Polder System Research Articles

The polder systems legacies in the early twentieth century affect the contemporary landscape in the Jianghan Plain of Hubei, China

The long-term interaction between the environment and humans has significantly shaped contemporary landscapes worldwide. As a heritage landscape, the polder is essential for understanding the impact of the past human activities on contemporary landscape on low-lying area of land. In this case study, we reconstructed the regional spatial distribution of the polders in the early twentieth century and examined their coherence over the past century using historical maps and other materials in the Jianghan Plain. The results revealed several key findings: (1) In the early twentieth century, there were 1571 polders covering an area of 11,826.1 km2 with a perimeter of 17,035.9 km. (2) These polders exhibited a J-shaped distribution, with few large-scale polders despite their substantial areas. (3) The shape of these polders reflected an ability to balance human needs and the natural environment. (4) Considerable regional differences in these polders were attributed to centuries-old interactions between humans and the environment. (5) The remnants of these early twentieth century polders have had a lasting influence on the contemporary landscape, particularly in rural areas. This case study underscores how taking into account a broader historical perspective can enhance our understanding of present-day landscapes and, more importantly, the role that past human actions have played in shaping them.

Design of Automatic Flood Control System in Kulon Progo, Special Region of Yogyakarta, Indonesia

The construction of Yogyakarta International Airport has resulted in land use changes and increased the topography in the area by up to 9 m, which has led to potential flooding. A spatial analysis was conducted on the flow direction after the airport's construction, specifically in the West Carik watershed. It was discovered that the flow direction was uncertain and trapped upstream of the airport. A polder system with automation is proposed as a solution to the drainage problem. An automation system is designed for pump operation with the objective of improving efficiency. The system is expected to reduce the volume of water in the polder by reducing human involvement and providing ease in monitoring. The results of hydrological analysis with a 50-year return period flood resulted in a design flood discharge of 11.73 m3/s. The polder has a storage capacity of 63,312 m3, a depth of 4 m, and a surface area of 1.58 ha. It requires a total pump capacity of 8.4 m3/s, consisting of three pump units, with one unit having a capacity of 1.8 m3/s and two units with 3.3 m3/s. One additional pump with a capacity of 3.3 m3/s is also planned as a backup pump. The pump automation system simulation employs an Arduino UNO as the controller, water level sensor, and relay. This system successfully fulfills the desired operational system for the pump. In the actual implementation, a Programmable Logic Controller (PLC) is required as the controller, with water level sensors, relays, and contactors as inputs and outputs.

PRIORITIZATION OF MITIGATION AND FLOOD RISK REDUCTION EFFORTS IN KOTA SINGKAWANG

Floods present significant challenges to urban areas globally, including Kota Singkawang in West Kalimantan, Indonesia, due to its susceptibility to seasonal monsoons. Mitigation efforts are crucial to protect lives, infrastructure, and sustainable development amidst urbanization and climate change. This research delves into flood risk reduction methods by integrating technical, economic, social, and environmental dimensions to aid policymakers and stakeholders in efficient resource allocation.This study aims to enhance flood resilience by identifying critical intervention areas through structured approaches, leveraging literature reviews, field surveys, and consultations with local authorities and communities. Prioritizing flood risk mitigation involves evaluating existing strategies and implementing structural measures like flood control buildings.The analysis findings indicate that the primary focus for mitigation efforts and flood risk reduction in Kota Singkawang should involve structural measures, specifically implementing flood control structures like polder systems and retention ponds to effectively address sea tides and heavy rainfall impacts. However, given the preliminary nature of this study, a more thorough investigation with comprehensive data is necessary to implement these structural technologies fully. This comprehensive analysis underscores the significance of comprehending flood risk complexities to formulate sustainable urban strategies in Kota Singkawang.

Design of Water Pump Station Control in Polder System using IoT and Hybrid Fuzzy Neural Network

This study aims to design a prototype of water pump station control in a polder system using IoT and artificial intelligence at Sringin Polder system. The design includes the hydrology analysis to define how the pump station should be set to control the level of the embankment. The second part is the IoT system that contain the sensor and interface to the water pump in the pump station. The third part is the core of control system using hybrid fuzzy neural network as the heart of the control system. Pump station has important role in the polder system to control the level of water in embankment. Five pumps in the system should operate based on schedule determined by the hydrology modelling. Since it is located in separate area with the control room, MQTT protocol is used to deliver data communication in both ways, to send the water level detected by ultrasonic sensor and also to switch on and off the pump by control system. In other hand, hybrid fuzzy neural network is used to control the pump scheduling based on water level detected.

