Groundwater Discharge Research Articles

Surface Runoff Estimation in Kubaisa Watershed Using SWAT, Western Desert, Iraq

Surface runoff significantly impacts the region's ecosystem and is closely linked to sustainable development and human well-being. The mathematical model SWAT running in an ArcGIS environment was used to estimate surface runoff in the Kubaisa watershed. The period simulated in this model is 31 years, sufficient to give good results regarding surface runoff volume and water yield contribution from each sub-watershed. The Kubaisa watershed has an area of 2485.50 km2 and comprises six sub-watersheds. The watershed consists of 172 HRUs distributed among sub-watersheds depending on the variation in soil layers, land use, vegetation, and slope. The volume of water calculated from the annual surface runoff rate for the simulation period was about 2699 million cubic meters, and its value was higher than the volume of water calculated from surface runoff due to lateral flow, groundwater discharge, and the contribution of each sub-watershed to the water yield. The highest value in water yield was in 2012, and the largest contribution was made by the sixth sub-watershed; thus, the difference is temporal and spatial in terms of water yield. Finally, through the analysis of multiple scenarios, the difference in hydrological response units is controlled by the difference in surface runoff, discharge, and water yield to the watershed. For this purpose, the SWAT model divides the watershed into many different units and starts the simulation on their basis.

Greensand formation, siliceous earth deposition and coastal metal drawdown in the Danubian Cretaceous Basin (Bavaria, Germany)

The Late Cretaceous epoch was characterized by extreme greenhouse climates and widespread glaucony formation in shallow marine settings. However, in the Danubian Cretaceous Basin (DCB, Bavaria, SE-Germany), contemporaneous shallow marine deposition of glauconitic sandstones of the Regensburg Formation in the eastern and the glaucony-free Neuburg Siliceous Earth deposits of the Wellheim Formation in the western parts of the basin during Cenomanian–early Turonian times is puzzling. An integrated approach of sedimentology, stratigraphy, mineralogy and geochemistry reveals that the striking lithological and mineralogical differences can be attributed to the geological structure of the hinterland and the nature of element input: in the eastern DCB, deeply chemically weathered granites and gneisses of the Bohemian Massif were leached due to the warm climate and high precipitation rates as indicated by high values of the modified Chemical Index of Alteration (CIXrev). Elements crucial for glaucony formation (K, Fe, Si, Al) were amply supplied by rivers, fueling a shallow marine glaucony factory. Slightly reducing conditions and a temporally increased primary productivity further promoted favorable conditions for geologically fast shallow water glaucony authigenesis. In the western DCB, in contrast, a hinterland consisting of karstified Upper Jurassic carbonates devoid of essential elements for glaucony formation and a lack of significant fluvial input inhibited shallow marine glaucony formation. Furthermore, we suggest that the depositional environment of the Neuburg Siliceous Earth was affected by submarine discharge of silica-rich groundwater resulting in intense early diagenetic silicification. Additionally, our geochemical data provide the first evidence of a trace metal drawdown during Oceanic Anoxic Event 2 (approx. 94 Ma) in shallow water/coastal settings in Germany, as shown in very low V/Al, Ni/Al, Cu/Al and As/Al ratios, much lower than the average shale and upper continental crust.

Integrated Modeling of Flow, Soil Erosion, and Nutrient Dynamics in a Regional Watershed: Assessing Natural and Human‐Induced Impacts

AbstractCurrent integrated modeling frameworks for simulating nutrient sources and dynamics are inadequate for large regional watersheds dominated by groundwater‐surface water interactions due to their simplistic representations of groundwater. In this study, we develop a coupled model that integrates comprehensive surface water, 3‐D groundwater, soil erosion, and nutrient processes. The model is intended to enhance the understanding of nutrient dynamics and sources in the Pearl River Basin (PRB). The model exhibits satisfactory performance in simulating streamflow and sediment transport patterns, capturing essential seasonal variations in water quality indicators. Hydrological budget assessments from 2002 to 2020 in the PRB reveal that 54% of precipitation drains into the South China Sea as surface water, while groundwater discharge as baseflow accounts for 18% of the streamflow. The nutrient budget for the PRB indicates that non‐point sources are the dominant contributors to both nitrogen (N) and phosphorus (P), ranging between 64% and 90%. Improved sewage collection and treatment have reduced point source nutrient contributions over the evaluation period. Groundwater remains a significant and consistent source of N, contributing between 11% and 19%. Natural disturbances and fertilization have led to an upward trend in river N inputs, while afforestation and sewage reduction efforts have resulted in a downward trend in river P inputs. Increased fertilization emerges as a central concern for the PRB, suggesting cost‐effective mitigation of fertilizer usage a pragmatic solution. The coupled simulation model developed in this study offers a novel systems approach for basin‐wide nutrient analysis and pollution control strategies, considering both natural and human‐induced disturbances.

