, D4) through the groundwater simulation system in 2021 and 2022 and explored the relationships between groundwater depth and groundwater recharge, irrigation, growth dynamics of soybean plants, and yield. We used the Logistic regression model to simulate the dynamics of soybean growth indices, including plant height, leaf area index, and dry matter accumulation. The results showed that compared with D1 treatment, the amount of groundwater recharge under D2, D3, and D4 treatments decreased by 81.1%, 96.8%, 97.5% and 80.7%, 96.7%, 97.3% in the two years, respectively. The groundwater in D1 treatment could meet water needs of soybean throughout the whole growth period, except that irrigation was needed in the sowing stage. The amount of irrigation under D1 treatment was decreased by 91.7%, 93.0%, 94.2%, and 90.9%, 92.9%, 94.0% in the two years, respectively, compared with D2, D3, D4 treatments. Among the four treatments, D1 treatment took the shortest time for entering the rapid growth stage and reach the maximum growth rate, which had the highest maximum growth rate. At the mature stage of soybean, the dry matter distribution ratio of stem in D1 treatment was the highest. D1 treatment promoted the translocation of post-flowering assimilates in soybean, and its post-flowering assimilate contribution to seeds increased by 15.5%, 16.2%, 32.6% and 45.5%, 48.7%, 63.3% in the two years, respectively, compared with D2, D3, D4 treatments. D1 treatment had the highest plant height, leaf area index, and dry matter accumulation, follo-wed by D4 treatment, while D3 treatment had the lowest. Soybean yield, number of pods per plant, number of grains per plant, and 100-grain weight all decreased and then increased with increasing groundwater depth, following an order of D1>D4>D2>D3. Soybean yield was significantly positively correlated with groundwater recharge, which was positively correlated with plant height, leaf area index, and dry matter accumulation. Our results indicated that the D1 treatment with adequate groundwater recharge increased plant height, leaf area index, and dry matter accumulation, coordinated the distribution and translocation of dry matter among all plant parts in the late soybean growth period, and ultimately achieved the highest yield. When groundwater depth was deep (D4), groundwater recharge was small. In such case, the growth and development status and yield of soybean could also reach a high level if there was sufficient water supply.

Impact of deeper groundwater depth on vegetation and soil in semi-arid region of eastern China.

Understanding the impact of deep groundwater depth on vegetation communities and soil in sand dunes with different underground water tables is essential for ecological restoration and the conservation of groundwater. Furthermore, this understanding is critical for determining the threshold value of groundwater depth that ensures the survival of vegetation. This paper was conducted in a semi-arid region in eastern China, and the effects of deep groundwater depth (6.25m, 10.61m, and 15.26m) on vegetation communities and soil properties (0-200 cm) across three dune types (mobile, semi-fixed, and fixed dunes) were evaluated in a sand ecosystem in the Horqin Sandy Land. For vegetation community, variations in the same species are more significant at different groundwater depths. For soil properties, groundwater depth negatively influences soil moisture, total carbon, total nitrogen, available nitrogen, available phosphorus concentrations, and soil pH. Besides, groundwater depth also significantly affected organic carbon and available potassium concentrations. In addition, herb species were mainly distributed in areas with lower groundwater depth, yet arbor and shrub species were sparsely distributed in places with deeper groundwater depth. As arbor and shrub species are key drivers of ecosystem sustainability, the adaptation of these dominant species to increasing groundwater depth may alleviate the negative effects of increasing groundwater depth; however, restrictions on this adaptation were exceeded at deeper groundwater depth.

