Varamin Plain Research Articles

Sensitivity analysis to determine the importance of input variables in groundwater stress

This study aims to provide insights into groundwater stress and subsidence in the Varamin plains, located in central Iran. To achieve this, groundwater distributed modeling was simulated, and the groundwater stress index was evaluated accordingly. The area was divided into nine sections based on the intensity and spatial distribution of stresses and subsidence. The study conducted sensitivity analysis to reduce uncertainty, focusing on the importance of abstraction, recharge, and environmental flow at groundwater footprint and stress index equation. The impact of input variables on the groundwater stress index was evaluated using two methods of local and global sensitivity analysis, and the results were compared. A partial correlation criterion was also applied to measure the relationship between stress-dependent variables and three independent variables. The analysis revealed that the correlation analysis and global sensitivity analysis are consistent with each other and differ from the local sensitivity analysis. The abstraction based on global sensitivity analysis and correlation analysis has the greatest impact on stress, while a local sensitivity analysis showed that the reduction in recharge had the greatest effect. The study found that the impact of environmental flow requirements is negligible. Based on the results, the study presented different scenarios for reducing stresses in critical areas and proposed corresponding scenarios to reduce stress in the future management of the plain. A review of three different scenarios for the Varamin plain revealed that recharge is the most effective parameter that correlates with local sensitivity analysis. However, based on the results of the analysis of global sensitivity and correlation coefficient analysis, abstraction has the highest effectiveness.

Linking archaeology and paleoenvironment: Mid‐Holocene occupational sequences in the Varamin Plain (Iran)

AbstractEarly human habitation of the arid to semiarid Central Iranian Plateau was strongly connected to the availability of water and associated natural hazards, such as flooding and drought events. In this geoarchaeological study, we focus on the occupation at the prehistoric site of Ajor Pazi within the formerly active fluvial environment of the Varamin Plain. Through radiocarbon and luminescence‐dated sediment cross‐sections, we apply multivariate statistics to sedimentological characteristics of bulk samples collected during a rescue excavation in 2018. Based on facies interpretations, we differentiate depositional processes and present their implications for the environs of Ajor Pazi. Our results show evidence of settlement activities between 6.4 and 5.6 ka cal B.P. (4.4–3.6 ka cal. B.C.E.). Phases of reduced geomorphodynamics can be distinguished when soil‐forming processes take place. Our findings provide insights into the hitherto scarcely explored Transitional Chalcolithic II period during which the site of Ajor Pazi emerged and began to shape its environs.

Were climatic forcings the main driver for mid-holocene changes in settlement dynamics on the Varamin Plain (Central Iranian Plateau)?

Settlement crises in ancient cultures of Western Asia are commonly thought to be caused by climatic events such as severe droughts. However, the insufficient climate proxy situation in this region challenges the inference of clear relationships between climate and settlement dynamics. We investigate the Holocene climatic changes on the Varamin Plain in the context of the climatic history of Western Central Asia by using a transient comprehensive Earth System Model simulation (8 ka BP to pre-industrial), a high-resolution regional snapshot simulation and a synthesis of pollen-based climate reconstructions. In line with the reconstructions, the models reveal only slightly varying mean climatic conditions on the Varamin Plain but indicate substantial changes in seasonality during the Holocene. Increased precipitation during spring, combined with lower temperature and potentially stronger snow accumulation on the upstream Alborz mountains may have led to an increased water supply on the alluvial fan during the vegetation period and thus to more favourable conditions for agricultural production during the Mid-Holocene compared to modern times. According to the model, dry periods on the Central Iranian Plateau are related to particularly weak Westerly winds, fostering the subsidence in the mid-troposphere and hampering precipitation over the region. The model reveals that dry periods have spatially heterogenous manifestations, thus explaining why they do not appear in all proxy records in the wider study region. In fact, the climatic signal may depend on local environmental conditions. The interaction of the topography with the atmospheric circulation leads to additional spatial heterogeneity. Although our results provide several indications for a connection between climate and settlement dynamics, the small overall changes in moisture call into question whether climate is the main driver for settlement discontinuities on the Central Iranian Plateau. To shed further light on this issue, more high-resolution long-term proxy records are needed.