Ce anomaly in hemiboreal headwater streams: An indicator of dominant groundwater flow paths through the catchment

The water quality in headwater streams depends on the groundwater origin and its transport pathways before it eventually discharges as surface water. In this case study, we present the Ce anomaly of over 100 hemiboreal headwater streams in Sweden and discuss the potential of the Ce anomaly to define two stream end members as proxies for the stream origin. The data show a relation between topography and the Ce anomaly, with more negative values (−0.8 to −0.4) in hilly catchments with distinct slopes, defined as an oxidized groundwater end member. The second end member is dominated by groundwater discharge from reduced, organic-rich riparian and wetland soils (reduced groundwater) having small Ce anomalies (−0.3 to zero). Element concentrations show a wide range, depending on the end member of the stream. For example, redox elements (Fe, Mn, S and N) show concentrations up to five times in streams with small negative Ce anomalies (reduced groundwater) compared to the concentrations in streams with large negative Ce anomalies. While element concentrations (of the classic redox elements Fe, Mn, S, N) show seasonal variations due to a summer drought period and the resulting reduced conditions, the Ce anomaly is constant, offering an excellent indicator of the dominant groundwater flow paths regardless of seasonal variations.

Seasonal dynamics of submarine groundwater discharge in Zhanjiang Bay: an investigative study utilizing 222Rn as a tracer

The spatial and temporal variation of submarine groundwater discharge (SGD) has been a topic of interest for researchers in recent years. However, the study of SGD is limited within the scope of Zhanjiang Bay. The Zhanjiang Bay is located in western Guangdong Province, known for its high temperatures and typhoon vulnerability. The present study examined the fluctuation patterns of seawater 222Rn in Zhanjiang Bay during the period from June 2021 to March 2022. This investigation employs the 222Rn tracing technique to establish a mass balance model to compute the SGD rate, and assess the significance of SGD in translocating nutrients into the bay. We conducted a four-month sampling, observing the differences between the rainy and dry seasons. Precipitation exerts a significant influence on the seasonal variations of SGD. The SGD rates of Zhanjiang Bay ranged from 7.14 to 10.75 cm·d-1 in the dry season and from 13.38 to 14.61 cm·d-1 in the rainy season. SGD-derived nutrients also exhibited seasonal fluctuations, the annual input of nutrients it supplies may be comparable to the quantity delivered by rivers, which significantly impacted the trophic status of the bay.

The Serendipity of Discovery: Life of a Geochemist.

My strategy for writing this autobiography is to use examples of how working on seemingly different projects can often lead to outcomes more important than originally envisioned. Serendipity is a happy accident-specifically, the accident of discovering something useful without directly looking for it. This often occurs when two research projects converge unexpectedly. The main text contains examples of how serendipity has led me to important discoveries, including (a) finding surprisingly high 228Ra activities in the ocean; (b) developing a means of rapidly and quantitatively extracting radium from seawater; (c) devising a rapid, sensitive method of measuring 224Ra and 223Ra; (d) realizing the scale and biogeochemical importance of submarine groundwater discharge; and (e) conceiving a method to estimate the total flux of submarine groundwater discharge to the Atlantic Ocean. The Supplemental Material fleshes out details of these discoveries and places them in the context of my other investigations.