Abiotic stress and human activities reduce plant diversity in desert riparian forests

Plant diversity is very important for maintaining the structure and function of ecosystems. Affected by climate change, it is increasingly sensitive to abiotic stresses, especially in ecologically fragile arid areas. Moreover, human activities have accelerated this process, resulting in a reduction in plant diversity. Revealing the spatial pattern and effecting mechanisms of plant diversity at the watershed level is beneficial to biodiversity conservation. Here, we analyze changes in plant diversity and its influencing factors in the desert riparian forest in the lower reaches of the Kashgar River using niche theory, interspecific association analysis, geographic information system (GIS) mapping, redundancy analysis (RDA), and structural equation modelling (SEM). We show that the niche breadth of plants varies from 0.174 to 0.451, and niche overlap ranges from 0.059 to 0.995, and Populus euphratica and Tamarix chinensis are the dominant species for improving the desert environment and maintaining the stable and ecological balance of communities. Plant diversity has spatial heterogeneity that woody plants are higher than herbaceous plants, which decreases with increasing groundwater depth and salt content. Groundwater depth was the key factor to stress plant diversity. In fact, it works by influencing soil moisture and salt content. LULC change significantly reduced plant diversity (P < 0.05) and destroyed ecosystem balance. Plant diversity and NDVI were significantly positively correlated (P < 0.01). It is useful for understanding and predicting changes in plant diversity. Therefore, the effective implementation of ecological water conveyance can promote groundwater rise, alleviate soil water shortage, reduce salt content, and promote the increase in plant diversity. In addition, reasonable control of LULC changes and improvement of vegetation cover will help maintain higher plant diversity.

Contrasting plant water-use responses to groundwater depth from seedlings to mature trees in the Gurbantunggut Desert

• Stable hydrogen and oxygen isotope method was used to research size-dependent water use pattern of Haloxylon ammodendron woodland. • Both development stage and groundwater depth had significant effect on water use pattern of H. ammodendron . • H. ammodendron used water from increasingly deeper soil layers throughout its development. • Water absorption depth decreased for seedlings but increased for mature trees with increasing groundwater depth. Haloxylon ammodendron is one of the major dominant woody species in Central Asian desert, and the size-dependent water use pattern of this species significantly affects the functioning of the local ecosystem. Here, we investigated the water-use responses of H. ammodendron to groundwater depth from seedling to mature tree in the Gurbantunggut Desert, China. Three sites were selected along a groundwater depth gradient, with 4.6 m, 7.4 m, and 11.0 m, respectively. Based on basal stem diameter, H. ammodendron plants were grouped into three development stages, namely seedling, sapling, and mature tree. Hydrogen isotope ratios ( δ 2 H) and oxygen isotope ratios ( δ 18 O) were measured for xylem water at different basal stem diameter (BSD) values, soil water in different soil layers, and groundwater. The water sources used by the seedlings, saplings, and mature trees were determined by applying the IsoSource model. The relationship between δ 18 O value of xylem for H. ammodendron and basal stem diameter reflected changes in the water source from upper soil water to deeper soil water as H. ammodendron grows. With the decline in groundwater depth, seedlings decreased their water uptake depth, whereas saplings increased at mid depth but decreased at maximum depth. However, mature trees constantly increased their water absorption depth across sites. Tree height, crown width, and BSD presented a decreasing trend as the groundwater depth declined. The above study promotes the evaluation of the impact of groundwater retrogression on H. ammodendron population in its natural habitat.

Effects of groundwater depth on functional traits of young Haloxylon ammodendron.

Groundwater is an important water source for phreatophytic shrubs in arid desert areas. In order to understand the impacts of groundwater depth on functional traits of phreatophytic shrubs, two groups of groundwater levels (2 and 3.5 m) were set up using lysimeter with automatic water replenishing instrument. We measured hydraulic traits, gas exchange characteristics, and root morphological parameters of young Haloxylon ammodendron during the growing season. The results showed that predawn assimilating branch water potential, osmotic potential at full turgor, and root length ratio of young H. ammodendron in the groundwater depth of 3.5 m were lower by 48.2%, 41.5% and 56.7% than that under groundwater depth of 2 m, respectively, while maximum net photosynthetic rate of late growing season, root volume, specific root length and specific root area of fine root were 75.7%, 41.0%, 273.7% and 67.7% higher, respectively. Midday water potential and water content of assimilating branch tended to decrease first in the early growing season and then increase in the late growing season. Root distribution of young H. ammodendron along soil profile showed a significant positive correlation between the average root diameter and soil depth, while the proportion of fine root surface area showed a significant negative correlation with soil depth at both groundwater levels. There was synergy of aboveground assimilating branch hydraulic traits and photosynthetic capacity with belowground root morphological traits in young H. ammodendron. Under the condition of increasing groundwater depth, young H. ammodendron adopted the ecological strategies of reducing predawn assimilating branch water potential and osmotic potential at full turgor, and increasing root diameter and length to enhance water deficit tolerance and expanding the area of water uptake to sustain their survival.