Assessing the Impact of Human Activities and Climate Change Effects on Groundwater Quantity and Quality: A Case Study of the Western Varamin Plain, Iran

In this paper, the effects of climate change and human activities on the groundwater level and the concentration of pollutants, such as total dissolved solids (TDS), chloride, and sodium, were investigated in the western part of the Varamin Plain. The groundwater flow and pollutant transport were simulated with the two models of MODFLOW and MT3D, respectively. To investigate the impacts of climate change, the two parameters of temperature and precipitation were downscaled under the three scenarios of RCP 2.6, RCP 4.5, and RCP 8.5. Four scenarios, including the current condition (Scenario 0), a 25% increase in the extraction from pumping wells (Scenario 1), the climate change effects (Scenario 2), and an increase in the incoming effluent (TDS) to the Shoor River due to industrial activities (Scenario 3), were investigated for a future period of 30 years (2025–2055). The results show that the highest groundwater decline and chloride and sodium concentrations occur under Scenario 1, while Scenario 3 leads to the maximum TDS concentration (milligrams per liter). In Scenario 1, the average and maximum groundwater decline at the end of the simulation period will be 2.5 m and 7.3 m, and the chloride and sodium concentrations will increase by 7 and 5 milligrams per liter, respectively.

Research on the status of the irrigation and drainage network in Iran based on the viewpoint of water users and experts using SWOT analysis

AbstractIn many arid and semi‐arid regions, such as Iran, traditional irrigation systems consume a large proportion of surface water and groundwater resources. In the last few decades, due to the undesirable effects of climate change and continuous drought, many studies have been conducted on reducing threatening factors using the capacity of irrigation and drainage networks and participating water users in irrigation management. This study aimed to evaluate the performance of the irrigation and drainage network of the Varamin Plain using the SWOT model that is an analytical model used based on the results of the strengths, weaknesses, opportunities and threats of the subject, considering the cooperative management of water users. In addition to conducting interviews, a questionnaire was used to collect data from two groups of network water users and experts. According to the results, the farmers and water users of the network are in a relatively good situation in terms of access to water, possibly due to the available three annual planting seasons (due to the existence of irrigation and the drainage network and the use of grey water, conditions for irrigation and crop cultivation are available in four seasons of the year). The most critical concerns of the water users were the lack of knowledge about the organization of water collectors, the inconsistency among governmental organizations and the insignificant action of the Agricultural Jihad Organization (a government ministry in Iran to advance agricultural goals and all related matters) in solving issues related to water rights. Failure to inform the users about the costs of creating network infrastructure, maintaining facilities and canals, not accepting water shortage conditions and the state of groundwater resources, consecutive cropping seasons throughout the year, cultivation of crops with high water demand, people with political and social influence obtaining excess water rights, and the lack of coordination among related governmental policies were the most critical concerns of the experts.

Exploring Dynamics of Water, Energy, and Food Systems in Agricultural Landscapes Using Mental Modeling: A Case of Varamin Plain, Iran.

This study applies the mental model and cognitive mapping method to involve stakeholders in delineating the mutual relations between sources of water, energy, and food (WEF) production in the Varamin Plain (VP). Through involving farmers and managerial experts, the approach facilitates the deployment of community communication patterns to recognize and comprehend problems and move from single-loop learning to double-loop learning. The dynamic model was driven from the final mental model of the participants to reflect changes in the systems over time. The system dynamic (SD) model incorporates three scenarios for enhancing irrigation efficiency, managing groundwater extraction, and satisfying environmental needs. The results uncovered that the surface and underground water resources of the VP will gradually decrease within the next two decades in the range of 158 and 2700 million cubic meters (MCM) per year. Also, the plain suffers from water insecurity and a 162 MCM shortage. Consequently, focusing on understanding the nexus and nexus governance can enhance resource management and achieve sustainable development goals. Essentially, promoting collaborative governance, such as creating cooperative organizations and implementing double-loop learning, and instituting a water market, regulatory governance, and monitoring laws can improve the state of Varamin Plain's resources. These results carry important policy implications for using mental models to consider dynamics for discussions on participatory management of the WEF system nexus and environmental management.