Controlled drainage with subirrigation systems: Reduce water supply by automatic control

Controlled drainage with subirrigation (CDSI) is a viable measure to supply, retain or discharge groundwater, thereby contributing to freshwater availability in agriculture under changing environmental conditions. Relatively simple CDSI systems can be controlled manually to set a few drainage levels. More advanced systems can be controlled remotely to set any drainage level (between a technical maximum and minimum). CDSI potentially improves hydrological conditions for crop growth, but the required external water supply can be large. Therefore, the objective of this paper is to investigate whether external water supply for subirrigation can be reduced by automatic control of CDSI systems in relation to crop water demand. Field measurements of a CDSI pilot in the Dutch sandy Pleistocene uplands were combined with weather forecasts to simulate the optimal drainage level and day by day water demand and supply using the agro-hydrological Soil, Water, Atmosphere, Plant model (SWAP). Firstly, model simulations showed that the water requirement reduced by 60 mm (dry growing season), 253 mm (average growing season) and 348 mm (wet growing season) using a dynamically managed crest level (CDSI-dyn) compared to using a fixed crest level (CDSI-fix), with minor effects on crop yield. Secondly, model simulations showed that a higher hydraulic resistance to downward seepage, a higher ditch water level or deeper roots reduced the water supply (up to 100 mm). Thirdly, accepting 10 % daily crop drought and oxygen stress for CDSI-dyn reduced the water supply requirement with 235–628 mm (dry vs wet growing season) compared to CDSI-fix. In conclusion, the required water volume for CDSI could be substantially reduced by automated control of the drainage level and water supply rate, while maintaining crop yield or accepting minor reductions, which increases the potential of implementation of CDSI systems.

Estimating exploitable groundwater for agricultural use under environmental flow constraints using an integrated SWAT-MODFLOW model

Traditionally, the impact of groundwater abstraction on streamflow and aquifers has been assessed primarily in terms of water availability and depletion, often neglecting critical environmental flow (Eflow) requirements. To address this gap, our study employed an Eflow perspective to evaluate exploitable groundwater for agricultural use at a watershed scale using an integrated modeling approach. We used the SWAT-MODFLOW model, integrating the Soil and Water Assessment Tool (SWAT) and the Modular Finite-Difference Flow Model (MODFLOW), to analyze water balance segments and stream-aquifer interactions in the study region. Eflow for different management classes was determined using the flow duration curve (FDC) approach. The FDC method classifies Environmental Flow Management Classes (EMC) from “A” (natural or minor alteration) to “F” (critically modified). The relationship between the selected EMC and simulated streamflow, under the pressure of agricultural groundwater pumping, served as a constraint in evaluating exploitable groundwater alongside groundwater level fluctuations. As the traditional approach depends on average recharge for determining exploitable groundwater, this study also assessed the spatiotemporal distribution of groundwater recharge. Simulation periods were aligned with agricultural pumping schedules. Results indicated that groundwater pumping significantly impacts streamflow at the start of the irrigation period. Under moderately affected EMC and with an average groundwater level fluctuation of one meter, the exploitable groundwater during the irrigation period can reach up to 2.96 Mm3/day. With this amount of pumping, the average groundwater discharge to streams declined by around 13 %. While groundwater level fluctuations near the watershed outlet were minimal, in the upstream area, levels could decrease by as much as 2.5 m. The groundwater level fluctuation underscores the necessity of accounting for both spatial and temporal factors, particularly in highly affected regions.

Response of dissolved organic carbon in streams draining peatbogs to extreme rainfall-runoff events: a case study from Šumava (Bohemian Forest) National Park, Czech Republic

The presented study investigates the dynamics of DOC concentrations in headwater peatbog areas with respect to the extreme rainfall-runoff (R-R) events hydrometeorological catchment preconditions (23 variables in total). The main data sources were automatic devices for monitoring of groundwater level, discharges and rainfalls providing data in 10 min steps installed in the Vydra River catchment and one automatic water sampler ISCO in sub-catchment of the Rokytka River basin in the Šumava (Bohemian Forest) National Park. The study period was 2018–2021, in which 18 R-R events were analysed. Data of DOC variability and catchment conditions were analysed using Spearman’s correlation coefficient, Principal Component Analysis and DOC/Q hysteresis loops. Changes in groundwater level and discharges had the greatest influence on DOC concentrations. Higher mean and maximum DOC were measured during events after a longer period without an extreme R-R event. The greater lag time of maximum DOC after peak flow and the higher mean DOC during the event were primarily due to hydrometeorological preconditions of the catchment. The highest DOC was in autumn after the previous summer period with low discharges and low groundwater levels. DOC was also positively correlated with air and water temperatures.