地下水埋深和季节性干旱对古尔班通古特沙漠南缘梭梭生理和生长的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 地下水埋深和季节性干旱对古尔班通古特沙漠南缘梭梭生理和生长的影响 DOI: 10.5846/stxb202101190206 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 新疆维吾尔自治区天山青年计划项目（2020Q025）；国家自然科学基金项目（32171874）；新疆维吾尔自治区自然科学基金项目（2019D01A98） Effects of groundwater depth and seasonal drought on the physiology and growth of Haloxylon ammodendron at the southern edge of Gurbantonggut Desert Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:干旱区因降水稀少，地下水成为荒漠植被重要且稳定的水源。选取古尔班通古特沙漠南缘建群种植物梭梭（Haloxylon ammodendron）为研究对象，通过测量不同地下水埋深（3.45、9.08、10.47、13.27 m和15.91 m）下生长季前期和后期同化枝生理生化指标（黎明水势、正午水势、含水量、氯离子、钠离子、脯氨酸和非结构性碳水化合物）和生长与形态特征（生长速率和胡伯尔值），旨在认识荒漠植物对地下水埋深增加和季节性干旱的响应特征和调节适应机制。结果表明：（1）梭梭应对地下水埋深变化的生理调节对策，是采取先降低后升高黎明前同化枝水势、降低新枝形成期同化枝生长速率、增大胡伯尔值和积累非结构性碳水化合物的策略；（2）梭梭应对生长季大气干旱的生理调节对策，是通过降低黎明前同化枝水势、维持较高胡伯尔值、积累钠离子和消耗淀粉抵御季节性干旱；（3）在大气干旱与地下水水文干旱交互作用下，梭梭是采取降低正午同化枝水势、维持较高的同化枝含水量和积累可溶性糖的生态策略。综上所述，梭梭在响应地下水水文干旱和季节性大气干旱的生理特征间存在差异。研究结果丰富了水文和大气干旱对梭梭生理和生长影响的认知，可以为基于地下水资源管理的干旱区荒漠植被保育提供参考。 Abstract:Groundwater is an important and stable water source for desert vegetation due to the scarcity of precipitation in arid areas. Currently, anthropogenic withdraw of groundwater and climatic drought further limits the water availability for phreatophytic vegetation in arid regions and results in the decline of native vegetation. However, how phreatophytic plant response and adaptation to reduced water availability along the groundwater depth gradient are not clear. Haloxylon ammodendron was widely distributed in northwest arid zone of China and played an important role in desertification control. Groundwater was an important water resource for H. ammodendron. The declining groundwater depth has caused the partially decline even death of H. ammodendron in their native distribution area. In this study, we selected H. ammodendron, one of the main constructive species, at the southern edge of Gurbantonggut Desert as the taget species. In order to understand the response characteristics and regulatory adaptation mechanisms of desert plants to increasing groundwater depth and seasonal drought, we measured physiological and biochemical traits (including predawn water potential, midday water potential, water content, sodium ion, chloride ion, proline and non-structural carbohydrates), growth and morphological characteristics (including growth rate and Huber value) of assimilation branches in the early and late growing season under different groundwater depths (3.45, 9.08, 10.47, 13.27 and 15.91 m). The results showed that:(1) H. ammodendron adopted the strategy of first decreasing and then increasing predawn assimilation branch water potential, decreasing assimilation branch growth rate during new branch formation, increasing the Huber value and accumulating non-structural carbohydrates in response to increase in groundwater depth. (2) With the prolonged drought, H. ammodendron reduced predawn assimilation branch water potential, maintained high Huber values, accumulated sodium ions and consumed starch to resist seasonal drought during the growing season. (3) Reducing midday assimilation branch water potential, maintaining high assimilation branch water content and accumulating soluble sugars are ecological strategies in H. ammodendron to acclimate atmospheric and hydrological drought. In summary, there are differences in the physiological characteristics of H. ammodendron in response to groundwater hydrological drought and seasonally atmospheric drought. The results of the study enriched the knowledge of the effects of hydrological and atmospheric drought on the physiology and growth of H. ammodendron, and provided a reference for the conservation of desert vegetation based on groundwater resource management in arid areas. 参考文献 相似文献 引证文献