Analysis of Mini Polder Performance for Improved Water Management in the Dadahup Swampy Irrigation Area

The Indonesian government has chosen the Dadahup Swampy Irrigation Area as one of its Food Estate regions to address food security concerns. Following the rehabilitation of the irrigation network and the construction of gates from 2020 to 2022, block A5 is chosen as a pilot project using a mini polder as its water system management. In the right block A5, mini polders small-scale embankments are designed to regulate water flow and optimize land utilization. This research comprehensively evaluates the mini polder’s effectiveness in right block A5 in improving water control within the swampy landscape, considering the influence of rainfall patterns. Our study aims to comprehensively evaluate how mini polders effectively improve water control in the Dadahup Swamp Irrigation Area. To achieve this, we consider the influence of rainfall patterns, which play a significant role in the region’s hydrological dynamics. In addition to traditional data analysis, we integrate hydraulic simulations into our study using HEC-RAS software. These simulations allow us to model and visualize how mini polders interact with the complex water flow patterns in the Dadahup Swampy Irrigation Area. By combining empirical data with hydraulic simulations, we can better understand how mini-polders function in response to varying rainfall scenarios. Based on the simulation results, it was found that without the polder system, the water level would be on dry land during the dry season. The simulation provides direction or guidelines for polder operations to maintain the water level as long as possible during the dry season and prevent flooding during the rainy season. This research can provide valuable insights into sustainable agriculture practices in this unique and challenging environment.

Circles of Coastal Sustainability and Emerald Growth Perspectives for Transitional Waters under Human Stress

Emerald Growth is an overarching sustainable development framework for transitional waters situated between rivers and open sea. The emphasis on connectivity and ecosystem-based management as the underlying principles differentiates Emerald Growth from conventional approaches to managing transitional waters. The study’s primary objective was to conjoin the Emerald Growth concept with the Coastal Circles of Sustainability methodology, an analytical framework to assess indicators of critical processes determining the sustainability of the coastal zone. We hypothesized that applying the CCS is an apt approach to categorizing the Emerald Growth’s aspects using Lake Liepāja, a fresh-to-brackish water lagoon on Latvia’s Baltic Sea coast, as a case study. Based on the document scoping findings on Lake Liepāja’s hydrology, ecology, biodiversity, nature conservation, and management, we addressed the knowledge gaps through the field survey, 4 workshops, and 18 in-depth semi-structured interviews with local stakeholders. The research results show that the challenging socio-economic situation is a crucial obstacle to Emerald Growth in the Lake Liepāja area. Subsistence salary and Housing affordability (Economic Welfare aspect), Population growth and Aging population (Demographic aspect), and Traditional practices (Identity aspect) received the lowest sustainability score (Bad). The results imply that considering the Emerald Growth conditions and drivers for transitional waters worldwide, finding a ‘one-fits-all’ recipe to ensure their sustainability is impossible. The decision-makers, stakeholders, and external experts agreed that for Lake Liepāja, the priority was to bring back to nature part of the polder system, clean the bottom sediments from Soviet-era pollutants, and enhance the transboundary cooperation with Lithuania. These measures would set the right conditions for future Emerald Growth in the area.

Justification of Technological and Design Parameters of Polder Drainage Systems by an Optimization Approach

Abstract A system optimization method was used, which consists in the consistent justification of optimal technological and constructive solutions and parameters of drainage polder systems during the development of their projects. This is done in compliance with modern economic and environmental requirements according to criteria and models for di erent levels of management decision-making over time (project, planned operation). Based on the performed relevant predictive and optimization calculations for the conditions of the real object, the following three tasks have been accomplished. (1) The optimal pump module at the stage of operation for the existing polder drainage system has been substantiated. (2) The design of the pumping unit and the parameters of its components during the reconstruction of the polder drainage system have been improved. This made it possible to reduce the load on the pumping equipment, the duration of its operation, and the cost of electricity by 20–40%, depending on the water level of the year. The improvement was carried out by the diversion of the corresponding part of the surface runo with additionally introduced gravity elements in the form of a puncture in the body of the protective dam and a siphon intake. (3) We have substantiated the optimal water regulation technology for the existing polder drainage system in modern and forecast weather and climate conditions, which will ensure the maintenance of the necessary water-air regime of the drained soils in di erent phases of the growing season of agricultural crops. This will make it possible, on demand, to increase the energy and general environmental and economic e ciency during their creation and functioning of the polder drainage system in accordance with modern changing conditions.