An evolutionary bargaining framework for allocating water and reclaimed wastewater in agricultural regions

Abstract This paper presents a new framework for modeling the bargaining process among stakeholders by coupling social choice and bargaining methods. Based on this framework, two methods of evolutionary bargaining coupled with Borda count (BBC) and evolutionary bargaining coupled with pairwise voting (BPV) are proposed, and the results of applying them to resolve the challenging problems of allocating water and reclaimed wastewater in agricultural regions are analyzed. After proposing some candidate scenarios of allocating water and reclaimed wastewater, non-dominated scenarios are determined. Then, in the first level of bargaining, using a social choice technique, each stakeholder chooses the most desirable scenario out of the non-dominated ones, regardless of the utilities of other stakeholders. The selected scenarios by all stakeholders can provide them an estimate of other stakeholders' expected utilities. This enables each stakeholder in the next step of bargaining to suggest a scenario that improves their own utility, while providing a minimum acceptable utility of other stakeholders. If the bargaining results in more than one scenario, a social choice method is applied to find the most preferred scenario. The applicability and performance of the proposed framework are evaluated by applying it to the Varamin plain, in the south-east of Tehran, Iran.

Waste load allocation by integrated GMS modeling and economic evaluation for nitrate reduction in Varamin aquifer

Abstract Groundwater is a dependable freshwater resource in arid and semi-arid areas where its quality management is essential. However, untreated wastewater is a risk to safe water supply from aquifers. Wastewater treatment plants (WWTPs) can reduce pollutants, but their impact on groundwater quality versus their operating costs requires case studies. This research uses the two modules of the groundwater modeling system (GMS) to simulate the Varamin Plain, south-eastern Tehran, Iran. The MODFLOW and MT3D were used for groundwater quantity and quality modeling, respectively. Through these modules, the effectiveness of two WWTPs (Pakdasht and Varamin) in six waste load allocation (WLA) scenarios was compared based on nitrate reduction in 3-year and 10-year periods. The construction and operating costs of each WLA scenario were also calculated. The best WLA is a scenario with the lowest marginal cost. Therefore, constructing Varamin WWTP with 25% nitrogen removal was the selected option. Here, the average nitrate concentration in the aquifer is reduced from 28.4 mg/L (±4.1) to less than 25 mg/L (±2.4) with an annual marginal cost of 8 M$.L/mg. It implies that constructing more WWTPs or tertiary units for nitrate removal is not recommended as they do not add significant nitrate abatement in groundwater regarding the related costs.

Development of an integrated multi-objective approach to formulate optimal harvesting policies with the aim of sustainable management of groundwater resources: study area: Varamin Plain

Abstract In the present study, an integrated multi-objective simulation -optimization model was developed using a NSGA-II optimization algorithm and GMS simulation software with the aim of sustainable harvesting from the Varamin aquifer. For this purpose, the quantitative model of the aquifer was simulated. Based on the calibrated model and integrated with the optimization algorithm, a two-objective management model was developed to control the exponential drop in the groundwater level and ensure sustainable harvesting from the wells. Then, according to multi-criteria decision-making methods and the Borda aggregation method, the optimal harvesting scenario of wells in operation was analyzed. The results show that more than 69% of the wells in the study area have experienced a rise of up to 30 m in the groundwater level during a 1-year period. It indicates the relative improvement of the aquifer in the direction of restoring its saturated thickness. Also, the comparison of the results obtained from the optimal and existing conditions showed that by applying the proposed model, an average of 46.9% can be saved in the harvesting of groundwater resources. For this purpose, it is necessary to reduce the allowed amount of harvesting from wells in this area from 21 to 11 L/s.

Developing a decision-making model for improving the groundwater balance to control land subsidence

This study aimed to control land subsidence by improving the groundwater balance in the Varamin plain using the Groundwater Modelling System software and a multi-criteria decision model. For this purpose, aquifer level quantification and subsidence rate simulation were performed with the MODFLOW model and SUB package, respectively. The results showed a 6 m decrease in the aquifer level over a 5-year period and the subsidence rate in the central parts was 37 cm. Accordingly, the aquifer was evaluated by considering eight different restoration strategies based on reduced exploitation and artificial feeding. The results showed that the environmental criterion related to the subsidence adjustment index had the highest weight (0.27) and was introduced as the most important decision-making criterion. The evaluation of the results and priorities using the Complex Proportional Assessment (COPRAS) method showed that a 30% reduction in exploitation with artificial feeding is the best restoration strategy and can improve the subsidence rate and aquifer level by 36% and 76%, respectively, over a 5-year period (2024).

