Reservoir Region Research Articles

Analysis of flood frequency using plotting position methods and Gumbel’s method: Vaigai River basin, Southern India

<p>Estimation of annual peak flood flow has considerable financial influence, as this can pave numerous ways for water resources planning, design of hydraulic structures, and sustainable management of these valuable resources at an optimum benefit. Six standard plotting position methods coupled with Linear Log-regression Model (LLM) are employed for the Return Period (RP) of 15, 20, 25, 30, 60, and 1000 years to forecast peak flood flow magnitudes and for this daily annual peak flood flow data for the periods of 24 years were used to illustrate the proposed methods. Another method, Gumbel's Analytical Approach (GAA), is also used to forecast the peak flood flow magnitude for the same RP, and it was taken as the benchmark to compare all the six methods of plotting position. The degree of accuracy of the plotting position is based on the coefficient of determination R2. The R2 values for Adamowski, Hazen, Beard, Chegodajev, Tukey, and Benard, are 0.9073, 0.9284, 0.9119, 0.9111, 0.9135, and 0.9111, respectively. Out of the six-plotting position methods, Hazen method predicted a very closer value in comparison with GAA. It is advantageous to have the additional method to forecast peak flood flow for longer RP. The achieved result assuredly facilitates effective planning, regulation, and maintenance of huge reservoirs across the frequent occurrence of floods in river basin areas, considering safety and particularly downstream inhabited areas to save loss of human life, animals, damages to properties, and an impair of cultivated crops.</p>

Study of the impact of reservoir water level decline on the stability treated landslide on reservoir bank

Affected by the reservoir water and rainfall and other factors, the stability of the landslide changes with the different reservoir water level decline rate. Using the Baota landslide in Yunyang County in Three Gorges Reservoir region as an example, and applying the saturated-unsaturated numerical simulation method, the groundwater seepage field is calculated when the reservoir water level rises and declines. The calculated pore water pressure is used for the ultimate equilibrium analysis of landslide, and the effect of reservoir water level drop on landslide stability was discussed. The calculation results show that with the water level falling from 159 m to 145 m, and the increase of the rate of decline in the reservoir water level, the decline in groundwater seepage lines has increased somewhat. The landslide stability coefficient calculated under each working condition is larger than that in the original design, and the decrease rate of reservoir water has a qualitative effect on the stability of the landslide, but the influence is very small. Through the SBAS-InSAR time series monitoring of Sentinel-1 data in the 13th period and field verification, it is found that the cumulative deformation of the slope is small, which prove the stability of the landslide.

Quantifying and reducing the uncertainty in multi-source precipitation products using Bayesian total error analysis: A case study in the Danjiangkou Reservoir region in China

Quantifying and reducing the errors and uncertainties in high-resolution gridded precipitation products are essential for hydrological modelling to generate realistic parameter estimates and accurate model predictions. Formal Bayesian inference provides a powerful tool to quantify rainfall uncertainty and other uncertainty factors in hydrological modelling. An efficient way to reduce rainfall uncertainty in hydrological modelling is to merge precipitation estimates from multiple sources prior to their real-world applications. However, it remains a challenging task to optimally determine the weightings of different precipitation estimates based on the predictive ability of a hydrological model, especially from a probabilistic perspective. This study addresses this issue by evaluating and merging six gridded precipitation products using formal Bayesian inference schemes, including the Markov Chain Monte Carlo (MCMC) approach and Bayesian total error analysis (BATEA). MCMC describes the total predictive uncertainty using residual error models. In contrast, BATEA utilizes the storm depth multiplier approach to separately account for rainfall uncertainty in probabilistic uncertainty quantification. A novel signature of model predictability that combines the reliability and precision of probabilistic model predictions is developed to quantify the predictive ability of a hydrological model from precipitation estimates. Using the proposed predictability signature, we examined the benefit of optimally merging multi-source precipitation estimates in improving the accuracy of hydrologic simulations by applying the method to the Danjiangkou Reservoir region (DRR) between 1998 and 2007. The analysis demonstrates that (1) BATEA, compared to MCMC, can provide more realistic estimates of model parameters and tighter predictive intervals of daily streamflow when using rain gauge observations as well as gridded precipitation products; and (2) the proposed predictability signature is efficient for optimally merging precipitation measurements as it can produce tighter uncertainty bands and better streamflow simulations than individual precipitation estimates. This study provides insights into improved quantification and reduction of uncertainty in multi-source precipitation products for hydrological modelling.