Responses of two dominant desert plant species to the changes in groundwater depth in hinterland natural oasis, Tarim Basin.

Groundwater is increasingly becoming a permanent and steady water source for the growth and reproduction of desert plant species due to the frequent channel cutoff events in arid inland river basins. Although it is widely acknowledged that the accessibility of groundwater has a significant impact on plant species maintaining their ecological function, little is known about the water use strategies of desert plant species to the groundwater availability in Daryaboyi Oasis, Central Tarim Basin. This study initially determined the desirable and stressing groundwater depths based on ecological and morphological parameters including UAV‐based fractional vegetation cover (FVC) images and plant growth status. Then, leaf δ13C values of small‐ and big‐sized plants were analyzed to reveal the water use strategies of two dominant woody species (Populus euphratica and Tamarix ramosissima) in response to the groundwater depth gradient. The changes in FVC and growth status of plants suggested that the actual groundwater depth should be kept at an appropriate range of about 2.1–4.3 m, and the minimum groundwater depth should be less than 7 m. This will ensure the protection of riparian woody plants at a normal growth state and guarantee the coexistence of both plant types. Under a desirable groundwater condition, water alternation (i.e., flooding and rising groundwater depth) was the main factor influencing the variation of plant water use efficiency. The obtained results indicated that big‐sized plants are more salt‐tolerant than small ones, and T. ramosissima has strong salt palatability than P. euphratica. With increasing groundwater depth, P. euphratica continuously decreases its growth status to maintain hydraulic efficiency in drought condition, while T. ramosissima mainly increases its water use efficiency first and decreases its growth status after then. Besides, in a drought condition, T. ramosissima has strong adaptability than P. euphratica. This study will be informative for ecological restoration and sustainable management of Daryaboyi Oasis and provides reference materials for future research programs.

Modelling groundwater-dependent vegetation index using Entropy theory

An ecosystem is vulnerable to the scarcity of water resources and sparse vegetation cover in arid regions. Groundwater plays an important role in maintaining ecological environment and strongly impacts the ecosystem through influencing vegetation structure and species distribution. It is therefore important to clearly understand the relationship between vegetation patterns and groundwater depth(GWD). In this paper, Tsallis entropy theory was applied to derive a functional relationship between GWD and vegetation distribution. The theory was tested using observed data from arid regions in northwestern China. Results showed that higher vegetation coverage exist at places of shallow GWD. The values of NDVI gradually increase with increasing GWD until reaching a maximum at the optimum depth, after which they decrease with increasing groundwater depth when GWD is less than approximately 10 m. Beyond that depth, a low level of vegetation coverage is maintained. The correlation coefficients between measured and simulated values of NDVI were above 0.9 (p < 0.01) in the Ejina, Qaidam and Hailiutu basins. The theory is applicable to different regions and vegetation types and may improve our ability to sustainably manage land and groundwater resources in arid regions, especially where the vegetation is groundwater-dependent.