Chalcolithic Painted Pottery of the Sialk III Period: Quantifying Stylistic Continuities and Changes on the Northern Central Plateau

ABSTRACT In the last two decades, the Varamin Plain to the southeast of Tehran has been the focus of intensified archaeological work, with surveys and excavations yielding new insights into the region’s settlement history. New material from the dynamic network of Chalcolithic sites on the Jajrud alluvial fan now offers a more solid basis to assess the links between the Varamin Plain and the site of Tappeh Sialk in the Kashan Plain, which still remains a reference point for this period. In this paper, we use painted pottery to examine the dating and relationships of the settlements of Ahmadabad-e Kuzehgaran and Chaltasian on the Varamin Plain to Tappeh Sialk. Based on an in-depth quantitative analysis of motif assemblages at the three sites, we evaluate contemporaneities as well as stylistic similarities and differences. We highlight both shared temporal trends and regional specificities.

Investigation of the Relationship between Land Surface Changes due to Subsidence and Groundwater using Sentinel-1 Satellite Images and Statistical Models (Case study: Varamin plain)

Investigation of the Relationship between Land Surface Changes due to Subsidence and Groundwater using Sentinel-1 Satellite Images and Statistical Models (Case study: Varamin plain)

Investigation of groundwater quality changes in Varamin Plain of Tehran

ارزیابی کیفیت منابع آب به‌خصوص منابع آب آشامیدنی در سال‌های اخیر هم‌زمان با کاهش سرانۀ منابع آب در دسترس، از اهمیت زیادی برخوردار است. در این پژوهش، کیفیت منابع آب دشت ورامین با استفاده از شاخص کیفیت آب (WQI) در یک بازۀ 10 ساله (97-1387) مورد بررسی قرار گرفت. بدین‌منظور، تعداد 35 نمونه از نقاط مختلف دشت ورامین تهیه شد و غلظت پارامتر‌های تأثیرگذار بر کیفیت منابع آب زیرزمینی مورد بررسی قرار گرفت. طبق نتایج، کلسیم، سدیم و کلر با کل مواد جامد محلول (TDS) همبستگی مثبت و معنا‌داری دارند. هم‌چنین، بر اساس نقشه توزیع مکانی منطقۀ دشت ورامین در سال ۱۳۹۷ مساحت بیش‌تری را با کیفیت مناسب داشته است در حالی‌که قسمت‌های جنوبی و جنوب غربی منطقه در بازۀ ۱۰ ساله با وجود بهبود کیفیت آب منطقه، هم‌چنان دارای کیفیت نامناسب برای شرب هستند. به‌‌طور کلی نتایج حاصل از این پژوهش‌ نشان داد که در حال حاضر تعدادی از پارامتر‌های مؤثر در تعیین کیفیت منابع آب زیرزمینی، مانند TDS، هدایت الکتریکی (EC) در اکثر نقاط نمونه‌برداری شده و هم‌چنین مقدار pH در بعضی نقاط نمونه‌برداری شده دشت ورامین از حد مجاز استاندارد‌ها فراتر رفته است. علاوه بر این، کیفیت آب زیرزمینی نیز در بازۀ زمانی مورد مطالعه کمی بهبود یافته است. نتایج به‌دست‌آمده نشان داد که بر اساس WQI در سال 1387، 42/51 درصد منطقه دارای آب نامناسب از نظر شرب است که این مقدار در سال ۱۳۹۷ به ۴۵ درصد کاهش ‌یافته است. هم‌چنین، میزان 31 درصد آب با کیفیت بد در سال 1387 به 22 درصد کاهش یافته است، درحالی‌که مقدار میزان آب با کیفیت خوب در سال 1397 تا 25 درصد افزایش یافته است. به‌این‌ترتیب توجه به تأثیر گسترده عوامل انسانی بر کاهش کیفیت منابع آب زیرزمینی دشت ورامین و برداشت شدید آب زیرزمینی، بررسی دقیق چاه‌های غیرمجاز در منطقه، حفاظت کیفی از منابع آب زیرزمینی و مدیریت برداشت چاه‌های بهره‌برداری و هم‌چنین استفاده از سامانه‌های آبیاری مناسب در جهت کاهش برداشت و هم‌چنین زهکشی مناسب اراضی کشاورزی در طرح‌های آتی امری ضروری به‌نظر می‌رسد.