Salt precipitation in carbonates during supercritical CO2 injection: A pore-scale experimental investigation of the effects of wettability and heterogeneity

Salt precipitation in near-wellbore regions of subsurface reservoirs is a significant phenomenon that may occur during carbon dioxide (CO2) injection for carbon sequestration and enhanced oil recovery. It induces pore plugging, which causes reductions in porosity and permeability and hence well injectivity. Previous research in this area focused predominantly on salt precipitation in fully brine-saturated media, such as those found in deep saline aquifers. However, studies on salt precipitation in hydrocarbon reservoirs where brine and oil coexist are rare, particularly at the pore scale. In this study, we elucidate the occurrence and dynamics of salt precipitation at the pore-scale during supercritical CO2 injection into depleted carbonate and sandstone oil reservoirs at elevated temperature and pressure conditions. Specifically, we examine the influence of wettability and morphological heterogeneity on salt precipitation evolution and dynamics. Investigations on the influence of wettability reveal that the time required for the initiation of precipitation is shorter for an intermediate-wet carbonate compared to that for a weakly oil-wet carbonate. Relatively higher amounts of salt deposits are formed in the more hydrophilic (intermediate-wet) carbonate, leading to an intensification of porosity reduction in this rock. In addition, the presence of oil within the pore space does not hinder the precipitation process but suppresses the reverse flow of solutes toward the evaporation front, thereby creating localized precipitation at the front. The level of heterogeneity affects the precipitation initiation and its duration as well as the amount of precipitates formed, with lower salt saturations encountered in the less heterogeneous sandstone compared to the more heterogeneous carbonate. Heterogeneity also influences the salt cluster size distribution within pore elements such that, for the same wettability state, medium-sized clusters dominate in sandstone, while large clusters are the main contributors to the total salt volume in the more heterogeneous carbonate. This study forms a fundamental basis for the prediction of salt precipitation during three-phase flow processes induced by supercritical CO2 injection into porous media containing high-salinity brine and oil.

Influence of reservoir impoundment on rainfall erosivity in the Three Gorges Reservoir region of China

Abstract New dammed reservoirs are expected to have a significant effect on the regional hydrocycle, but the detailed patterns may not be well understood. Regional climate change is likely to cause soil erosion uncertainty by affecting rainfall erosivity. In the present study, local precipitation and rainfall erosivity were investigated to determine the impounding influence of the Three Gorges Reservoir. Daily erosive precipitation, from 1980 to 2020, was categorized into four intensity levels (light, moderate, heavy, and very heavy), as well as extreme rainfall, to understand their contribution to erosivity. It was found that the impoundment significantly affected local precipitation, with both heavy precipitation and the relative erosivity showing a substantial declining trend (Sen's slope = 2.141, p < 0.05). The Mann–Kendall test indicated an abrupt change point around the year 2002, evidencing the effect of the reservoir impoundment (since 2003). Reservoir impoundment redistributed the intensity levels of erosive precipitation, leading to a 24.3% decrease in the erosivity of heavy precipitation and an 8.2% increase in the moderate category. The unimodal distribution of monthly precipitation was altered to a bimodal distribution with peaks in July and September, resulting in a longer but lower-risky erosion period of high concern. The fluctuations of Rx1day and Rx5day were obviously flattened after impoundment, with a 54.2% peak reduction in relative erosivity on average. Results indicated that heavy rainfall (including extreme rainfall) was reduced, and annual precipitation and erosivity both had a more even seasonal distribution following reservoir impoundment.

Landslide susceptibility mapping based on the coupling of two correlation methods and the BP neural network model: A case study of the Baihetan Reservoir area, China

The correlation calculation model between landslide and mapping factors has a direct influence on the accuracy of landslide susceptibility mapping results. Using the Baihetan reservoir area as a case study, the effect of several correlation models on mapping landslide susceptibility is studied. The frequency ratio (FR) and the information value (IV) coupled BP neural network (BPNN) model was utilized to assess landslide susceptibility, with the mapping results of the single back propagation neural network (BPNN) model acting as a reference. The receiver operating characteristic (ROC) curve, the frequency ratio, and the susceptibility index distribution (mean value and standard deviation) are used to compare and assess landslide susceptibility values. The FR-BPNN coupling model is less precise than the IV-BPNN model. Findings from a single BPNN model for susceptibility mapping are less exact than those from a coupled model. Using the coupling model of the mapping factor correlation approach to assess landslide susceptibility has evident benefits, according to the study. The coupled model employing IV as the correlation method provides the most accurate and dependable susceptibility findings, and the mapping results are more consistent with the actual distribution of landslides in the study area. It can effectively direct disaster prevention efforts in the reservoir region.