Contrasting plant water‐use responses to groundwater depth in coastal dune ecosystems

AbstractGroundwater lowering can produce dramatic changes in the physiological performance and survival of plant species. The impact of decreasing water availability due to climate change and anthropogenic groundwater extraction on coastal dune ecosystems has become of increasing concern, with uncertainties about how vegetation will respond in both the short and long terms.We aimed to evaluate the water‐use responses of different plant functional types to increasing groundwater table depth and how this would affect their physiology in Mediterranean coastal dune systems differing in aridity.We modelled water‐table depth, quantified the contribution of different soil layers to plant water through Bayesian isotope mixing models and used a combination of spectral and isotope data to characterize plant ecophysiology. We found that increasing depth to groundwater triggered water uptake adjustments towards deeper soil layers only in the dry season. These adjustments in water source use were made by conifer trees (Pinus pinea,P. pinaster) and hygrophytic shrubs (Erica scoparia, Salix repens) but not by the xerophytic shrubCorema album. Moreover, we observed a greater use of groundwater under semi‐arid conditions. Accompanying the greater use of water from deep soil layers as a response to increasing groundwater depth, the semi‐arid dimorphic‐rooted conifer treeP. pineaand hygrophytic shrubE. scopariadeclined their water content (WI), without implications on photosynthetic parameters, such as chlorophyll content (CHL), photochemical index (PRI) and δ13C. Unexpectedly, under semi‐arid conditions, the shallow‐rooted xerophytic shrubC. album, associated with an absence of water source use adjustments, showed a decline inWI,CHLandPRIwith groundwater table lowering.We provide insight into how different species, belonging to different functional types, are acclimating to groundwater changes in a region experiencing climatic drought and a scarcity in groundwater due to anthropogenic exploitation. Greater depth to groundwater combined with limited precipitation can have a significant effect on plants’ water source use and ecophysiology in semi‐arid coastal dune ecosystems.Aplain language summaryis available for this article.

Water use strategies of Populus euphratica seedlings under groundwater fluctuation in the Tarim River Basin of Central Asia

Riparian vegetation in arid regions is subject to frequent water environmental fluctuations and plays an important role in maintaining the riparian ecosystem. In recent decades, Populus euphratica (P. euphratica) has been threatened by decreasing groundwater levels, and large areas of P. euphratica seedlings are withered. To better understand the adaptive mechanism underlying hydrological process alterations, we designed an experiment to identify the water sources of P. euphratica seedlings using δ18O values (plant stem xylem, soil water and groundwater) under different groundwater scenarios by simulating the wild habitats conditions in the Tarim River bank of Central Asia. Seedlings were grown in lysimeters (2.0 m tall × 0.4 m inner diameter) under varying hydrological conditions. P. euphratica seedlings generally took up water from the shallow soil layer (0–80 cm). Under the T. ramosissima disturbance, the water source depths of the P. euphratica seedlings moved down as the groundwater depth increased. With increasing groundwater depth, the proportions of groundwater used by P. euphratica seedling monocultures (only P. euphratica seedlings) and P. euphratica seedling mixtures (P. euphratica and T. ramosissima seedlings) were reduced. Under shallow groundwater treatment (W1: 25 cm), the plant height growth of the P. euphratica seedlings slowed, and their biomass accumulation decreased. The aboveground and total biomasses of the non-coexistence seedlings under W2 (75 cm) treatment were maximized, while the maximum value for coexistence seedlings occurred under W3 (125 cm) treatment. Thus, we suggest that shallow groundwater depth is not beneficial to P. euphratica seedling growth, and appropriately decreasing the groundwater depth may promote their growth. Such information will be valuable to provide the configuration of water resources for a typical river basin in an arid region of Central Asia based on the ecological water requirements of desert riparian vegetation.

Stable isotope analysis reveals prolonged drought stress in poplar plantation mortality of the Three-North Shelter Forest in Northern China