Evaluation of the climate change impact on the intensity and return period for drought indices of SPI and SPEI (study area: Varamin plain)

Abstract In this study, to investigate the climate change effect on meteorological drought in the next three decades of Varamin plain, the EC-EARTH model was selected from the (AR5) report with the high performance of temperature and precipitation simulation compared to the base period under RCP scenarios and then by LARS-WG software was downscaled. In addition, (intensity-duration) and the return period of drought indices of Standard precipitation index (SPI) and Standardized precipitation-evapotranspiration index (SPEI) in annual time series were evaluated. Results illustrated that the seasonal precipitation pattern has changed, and mean temperatures will increase by 1.4 °C compared to the base period. The results of the drought assessment showed that the intensity of drought in the future compared to the base period based on SPI and SPEI increased by 8 and 28%, respectively, indicating that the SPEI was more severe in all three scenarios than the SPI. It can mainly be explained by the contribution and effect of increasing the average temperature along with precipitation in the SPEI. Also, the return period of severe droughts under the RCP8.5 scenario for SPEI in the base and future periods is 8 and 6 years, respectively, which indicates a decrease in the return period of severe droughts and an increase in dry years in the future.

Evaluating the effects of climate change on groundwater level in the Varamin plain

Abstract In this research, a number of paired three-dimensional Atmosphere-Ocean General Circulation Models (AOGCM) from CMIP (Climate Model Inter Comparison Project) 5 group with the base period of 1989–2005 have been evaluated and the output of these models was micro-scaled and calibrated by LARS-WG software. The appropriate model was selected to simulate temperature and rainfall data under the emission scenarios of RCP (Representative Concentration Pathway) 2.6, RCP4.5 and RCP8.5 for the future period of 2020–2050, and then to model the groundwater level of the region, GMS software for both stable and transient states for one water year was calibrated and then was validated by observation data. The results in the future periods showed an increase of 1–1.5 degrees in temperature and an increase in rainfall in the early months of the year to late spring season and a decrease in rainfall in autumn season. Generally, the RCP4.5 scenario showed slightly more annual rainfall increase over the next 30 years compared to the base period than the other two scenarios. The time series investigation of the average of groundwater level shows that the implementation of RCP 2.6, RCP 4.5 and RCP 8.5 scenarios respectively leads to an average monthly increase of 4.2, 4.3 and 4.6 cm of the groundwater level.

Spatial Differentiation Characteristics of Groundwater Stress Index and its Relation to Land Use and Subsidence in the Varamin Plain, Iran

Distributed modeling provides for mapping of spatial and temporal patterns of highly stressed regions, and it offers local solutions to reduce stress in aquifers. In this study, the groundwater stress index (GWSI) is evaluated based on the groundwater footprint index over the Varamin aquifer in Iran. Using ArcGIS software, all necessary layers were produced and then input into the Groundwater Modeling System software to evaluate GWSI. The results show that distributed modeling offers a more accurate assessment of GWSI than water budget analysis. The minimum and maximum values of the GWSI calculated by the distributed model are 2.4 and 1.4 times, respectively, higher than those values obtained in previous studies. Besides, a significant agreement was observed between highly stressed areas and agricultural land use. Furthermore, the results obtained from comparison between stress pattern and land subsidence indicated that only 10% of the area under subsidence was caused by groundwater stress. Applying appropriate scenarios in the future can be useful to reduce water stress and its increasing trend.

Investigating the effect of additives’ size on the improvement of the tensile and compressive strengths and deformation characteristics of collapsible soils

In the present study, an experimental investigation was conducted to examine the effect of the size of additives on the improvement of collapsible soils. For this purpose, three collapsible soils with severe collapse potential were selected from Varamin Plain, Iran. The selected soils were then treated in the laboratory by the addition of alumina materials in two sizes of 70 nm (nano-alumina) and 685 nm (micro-alumina) and in different contents to improve their mechanical properties. Soil properties including collapse potential, uniaxial compressive strength, compressive modulus of elasticity, direct tensile strength, and tensile modulus of elasticity were selected as the target parameters for the improvement. These parameters were measured and determined before and after the treatment and the behaviors of treated soils were studied in both compression and tension states. The obtained results indicated that the nanometric and micrometric additives have diverse effects on the performance of the treated soil under compression and tension conditions.