Speciation Distribution Characteristic and Ecological Risk of Heavy Metals in Surface Sediments of Cascading Hydropower Dams in Lancang River

In order to study the speciation, contamination statues, and distribution characteristic of heavy metals in the surface sediments of cascading dams, the reservoir regions of Manwan and Dachaoshan cascading dams were sampled and investigated. The speciation and its contents of heavy metals (As, Cd, Cr, Cu, Pb, and Zn) were analyzed by the modified European Community Bureau of Reference (BCR) sequential method. The ecological risk assessment of heavy metals was performed by the ratio of secondary phase and primary phase (RSP) method. The source identification of heavy metals was performed by correlation analysis (CA) and principal component analysis (PCA). The results indicated that the values of RSPcd were from heavy pollution (IV level), and those of RSPZn were from light pollution (II level) to moderate pollution (III level) in the lacustrine zone of the upper-stream Manwan dam. The values of the RSP were no pollution (I level) in the downstream of the Dachaoshan dam. The ecological risk assessment of heavy metals (Cd, Zn, and As) at the upper stream of the Manwan dam was generally higher than that at the downstream Dachaoshan dam. Cascading dams operation showed significant accumulation effects on heavy metals in surface sediments. The source identification of heavy metals showed that Cd and Zn were mainly from anthropogenic activities; As, Cu, and Pb were affected by both natural processes and anthropogenic activities; and Cr was mainly from natural processes.

Soil erosion rates on sloping cropland fragment underlain by contrasting lithologies

This study examines the pattern of soil erosion rates in a cropland fragment underlain by limestone and purple sandstone (referred to as the CLP) in the Three Gorges Reservoir Region (TGRR) using the 137Cs and 210Pbex techniques. A total of 96 bulk cores and 18 sectioned cores were collected from the CLP. In addition, 4 bulk and 4 sectioned cores were extracted from each of two reference sites. Based on the 137Cs and 210Pbex inventories obtained from these cores, the soil erosion rates were assessed. The results indicate that the 137Cs and 210Pbex inventories both increase downslope, while the calculated soil erosion rates both decrease. This implies that the intensity of soil erosion decreases downslope, which is a combined response to the effect of gravity, the short slope length, and the gentle gradient of the cropland. The soil erosion rates derived from 137Cs were generally higher than those determined using 210Pbex. This shows that soil erosion was more intense from 1954 onward than during the period between 1920 and 1954, reflecting the impact of disturbance by cultivation, which has increased rapidly over the past 60 years. The soil erosion rate on the limestone area was similar to that in the purple sandstone area, but higher than that reported from karst regions experiencing rocky desertification. This suggests that, providing that there is no evidence of karst rocky desertification, the soil erosion intensity in cropland underlain by limestone is similar to that underlain by purple sandstone. This result provides a new insight into the pattern of soil erosion intensity on the cultivated hillslopes of karst and non-karst regions at the scale of the cropland fragment. It will also help to improve our understanding of surface processes in karst and non-karst areas subject to intense cultivation.

A review of the zooplankton studies in Paraguay’s freshwater environments

This study is a review of the zooplankton studies in Paraguay, where two of South America’s large hydrographic basins are found those of the Paraguay and Paraná rivers. After exhaustive searches, twenty-two published studies were found, with three of them being developed by national researchers, while all the others were carried out by foreign researchers or with their collaboration. The first studies were done between 1905 and 1936, involving field studies with taxonomy and ecological data and an experimental study testing the influence of temperature and dissolved oxygen on zooplankton. The studies resumed in 1960, presenting mainly data for rotifers, cladocerans and copepods in the plankton community, including taxonomic inventory, richness and species diversity, community composition, beta diversity, abundance and dominance. Most of the studies were carried out in the Paraguay River basin and in the limnetic region of lakes and reservoirs. Some copepod species were typical of lentic or lotic environments and low or high trophic conditions, Overall, 154 zooplankton species were registered (70 rotifers, 51 cladocerans and 33 copepods), with high diversity for Brachionidae and Lecanidae (rotifers), Chydoridae (cladocerans) and Cyclopidae (copepods). The zooplankton richness recorded in Paraguayan territory is lower than in larger and more explored South American countries such as Argentina and Brazil, but similar to found in other countries of a similar size. Moreover, there are few scientific centers and there is low funding for research, which are important factors for the development of scientific knowledge. Planktonic protozoan communities, functional diversity or molecular analyses of zooplankton were the main gaps in the zooplankton studies. Few studies for the zooplankton were published from Paraguay country and more are necessary, close to the main channels of the Paraguay and Paraná rivers, in the Itaipu reservoir and far from these rivers, inside the country, where few or none studies were made.