The Three-North Shelter Forest (TNSF) is an important ecological barrier in northern China. However, in the last decade, poplar tree degradation has occurred in ∼80% of TNSF stands in Zhangbei County (Hebei Province), with about one-third of trees either dead or dying. Using isotopic techniques, we investigated the corrected δ13C value of tree-ring (δ13Ccor) and intrinsic water-use efficiency (iWUE) differences between non-dieback and dieback trees of the same age to understand the cause of poplar dieback death associated with the TNSF. We found that the diameter of poplar trees within the same age group decreased as degradation progressed. From 1997, inter-group differences of δ13Ccor and iWUE occurred but were not statistically significant. The differences became significant from 2002. Comparisons suggest that the continuous occurrence of positive ΔiWUE (iWUEdieback−iWUEnon-dieback) may be the threshold for subsequent divergence of the two groups. Stepwise regression revealed that the impact of groundwater depth on iWUE was stronger than that of other environmental factors (e.g., temperature and evapotranspiration) and was the leading cause of poplar degradation. IsoSource model analyses indicated that non-dieback trees took water mainly from soil 30–80 cm below the ground surface, and that dieback trees did it mainly from 0–30 cm below the surface. Non-dieback trees used more water from the 80–150 cm soil layer and groundwater than did dieback trees. Groundwater depth, which increased in the experimental area and so aggravated local droughts, was strongly related to cumulative water consumption but not to evapotranspiration (ET0), indicating that increasing groundwater depth was primarily caused by the change in land use. Therefore, poplar dieback in the TNSF in Zhangbei County was attributed to groundwater overuse associated with changed land use, which enhanced the duration and intensity of water stress on the trees.

Groundwater Depth and Soil Properties Are Associated with Variation in Vegetation of a Desert Riparian Ecosystem in an Arid Area of China

Groundwater is a major driving force for plant community distribution in arid areas worldwide. Although it is well known that groundwater has a significant impact on soil and vegetation, there is little information on how groundwater depth affects soil and vegetation in an arid inland basin desert riparian ecosystem. Therefore, quantitative analysis of the relationships among groundwater depth, soil properties and plant community distribution is necessary. A desert riparian ecosystem in the lower reaches of the Heihe River in an arid area of Northwest China was used to determine quantitative relationships among groundwater depth, soil and vegetation. Groundwater depth significantly increased with increased distance from the river. Soil and vegetation characteristics showed a significant trend with increasing groundwater depth. With increasing groundwater depth, soil water content, soil total nitrogen, soil total carbon, soil available phosphorus and soil available potassium decreased, while the soil bulk density and soil carbon:nitrogen (C:N) ratio increased. Soil pH and soil electrical conductivity followed quadratic function relationships with groundwater depth. Species richness, aboveground biomass, community coverage, community height, foliage projective cover and leaf area index all significantly decreased with increased groundwater depth. Groundwater depth and soil were associated with vegetation variance, explaining 85.8% of the vegetation variance. Groundwater depth was more important in explaining vegetation variance than soil properties (soil bulk density) and soil pH. Our observations indicate that changes in groundwater depth would have a significant influence on desert riparian forest vegetation, and that maintaining appropriate groundwater depth is necessary to preserve the riparian ecosystem.

Ecophysiological response of a wetland plant (&lt;i&gt;Carex cinerascens&lt;/i&gt;) to different groundwater depths in Lake Poyang wetland