Prediction of Nitrate Distribution Process in the Groundwater via 3D Modeling

The main purpose of this study was to model nitrate and its spatial and temporal variations under hydrological tensions. For this purpose, the hydrodynamic characteristics and transmission of Varamin plain groundwater were modeled after collecting and organizing the field data. Qualitative data were imported to the quantitative GMS model constructed by using the MODLFLOW code. Then, quantitative and qualitative models were calibrated. Measured and model predicted values were compared and relatively acceptable results were obtained. The most important factor in deteriorating the groundwater quality was nitrate pollution related to the agricultural areas which will exceed the permissible limits of drinking water during a 10-year period. Also, according to the model maps of the unconfined aquifer, the highest content of nitrate was related to downstream areas of the big cities. Since the confined aquifer is overlaid by an impermeable layer, it is less affected by nitrate pollution and contains high-quality drinking water. However, population growth and subsequent increase in agricultural activities and human sewage disposal may lead to further pollution of the unconfined aquifer by nitrate.

Assessing climate change impacts on water resources and crop yield: a case study of Varamin plain basin, Iran

This research evaluated climate change impacts on water resources using soil and water assessment tool (SWAT) models under representative concentration pathway scenarios (RCP 2.6, RCP 6, RCP 8.5). First, drought intensity was calculated using the standardized precipitation index (SPI) for the period 1987-2016. Then, the coefficients of precipitation as well as minimum and maximum temperature changes were simulated as SWAT model inputs. The results revealed that temperature will rise in future periods and the precipitation rate will be changed consequently. Then, changes in runoff during periods of 2011-2040, 2041-2070, and 2071-2100 were simulated by introducing downscaled results to SWAT model. The model was calibrated and validated by SWAT calibration and uncertainty procedures (SWAT-CUP). Nash-Sutcliffe (NS) coefficients (0.57 and 0.54) and R2 determination coefficients (0.65 and 0.63) were obtained for calibration and validation periods, respectively. The results showed that runoff will rise in fall and spring while it will drop in winter and summer throughout future periods under all three scenarios. Such seasonal shifts in runoff levels result from climate change consequences in the forms of temperature rise, snowmelt, altered precipitation pattern, etc. Future-period evapotranspiration will rise under all three scenarios with a maximum increase in the period 2070-2100 under RCP 8.5 scenario. Additionally, rainfed crop yields will decline without considerable changes in irrigated and horticultural crop yields.

Ground surface response to continuous compaction of aquifer system in Tehran, Iran: Results from a long-term multi-sensor InSAR analysis

Most of the developed groundwater basins in Iran are subject to land subsidence hazards resulting from the over-extraction of groundwater. Several areas in Tehran, the capital city and a provincial center in north-central Iran, have been reported to be subsiding at different rates. In this study, we present the results of an Interferometric Synthetic Aperture Radar (InSAR) time series analysis of Tehran using different SAR data between 2003 and 2017. By constructing more than 400 interferograms derived from 39 Envisat ASAR (C-band), 10 ALOS PALSAR (L-band), 48 TerraSAR-X (X-band), and 64 Sentinel-1 (C-band) SAR datasets, we compile displacement time series from interferometric observations using the Small Baseline (SB) technique. Our analysis identifies 3 distinct subsidence features in Tehran with rates exceeding 25 cm/yr in the western Tehran Plain, approximately 5 cm/yr in the immediate vicinity of Tehran international airport, and 22 cm/yr in the Varamin Plain to the southeast of Tehran city. The temporal pattern of land subsidence, which is dominated by a decreasing trend, generally follows the regional decline in groundwater level, which suggests that anthropogenic processes caused by excessive groundwater extraction are the primary cause of land subsidence. Integrating a decadal time series of subsidence constructed from multi-sensor InSAR with in-situ observations suggests that inelastic and permanent compaction dominates the main deformation regime of the Tehran aquifer, and the ratio between elastic and inelastic deformation is approximately 0.4. A geological analysis indicates that the shape of the subsidence bowl in the western Tehran Plain does not follow the trend of major mapped faults in the region. In contrast, the subsidence bowl in Varamin is controlled by the Pishva Fault, which suggests that either this fault acts as a hydrologic barrier to the groundwater flow in this region or that the differences in sediment thickness causes the discontinuity in land subsidence.