Effects of overconsolidation on the reactivated residual strength of remoulded deep-seated sliding zone soil in the Three Gorges Reservoir Region, China

Due to the landslide-prone geological conditions, many landslides have occurred in the Three Gorges Reservoir Region (TGRR), most of which are slow-moving ancient landslides characterized by preexisting shear surfaces. In this study, with the purpose of making full use of reactivated residual strength to prevent and treat the slow-moving landslides, such as designing anti-slide piles, the Huangtupo landslide as a typical reactivated ancient landslide in the TGRR was selected to study the overconsolidation (OCR) effect on the reactivated residual strength of deep-seated sliding zone soil by conducting a series of laboratory ring shear tests. It was found that after a short rest of one day, the reactivated residual strength increased with the increase of OCR under a given normal effective stress, but it was lost after a small shear displacement. The overconsolidation effect on reactivated residual strength at a lower stress was more prominent than that at a higher stress. Very coarse sand with a size of 1–2 mm inside the remoulded deep-seated sliding zone soil sample exerts direct control on the reactivated residual strength, and it significantly influences the overconsolidation effect on reactivated residual strength. The results could provide a reliable scientific basis for improving the stability analysis and reinforcement measures of the slow-moving landslides in the TGRR, as well as can be applied to similar slow-moving landslides in other areas.

A multi-fractured well performance model for unconventional reservoirs

Due to the ultra-low permeability of unconventional reservoirs, transient state prevails for a considerable period. Despite this, fracture interference can cause an apparent no-flow boundary. Consequently, the Duong's model, which was developed for transient-state period, yields unreliable estimates during the late-time period. In this paper, the Duong's model is modified to account for boundary effects caused by fracture interference and/or unstimulated reservoir regions that serve as no-flow boundaries. Specifically, an empirical correction function, which assumes an exponential decline, has been used as a “modifier” to extend the Duong's model to boundary-dominated flow period. The correction function ensures that during boundary-dominated flow period, an exponential-decline behaviour dominates. The proposed rate-decline model encompasses a gamma function, which converges at large times. Results show that a fractured-well production behaviour is characterised by a decaying power-law during early-time period and tends to exponential decline during late-time period. The results also suggest that although the conventional Duong's model gives good estimates during the transient-state period, it yields optimistic estimates during the boundary-dominated flow period. The proposed model gives a good match and estimates not only in the transient-state period, but also in the boundary-dominated flow period. A major advantage of the proposed model is that it converges to estimated ultimate recovery at large times without imposing any rate and time limits. A good agreement of the estimated ultimate recovery with analytical and semi-analytical models was obtained. Also, results suggest that the proposed model gives conservative estimates. The proposed model will be useful for analysing and predicting both the early- and late-time production performance of a multi-fractured well producing from an unconventional reservoir.

A New Hybrid Framework for Error Correction and Uncertainty Analysis of Precipitation Forecasts with Combined Postprocessors

With the rapid development of meteorological models, numerical weather prediction is increasingly used in flood forecasting and reservoir regulation, but its forecasting ability is limited by the large amount of uncertainty from meteorological systems. In this paper, a new, hybrid framework is developed to improve numerical precipitation forecasting by combining the multimodel ensemble and probabilistic postprocessing methods. The results show that the multimodel ensemble method used in this paper is an efficient way to reduce prediction errors, especially missing alarm errors. In a comparison of the probabilistic postprocessors based the generalized Bayesian model (GBM) and bivariate probabilistic model (BPM), the GBM shows better performance from the aspects of indicators and is more suitable for real-time applications. Meanwhile, the assessment of probabilistic results shows that the skill of probabilistic precipitation forecasts is related to the quality of their inputs. According to these results, a new hybrid framework is proposed by taking the results from multimodel ensemble as the input of probabilistic postprocessor. Compared to using the raw numerical in GBM, the hybrid framework improves the accuracy, sharpness, reliability, and resolution ability from different lead times by 2–13%, 1–22%, and 0–12% respectively, especially when the lead time is less than 4 d, the improvement can reach 9–13%, 10–22%, and 5–12% respectively. In conclusion, the hybrid two-step framework can provide a more skillful precipitation forecast, which can be useful for flood forecasting and reservoir regulation.