A global coastal permeability dataset (CoPerm 1.0)

The permeability of aquifers strongly influences groundwater flow characteristics. Worldwide, coastal groundwater is often the primary freshwater source for coastal communities and ecosystems but is also particularly vulnerable to abstraction since saltwater intrusion may threaten its quality. Thus, understanding coastal permeability is crucial to the sustainable use of coastal groundwater. Here, we present the first global dataset of coastal permeability (CoPerm 1.0), which provides data on coasts’ landward, shoreline, and seaward permeability. CoPerm accounts for shoreline characteristics such as cliffs and beaches and contains information on four million segments representing more than two million kilometers of global coastline. Rocky Shores are the most abundant shoreline class, followed by mangroves, beaches, and muddy coasts. Permeability differs between the immediate shoreline (median permeability: 10−12.3 m2), the seaward (median: 10−13.3 m2), and the landward (median: 10−13 m2) sides of the coast. CoPerm provides input data for global coastal groundwater assessments and regional studies of submarine groundwater discharge or saltwater intrusion that can radiate into ecological and economic studies.

Climate change and watershed hydrology: assessing variability in water balance components and groundwater flow patterns

ABSTRACT This study aimed to quantify the effect of climate change on water balance and groundwater flow systems over the Walga–Darge watersheds. In this paper, Water and Energy Transfer between Soil, Plants, and Atmosphere under quasi-Steady State and MODFLOW were used to assess the hydrological impact of climate change. The Mann–Kendall test and Sen's slope estimator were used to analyze climate change. The mean annual temperature shows increasing trends, whereas the mean annual precipitation indicates decreasing trends. The simulated annual mean surface runoff is projected to decrease by 17.18, 22.04, and 31.37 for 2040, 2060, and 2080, respectively, compared to the 1980s. The model also indicates a reduction in precipitation and increased temperature causes a relative change in recharge, ranging from a decrease of 2.65, 38.82, 50.91, 61.57, and 75.49 in the 2000s, 2010s, 2040s, 2060s, and 2080s, respectively. The MODFLOW outputs furthermore show that annual groundwater discharge to the stream has decreased by −26.46% from 1985 to 2020 and is expected to decrease by −34.02% by 2080 due to climate change. The results of the study indicate that the increasing trends in temperature and decreasing rainfall amounts pose a significant threat to the sustainable use of water resources.

Environmental DNA Reveals the Impact of Submarine Groundwater Discharge on the Spatial Variability of Coastal Fish Diversity.

Submarine groundwater discharge (SGD) has recently been recognized as an influential factor in coastal ecosystems; however, little research has been conducted on its effects on coastal fish diversity. To investigate the relationship between SGD and fish diversity, we conducted a survey at the coastal island scale using the environmental DNA (eDNA) method. Our findings indicate that fish species richness and functional richness peak at stations with high SGD. Environmental variables, such as salinity, dissolved inorganic nitrogen (DIN) concentration, and SGD, significantly influence fish diversity. Carnivore fish richness was negatively correlated with salinity, while planktivore fish richness was positively correlated. Additionally, SGD and DIN concentrations were found to be crucial in shaping omnivorous and pelagic communities, respectively. This study highlights the role of SGD in enhancing nutrient conditions favorable for diverse fish communities and demonstrates the effectiveness of eDNA metabarcoding for rapid marine biodiversity assessment. These findings provide valuable insights for coastal ecosystem monitoring and management.

Seasonal and interannual variations of nutrients in the Subei Shoal and their implication for the world's largest green tide

The world's largest “green tide” (Ulva prolifera) has occurred every year since 2007 in the Yellow Sea. The Subei Shoal area is thought to be the origin of the green tide. Based on field data from 2016 to 2023, seasonal and interannual variations of dissolved nutrients and their ecological effects in the Subei Shoal were analyzed. Spatial distribution of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicate (DSi) showed clear terrestrial sources, while ammonia (NH4-N) and dissolved organic nitrogen (DON) were not solely controlled by terrestrial sources. The seasonal variations of NH4-N, DIN, DON, DIP and DSi concentrations were significant, and the interannual variations of DIN, DON, DIP and DSi concentrations showed general decreasing trends from 2016 to 2023. The key factors affecting the seasonal and interannual variations of DIN and DIP concentrations were terrestrial input, aquaculture wastewater discharge, atmospheric deposition, submarine groundwater discharge and macroalgae absorption, while the dominant factor determining the variations of DSi concentrations was terrestrial input. NH4-N and DON concentrations were mainly influenced by aquaculture wastewater discharge and the absorption and release of macroalgae. The high nutrient concentrations in the Subei Shoal throughout the year provided sufficient material basis for the growth of Ulva prolifera in the source area of green tide outbreak.