地下水位在非淹水期对湿地植物的生长影响较大，但目前相关研究十分缺乏.本文选择鄱阳湖典型植被灰化薹草（Carex cinerascens）为研究对象，研究不同地下水位（地下水位埋深10、20、40、80和120 cm）对灰化薹草形态指标、地上生物量和生理指标的影响.结果表明，随着地下水位埋深的增加，灰化薹草的株高、叶长和生物量均显著降低，地下水位10 cm处理组的灰化薹草生物量为0.371±0.017 g，为地下水位120 cm处理组（0.084±0.004 g）的4.4倍；处理组间灰化薹草叶片中超氧化物歧化酶活性、过氧化物酶活性、游离脯氨酸含量和叶绿素含量均存在显著差异，其中游离脯氨酸含量由地下水位10 cm处理组的6.29±0.70 μg/g增加到地下水位120 cm处理组的8.54±1.37 μg/g，表明随着地下水位埋深的增加，灰化薹草面临一定程度的干旱胁迫.灰化薹草的生理生态响应综合表明，地下水位埋深20 cm以内适宜灰化薹草的生长，地下水位埋深80 cm以上的干旱胁迫会阻碍灰化薹草的生长.;Groundwater depth is important to wetland vegetation during non-inundation. However, the relationship between groundwater and wetland plants is still scarce. In this study, we investigated the effects of groundwater depth on the growth of Carex cinerascens, one of the dominant species in the Lake Poyang wetland based on controlled experiments. The experiment includes five groundwater depth treatments (10, 20, 40, 80 and 120 cm). Morphological and physiological characteristics of C. cinerascens as well as aboveground biomass were monitored. The results showed that the plant height, leaf length and aboveground biomass decreased with increasing groundwater depth. The biomass in 10 cm treatment (0.371±0.017 g) is 4.4 times of that in 120 cm treatment (0.084±0.004 g). The groundwater depth has significant effect on the enzyme activities, proline and chlorophyll. The proline content increased from 6.29±0.70 μg/g in 10 cm treatment to 8.54±1.37 μg/g in 120 cm treatment. These physiological differences among treatments suggested C. cinerascens would encounter some drought stress along with increased groundwater depth. All the results indicated areas with groundwater depth of less than 20 cm were suitable for C. cinerascens growth, but the growth of C. cinerascens would be impeded when the groundwater depth was below 80 cm.

Nitrate-nitrogen contamination in groundwater: Spatiotemporal variation and driving factors under cropland in Shandong Province, China

High groundwater nitrate-N is a serious problem especially in highly active agricultural areas. In study, the concentration and spatialtemporal distribution of groundwater nitrate-N under cropland in Shandong province were assessed by statistical and geostatistical techniques. Nitrate-N concentration reached a maximum of 184.60 mg L-1 and 29.5% of samples had levels in excess of safety threshold concentration (20 mg L-1). The median nitrate-N contents after rainy season were significantly higher than those before rainy season, and decreased with increasing groundwater depth. Nitrate-N under vegetable and orchard area are significantly higher than ones under grain. The kriging map shows that groundwater nitrate-N has a strong spatial variability. Many districts, such as Weifang, Linyi in Shandong province are heavily contaminated with nitrate-N. However, there are no significant trends of NO3--N for most cities. Stepwise regression analysis showed influencing factors are different for the groundwater in different depth. But overall, vegetable yield per unit area, percentages of orchard area, per capita agricultural production, unit-area nitrogen fertilizer, livestock per unit area, percentages of irrigation areas, population per unit area and annual mean temperature are significant variables for groundwater nitrate-N variation.

Assessing Groundwater Geospatial Variation Using Microgravity Investigation in the Arid Riyadh Metropolitan Area, Saudi Arabia: a Case Study

A combination of relative microgravity measurements at ground surface, and depth to water and water table measurements from adjacent wells were used to estimate geospatial variation of groundwater. A highly accurate portable Grav-Map gravimeter was used for gravimetric measurements at locations nearby a 42 well water table monitoring program. To efficiently correlate the two data sets, wells were clustered into five groups by geological unit and water saturation. Microgravity data was processed, interpreted, and correlated with both the depths to groundwater and the water table levels. Regression analyses revealed a strong negative correlation for microgravity and depth to groundwater in all five clusters; correlation coefficients varied between 0.70 and 0.97, and measured 0.78 over the entire study area. Microgravity values increased as groundwater depth decreased, likely because rising groundwater fills voids and fractures within soil and rocks, increasing rock density and therefore relative gravity. To validate the correlation, we superimposed a map of depths to water on the first derivative of microgravity measurements. The shallowest groundwater depths were positively related to the zero first derivatives, having intersection areas within a 75 % significance interval. Negative first derivatives covered the rest of the study area, with relative gravity decreasing with increasing groundwater depth. This technique can precisely and efficiently determine changes in subsurface geology and geospatial changes in depths to the groundwater table. Distances between microgravity stations should be small, to better detect small changes in gravity values, reflecting density contrasts underground.