Melting, Compaction and Reactive Flow: Controls on Melt Fraction and Composition Change in Crustal Mush Reservoirs

Abstract Changes in melt fraction and local bulk composition in high-crystallinity, crustal mush reservoirs are essential to produce the large volumes of low-crystallinity, silicic magma that are emplaced to form plutons, or erupted to surface. Heating (and cooling) is well understood and widely invoked in driving melt fraction change, but does not cause chemical differentiation because there is no separation of melt and crystals. Fractional crystallisation at high melt fraction is widely assumed to explain differentiation, but is inconsistent with the evidence that large-scale, long-term magma storage and evolution occurs in high-crystallinity mush reservoirs. Compaction has been suggested to explain melt fraction change and differentiation at low melt fraction, but compaction (and decompaction) causes simple unmixing (and mixing) of melt and solid crystals: to produce very refractory bulk composition by compaction, melt fraction must be driven down to very low values. Yet microstructural evidence demonstrating widespread compaction in crustal mush reservoirs at low melt fraction is lacking. Here we show that melt fraction change can be expressed in terms of heating/cooling and compaction, plus an additional term that we call ‘reactive flow’. Similarly, composition change can be expressed in terms of compaction and reactive flow. Reactive flow changes the local bulk composition, which causes ‘chemical’ melting (dissolution) and freezing (precipitation), distinct from ‘thermal’ melting/freezing caused by changes in enthalpy. We use numerical modelling to show that the contributions of compaction and reactive flow in a crustal magma reservoir are similar in magnitude. However, reactive flow opposes melt fraction and composition changes caused by compaction when compaction occurs in a temperature gradient that increases upwards at, for example, the base of a sill intrusion, or decompaction occurs in a temperature gradient that decreases upwards at, for example, the top of a sill intrusion. If compaction causes melt fraction decrease and creates a more refractory bulk composition, then reactive flow causes melt fraction increase and a more evolved bulk composition, and vice versa. Reactive flow means that very small melt fraction is not required to produce very refractory composition in a crustal magma reservoir, consistent with the relatively scarce microstructural evidence for widespread compaction. The apparent lack of compaction in crustal magma reservoirs, as compared to other natural and engineered systems in which reaction does not occur, is also explained by the contribution of reactive flow. Reactive flow also means that melt loss in compacting regions of a crustal magma reservoir may instead be accompanied by evidence for mineral dissolution, which facilitates ongoing melt fraction loss by preserving connected melt flow paths through the mush pore space. Reactive flow in decompacting regions can also explain why interstitial mineral phases display textures that mimic those of interstitial melt.

Reservoir Regulation for Ecological Protection and Remediation: A Case Study of the Irtysh River Basin, China.

Hydrological processes play a key role in ecosystem stability in arid regions. The operation of water conservancy projects leads to changes in the natural hydrological processes, thereby damaging the ecosystem balance. Ecological regulation is an effective non-engineering measure to relieve the influence of water conservancy projects on ecosystems. However, there are still some problems, such as an insufficient understanding of hydraulic processes and difficulty evaluating the application effects. In this study, the theory of ecological reservoir regulation coupled with hydrological and ecological processes was examined and ecological protection and remediation were investigated using the valley forests and grasslands in the Irtysh River Basin as a case study. The results demonstrated that (1) to meet the demand of the hydrological processes in the valley forests and grasslands, in terms of ecological regulation, the peak flow and flood peak duration of the reservoir, named 635, in the Irtysh River Basin should be 1000 m3 s−1 and 168 h, respectively, and the total water volume of ecological regulation should be 605 million m³. Ecological regulation can guarantee that the floodplain range reaches 64.3% of the core area of ecological regulation and the inundation duration in most areas is between 4–8 d; (2) an insufficient ecological water supply would seriously affect the inundation effects. The inundation areas were reduced by 2.8, 5.1, 10.3, and 19.3%, respectively, under the four insufficient ecological water supply conditions (528, 482, 398, and 301 million m3), and the inundation duration showed a general decreasing trend; (3) the construction of ecological sluices and the optimization of the reservoir regulation rules could effectively relieve the influences of an insufficient ecological water supply. At water supply volumes of 528 and 482 million m3, the regulation rules should assign priority to the flood peak flow; at water supply volumes of 398 and 301 million m3, the regulation rules should assign priority to the flood peak duration. Consequently, this study provides a reference for ecological protection in arid regions and the optimization of ecological regulation theories.