Spatiotemporally non-stationary evolution of groundwater levels in Poyang Lake Basin driven by meteorological and hydrological factors

The complex relationship between wet-dry transition in the Poyang Lake basin and groundwater storage significantly affects the lake's hydrology, downstream ecological state, and overall security along the Yangtze River in China. There is, however, a notable lack of systematic exploration into how various factors drive spatiotemporal variability in groundwater level (GWL). Using local indicators of spatial association (LISA), spatial non-stationarity models, and multi-source data, our analysis explores the spatial distribution of GWL and quantifies the influence of driving factors on its spatiotemporal non-stationarity at annual and monthly scales. We also compare driving factor contributions in hilly, plain, and local areas within the Poyang Lake basin. Our findings reveal significant local clustering of GWL, indicating substantial spatial autocorrelation and geographic heterogeneity in GWL. Influencing factors exhibit non-stationary effects on GWL at spatial and temporal scales, with precipitation (P), ground surface elevation (GSE), and soil moisture (SM) being primary contributors, generally exerting positive effects. SM contributes most during dry years and normal periods. P and the Palmer Drought Severity Index (PDSI) have greater impacts in hilly areas, while GSE shows the opposite trend. Rainfall is a source of groundwater recharge, with a lagged response observed in GWL to rainfall in this basin. The lag time is about 1–2 months. Evapotranspiration is not the dominant discharge pathway. The decrease in GWL during the dry season is mainly due to reduced precipitation recharge and increased lateral groundwater discharge from areas of high hydraulic head to areas of low hydraulic head.

Evidence for subsea permafrost in subarctic Canada linked to submarine groundwater discharge

Evidence for subsea permafrost in subarctic Canada linked to submarine groundwater discharge

Operational Mapping of Submarine Groundwater Discharge into Coral Reefs: Application to West Hawai‘i Island

Submarine groundwater discharge (SGD) is a recognized contributor to the hydrological and biogeochemical functioning of coral reef ecosystems located along coastlines. However, the distribution, size, and thermal properties of SGD remain poorly understood at most land–reef margins. We developed, deployed, and demonstrated an operational method for airborne detection and mapping of SGD using the 200 km coastline of western Hawai‘i Island as a testing and analysis environment. Airborne high spatial resolution (1 m) thermal imaging produced relative sea surface temperature (SST) maps that aligned geospatially with boat-based transects of SGD presence–absence. Boat-based SST anomaly measurements were highly correlated with airborne SST anomaly measurements (R2 = 0.85; RMSE = 0.04 °C). Resulting maps of the relative difference in SST inside and outside of SGD plumes, called delta-SST, revealed 749 SGD plumes in 200 km of coastline, with nearly half of the SGD plumes smaller than 0.1 ha in size. Only 9% of SGD plumes were ≥1 ha in size, and just 1% were larger than 10 ha. Our findings indicate that small SGD is omnipresent in the nearshore environment. Furthermore, we found that the infrequent, large SGD plumes (>10 ha) displayed the weakest delta-SST values, suggesting that large discharge plumes are not likely to provide cooling refugia to warming coral reefs. Our operational approach can be applied frequently over time to generate SGD information relative to terrestrial substrate, topography, and pollutants. This operational approach will yield new insights into the role that land-to-reef interactions have on the composition and condition of coral reefs along coastlines.