Effects of different groundwater depths on the distribution characteristics of soil-Tamarix water contents and salinity under saline mineralization conditions

The objective of this study was to explore the correlations between Tamarix and soil water salinity parameters and groundwater depth under saline mineralization conditions and to reveal the transport rules and exchange effects of salinity in soil-Tamarix systems with different groundwater depths. Soil columns planted with Tamarix chinensis (T.-soil column) grown in the Yellow River Delta were chosen for modeling. Six different groundwater depths were simulated in the models. We measured and analyzed the water and salinity contents of different soil profiles with different groundwater levels and Na+ contents in new T. chinensis branches and leaves. The results showed the following: (1) under shallow and medium groundwater levels, with increasing soil depth, the differences in the water contents between the T.-soil column and the control soil layer gradually decreased. Under deep groundwater levels, the differences increased and then decreased. Planting T. chinensis decreased the soil water content, and higher groundwater levels corresponded with a greater decrease in the soil water content of the T.-soil column. With increasing groundwater depth, the depth at which the water content of the T.-soil column suddenly changed increased. When the groundwater levels were 0.6m, 0.9m, 1.2m, 1.5m, and 1.8m, the critical depths at which the water contents suddenly changed in the T.-soil columns and control were 0.3m, 0.5m, 0.7m, 1.0m, and 1.3m, respectively. (2) Although planting T. chinensis clearly decreased soil salinity, the amplitude of the decrease increased and then decreased with increasing groundwater depth. The maximum decrease was reached at a groundwater level of 1.2m. Importantly, the groundwater level of 1.2m was the critical level for increasing the capillary water in the T.-soil column and was the depth at which the water and salinity significantly changed (the water and salinity turning depths). When the groundwater depths were 0.6, 0.9, 1.2, 1.5 and 1.8m, the water turning depths in the T.-soil columns and control were 0.3, 0.5, 0.7, 0.7 and 0.7m, respectively. Under medium (0.9m and 1.2m) and deep (1.5m and 1.8m) groundwater depths, the minimum soil salinities in the T.-soil column occurred at depths of 0.5m, 0.7m, 0.7m, and 1.0m, respectively. (3) With increasing groundwater depth, the water contents of the T.-soil column, T. chinensis leaves, and new branches gradually decreased. The salinity of the T. chinensis leaves decreased and then increased, and the salinities of the T.-soil column and new branches increased and then decreased. Under different groundwater depths, the water contents of the T. chinensis leaves were significantly higher than the water contents of the branches, and the salinity contents of the leaves were significantly lower than the salinity contents of the branches. With increasing groundwater depth, the water transported from the branches to leaves of T. chinensis significantly increased, and the ratio of Na+ that migrated decreased and then increased.

Variation in Populus euphratica foliar carbon isotope composition and osmotic solute for different groundwater depths in an arid region of China.

Water use efficiency (WUE) is an important trait associated with plant acclimation caused by water deficits, and δ13C is a good surrogate of WUE under conditions of water deficits. Water deficiency also enhances the accumulation of compatible solutes in the leaves. In this study, variations in foliar δ(13)C values and main osmotic solutes were investigated. Those included total soluble sugar (TSS), sucrose, free proline, glycine betaine (GB), and inorganic ionic (K+, Ca2+, and Cl-) content of Populus euphratica for different groundwater depths in a Ejina desert riparian forest, China. Results indicated that foliar δ13C values in the P. euphratica for different groundwater depths ranged from -29.14±0.06 to -25.84±0.04 ‰. Foliar δ13C signatures became richer as groundwater levels declined. TSS, sucrose, free proline, GB, and K+ were accumulated in P. euphratica foliage with developing plant growth and increasing groundwater depth. Ca2+ and Cl- content increased under stronger P. euphratica transpiration rates for shallower groundwater depths (1-2.5 m) and decreased for deeper groundwater depths (greater than 3.0 m). Moreover, correlations between δ13C, osmotic solutes, and groundwater depths showed that the primary osmotic solutes were TSS, sucrose, proline, GB, and K+. Correlations also showed that δ13C was not only a useful measure for P. euphratica-integrated WUE but also could be used as an indicator reflecting some physiological osmotic indexes.