Analysis on the ecological impact of the Xiaolangdi Reservoir on the Yellow River Delta wetland and coastal areas

An environmental water supplement through reservoir regulation is an important way to restore the deltaic coastal wetland. In order to quantify the impact of the reservoir on the deltaic coastal wetland ecosystem, this article proposes a quantitative analysis method for the ecological impact and contribution rate of the reservoir, which compares the ecological status in two scenarios with the presence or absence of the reservoir during the assessment period, and reveals the reservoir’s impact on and contribution rate to the ecological status and environmental water supplement conditions. The results show that during 2000–2019, through the regulation of the Xiaolangdi Reservoir, the average annual drying up days at the Lijin section reduced by 81.15 days, the average annual assurance rate of ecological base flow at the Lijin section increased by 20.60%, and the volume of water flowing into the sea from April to June increased by 2.37 billion m³. During 2008–2015, the Xiaolangdi Reservoir increased the environmental water supplement of the Yellow River Delta by 19.95 million m³. Compared with the flow and water withdrawal during 1980–1999, the natural flow of the Yellow River decreased by 10.59% and the measured flow in the lower reaches decreased by 22.15%, and the water withdrawal in the lower reaches increased by 0.32 billion m³ during 2000–2019, which is detrimental to provide environmental water to the Yellow River Delta wetland and coastal areas. The Xiaolangdi Reservoir reversed the adverse effects of flow and water withdrawal and ensured the continuous ecological improvement in the Yellow River Delta and coastal areas, providing the respective contribution rate of 187.85%, 137.24%, and 125.83% to the prevention of drying up at the Lijin section, the increase in the assurance rate of ecological base flow at the Lijin section, and the increase in the volume of water flowing into the sea in the critical period.

Gaussian Decline Curve Analysis of Hydraulically Fractured Wells in Shale Plays: Examples from HFTS-1 (Hydraulic Fracture Test Site-1, Midland Basin, West Texas)

The present study shows how new Gaussian solutions of the pressure diffusion equation can be applied to model the pressure depletion of reservoirs produced with hydraulically multi-fractured well systems. Three practical application modes are discussed: (1) Gaussian decline curve analysis (DCA), (2) Gaussian pressure-transient analysis (PTA) and (3) Gaussian reservoir models (GRMs). The Gaussian DCA is a new history matching tool for production forecasting, which uses only one matching parameter and therefore is more practical than hyperbolic DCA methods. The Gaussian DCA was compared with the traditional Arps DCA through production analysis of 11 wells in the Wolfcamp Formation at Hydraulic Fracture Test Site-1 (HFTS-1). The hydraulic diffusivity of the reservoir region drained by the well system can be accurately estimated based on Gaussian DCA matches. Next, Gaussian PTA was used to infer the variation in effective fracture half-length of the hydraulic fractures in the HFTS-1 wells. Also included in this study is a brief example of how the full GRM solution can accurately track the fluid flow-paths in a reservoir and predict the consequent production rates of hydraulically fractured well systems. The GRM can model reservoir depletion and the associated well rates for single parent wells as well as for arrays of multiple parent–parent and parent–child wells.

Methodology for including reservoir regulation in water scarcity evaluation

Water scarcity evaluation is essential for the sustainable development of water resources. Reservoirs can regulate water temporally and spatially to meet human needs and mitigate water scarcity. No approach has considered reservoir regulation in water scarcity evaluation. We propose a simulation model of a water resource system that explicitly incorporates reservoir regulation functions into water scarcity evaluation. The model is validated using data from the Haihe River basin, northern China, using an improved water scarcity indicator to describe the severity of water scarcity. The results show that, from 2000 to 2005, the annual Water Stress Index of the Haihe River basin was 1.02 without considering reservoir regulation, and 0.92 considering reservoir regulation. That is, incorporating reservoir water availability into the evaluation reduces the estimated water scarcity. Water scarcity was overestimated temporally and spatially when the effect of reservoir regulation was not accounted for. Our results show that reservoirs can have a significant influence on water scarcity evaluation, especially in smaller and subdivided units. We recommend consideration of the role of reservoirs when evaluating water scarcity to better inform water resource management decisions.