Hydrogeology and groundwater control at Chambishi mine, Zambia

The hydrogeological conditions in the Chambishi mine area in Zambia. were investigated. Chambishi is a very wet mine and groundwater discharge costs are high. The research goal was to identify methods to reduce the discharge costs without greatly affecting the mining process. Hydrogeological data was obtained from measurements in drill-holes and from drill cores. The differences in structural deformation in different areas determine whether a layer of rock is an aquifer. In the mine region, the Lower Roan Group ore shale and quartz-sandstone unconformably lie on the basal schist and granite aquifuge. The Upper Roan Group dolomite is continuously distributed over a large area around the orebody. This dolomite is the main target of groundwater control efforts, and its water yield is also influenced by the deformation intensity. Based on the observations, backfilling was gradually adopted to appropriately safeguard the integrity of the aquifuge above the orebody. The overlapping relationship between the aquifer and aquifuge has been utilized to increase mine drainage efficiency. This measure has not only greatly reduced the costs of mine drainage, but also improced protection of the groundwater environment. The old drill-holes that were not sealed after exploration drilling were grouted. Further, possible water-conducting fracture zones were investigated. Cover drilling towards the suspected zone was conducted to prevent the high-pressure groundwater from shallower aquifers from entering stopes in deep areas of the mine.

Multiple Overspill Flood Channels from Young Craters Require Surface Melting and Hundreds of Meters of Midlatitude Ice Late in Mars’s History

Mars’s tadpole craters are small, young craters whose crater rims are incised by one or more exit breaches but lack visible inlets. The tadpole-forming climate records the poorly understood drying of Mars since the Early Hesperian. A third of tadpole craters have multiple breaches; therefore, the climate must have been able to generate crater rim incision in multiple locations. We use HiRISE data for four multiple-breach tadpole craters to measure their crater fill, rims, and exit breaches. We compare these measurements and other data with our calculations of liquid water supply by rain, surface melting, groundwater discharge, and basal ice sheet melting to discriminate between four proposed formation hypotheses for tadpole breaches, favoring scenarios with ice-filled craters and supraglacial melting. We conclude that multiple-breach tadpole craters record hundreds of meters of midlatitude ice and climate conditions enabling intermittent melting in the Late Hesperian and Amazonian, suggesting that liquid water on Mars has only been available in association with water ice for billions of years.

Effects of groundwater pumping on pore water flow and salt transport in tide‐controlled unconfined coastal aquifers

AbstractUnconfined coastal aquifers are a main pathway for land‐sourced solutes to enter the oceans. The migration of these solutes in aquifers is highly affected by the groundwater flow and salt transport processes, which are, to a great extent, controlled by tides. While many studies have examined how tidal oscillations would influence the subsurface hydrodynamics in coastal aquifers, most of them ignored the potential impact of groundwater pumping, a common practice in coastal areas to satisfy the demand for freshwater. This study, by means of laboratory experiments and numerical simulations, explored the combined effects of tides and groundwater pumping on the pore water flow and salinity distributions in an unconfined coastal aquifer. The results show that, in a tide‐controlled aquifer, the addition of groundwater pumping would exacerbate the degree of seawater intrusion and lead to wider spreading and deeper penetration of the upper saline plume. Moreover, groundwater pumping would enhance the tide‐driven circulation in the upper saline plume and weaken the density‐driven circulation in the saltwater wedge, ultimately leading to the reduction in total submarine groundwater discharge. These findings may promote a deep insight into the complex coastal groundwater systems experiencing human activities, and provide guidance for better evaluating the environmental impact of groundwater pumping.

Time-dependent variations of groundwater radon: Insights from a twelve-year study in the Baikal region, East Siberia, Russia

Time-dependent variations of 222Rn concentration (Q) in groundwater have been monitored for twelve years (2012–2023) at eight sites of groundwater discharge within the Baikal region in East Siberia, Russia. The concentrations of radioactive gas at different sites vary from 30% to 60% of average values (Qav). The sampled waters are of three groups with Qav ≈ 15 Bq/l (I), ≈30 Bq/l (II1), and ≈50 Bq/l (II2). Cluster analysis shows closest linkage between the two subgroups of group II due to similarity in the discharge mechanisms. Fourier analysis of periodic 222Rn behavior reveals major cycles of 365, 180, 126, and 30 days correlated with variations of air temperature and pressure, as well as with the patterns of groundwater discharge. In addition, radon anomalies are related with seismicity. Earthquakes are reflected in the radon field as three distinct anomaly types, occurring either subsequent to or prior to the seismic event. The anomalies responding to earthquake nucleation can be considered as precursors and used in earthquake prediction. The revealed trends make basis for a model designed to predict Q variations in groundwater of the area to 80% average efficiency. The external and internal factors that affect the concentration of radioactive gas in groundwater are linked in a hierarchic system and are classified according to the degree and type of their influence.