Characteristic of soil fungal community and the influencing factors of Pinus massoniana forests in the Three Gorges Reservoir Region

Soil fungi are important components of belowground biodiversity and play important roles in soil carbon and nutrient cycling. We investigated fungal communities in the top soil (0-10 cm) of 22 Pinus massoniana forests in the Three Gorges Reservoir Region using high-throughput sequencing technique. We found that Ascomycota and Basidiomycota were the dominant fungi phyla, and Eurotiales, Russulales, and Tremellales were the most abundant fungi orders. The dominant functional groups in P. massoniana forests were saprophytic fungi, ectomycorrhizal fungi, and ericoid mycorrhizal fungi. Results of redundancy analysis showed that environmental variables but not spatial variables were the main drivers of soil fungal community structure across the 22 P. massoniana forests, which suggested that habitat filtering rather than dispersal limitation shaped soil fungal community structure. Aboveground biomass, soil conductivity, available phosphorus, soil bulk density, carbon to nitrogen ratio, nitrate concentration, and proportion of slit were the main factors explaining the variation in soil fungal community structure. It should be noted that the key factors influencing different fungal functional groups differed across forests.

Stress State Change and Fault-Slip Tendency Assessment Associated with Gas Injection and Extraction in the Hutubi (China) Underground Gas Storage

Summary The Hutubi (HTB) gas reservoir in northwest China was active in production from 1998 to 2012; beginning in 2013, it was used for gas storage. Several earthquakes occurred during the early operation of the Hutubi underground gas storage (HTB-UGS) project, so it is important to explore their relationship with gas injection and extraction from the viewpoint of stress state change and faulting susceptibility. We study the evolution of the stress state and slip tendency associated with the operation of the HTB-UGS to understand the relationship between pore pressure variation and induced seismicity. We constrain the stress state in the reservoir at the time of cessation of production in 2012 using image-logged wellbore breakouts. We then estimate stress state variation over time as a result of pore pressure changes, based on the assumption of poroelastic coupling between horizontal elastic stresses and pore pressure. The maximum principal stress in the reservoir region before the HTB-UGS operation is oriented N23°±6°E, which is consistent with that of the prevailing tectonic stress derived from earthquake focal mechanisms. Our results show that the vertical stress (Sv) and the minimum horizontal principal stress (Shmin) were initially similar in magnitude before petroleum production; the magnitude of Shmin gradually decreased during petroleum production. Gas injection, initiated in 2013, raised the Shmin magnitude, returning it to approximately that of Sv. We use the estimated variations in the reservoir stress state over time to calculate temporal changes in slip tendency of the main faults in the reservoir. The fault-slip tendency decreased continuously with petroleum production and then increased with gas injection. The first earthquake swarm associated with gas injection occurred approximately 2 months after the start of injection, possibly due to the slow pore pressure diffusion. Thereafter, earthquakes were induced whenever gas was injected. Our assessment of slip tendency suggests that earthquake swarms were induced during increasing pore pressure phases when slip tendency reached a value of 0.43 ± 0.04. The maximum allowable pore pressure for avoiding earthquakes (ML > 1.5) is 29.0 ± 4.5 MPa, which might increase through stress adjustments and a newly attained balance. This study provides a possible method for induced earthquake analysis associated with gas injection and extraction from the perspective of stress state change and faulting susceptibility.

Sensitivity study of C/O logging measurements by Monte Carlo method

Carbon oxygen ratio (C/O) logging has great importance in the accurate determination of hydrocarbon saturation in the reservoir region. This measurement is independent of the salinity of the formation water, unlike alternative logging methods. Analysis of the measurement requires modelling of the time-dependent coupled neutron-gamma field produced by the tool, which is most efficiently done by the Monte Carlo (MC) method. MC simulation can be used to generate the gamma spectrum at the detectors of the probe for a variety of rock physics conditions and borehole environments and thus the C/O can be determined by the processing of the simulated gamma spectrum. The simulation results are used to derive the interpretation diagrams for the basic petrophysical effects and to investigate the role of the side effects. Considering the industrial practice of log evaluation, the resolution and limitations of the method is quantified in the measurement space by defining a goodness factor based on the area of the interpretation chart. The focus of this paper is on detector arrangement, but it also covers the effect of porosity, lithology, and the casing.