Stakeholder engagement in urban water management: A SWOT analysis of the Banger polder system in Semarang

Semarang is a coastal city of more than 1.6 million people and is considered a global hotspot for land subsidence due to groundwater extraction. Large parts of the city are below sea level already. The Banger polder Project, Launched in 2016, aimed to reduce flood risk in the area by developing a technically well-functioning polder system, creating a sustainable water management organization and improving stakeholder participation. Stakeholder engagement is critical for the financial and operational support of the polder system. However, the results show that Semarang's stakeholder engagement scores the lowest on the OECD ladder of stakeholder engagement. This paper assesses the engagement of various stakeholder representatives in the water management organization and identifies key strengths, weaknesses, opportunities, and threats to the polder system through a SWOT analysis. The main threats are the lack of awareness among residents, that leads to solid waste being disposed of in waterways, the lack of adequate management and funding, and uncertainty about the future maintenance system.

How sediment dredging alters phosphorus dynamics in a lowland rural river?

China's lowland rural rivers are facing severe eutrophication problems due to excessive phosphorus (P) from anthropogenic activities. However, quantifying P dynamics in a lowland rural river is challenging due to its complex interaction with surrounding areas. A P dynamic model (River-P) was specifically designed for lowland rural rivers to address this challenge. This model was coupled with the Environmental Fluid Dynamics Code (EFDC) and the Phosphorus Dynamic Model for lowland Polder systems (PDP) to characterize P dynamics under the impact of dredging in a lowland rural river. Based on a two-year (2020–2021) dataset from a representative lowland rural river in the Lake Taihu Basin, China, the coupled model was calibrated and achieved a model performance (R2>0.59, RMSE<0.04 mg/L) for total P (TP) concentrations. Our research in the study river revealed that (1) the time scale for the effectiveness of sediment dredging for P control was ∼300 days, with an increase in P retention capacity by 74.8 kg/year and a decrease in TP concentrations of 23% after dredging. (2) Dredging significantly reduced P release from sediment by 98%, while increased P resuspension and settling capacities by 16% and 46%, respectively. (3) The sediment-water interface (SWI) plays a critical role in P transfer within the river, as resuspension accounts for 16% of TP imports, and settling accounts for 47% of TP exports. Given the large P retention capacity of lowland rural rivers, drainage ditches and ponds with macrophytes are promising approaches to enhance P retention capacity. Our study provides valuable insights for local environmental departments, allowing a comprehensive understanding of P dynamics in lowland rural rivers. This enable the evaluation of the efficacy of sediment dredging in P control and the implementation of corresponding P control measures.

Application of Sustainability Index of Tidal River Management (SITRM) in the Lower Ganges–Brahmaputra–Meghna Delta

The sustainability index (SI) is a relatively new concept for measuring the performance of water resource systems over long time periods. The purpose of its definition is to provide an indication of the integral behavior of the system with regard to possible undesired consequences if a misbalance in available and required waters occurs. Therefore, the tidal river management (TRM) approach has been implemented for the past three decades (from 1990 to 2020) within the polder system in Southwest Bangladesh to achieve water sustainability. TRM plan and watershed management plan (WMP) have commonalities as both are aimed at ensuring the sustainable use of watershed resources with the management of land, water, and the wider ecosystem of the watershed in an integrated way. The TRM plan focuses mostly on coastal regions, whereas the WMP focuses on both coastal and non-coastal regions. According to this, the aim of this study was to explore the application of the sustainability index of tidal river management (SITRM) in measuring the sustainability of tidal river management in the coastal area of the Lower Ganges–Brahmaputra–Meghna (GBM) delta. In order to quantify the sustainability of tidal river management, this research first provided the components and indicators of SITRM for the coastal region. The study follows a 5-point Likert scale for opinion survey of key informants and comprises households’ survey of farmers. In addition, it includes Landsat satellite images from Earth Explorer of the United States Geological Survey (USGS) and direct field observation to collect information regarding the indicators of SITRM. The study measures the index value of SITRM for identifying the water sustainability of Beel East Khukshia-TRM. The index value was 71.8 out of 100, showing good tidal river management for the Hari–Teka–Bhadra catchment. To achieve water sustainability and aid stakeholders and water managers in decision making, it may be possible to include the SITRM framework in tidal river management projects. In addition, the SITRM is more capable of facing drainage congestion, waterlogging, and climate change issues than watershed sustainability index (WSI), Canadian water sustainability index (CWSI), West Java water sustainability index (WJWSI), and water poverty index (WPI). Therefore, water professionals and policymakers can apply SITRM to assess the resilience of specific TRM schemes for greater sustainability in different coastal regions of the world.

Methods for addressing tidal floods in coastal cities: an overview

Coastal cities are prone to tidal floods because coastal cities are centers of economic activities, trades and services; in addition, there are residential and housing centers. In general, this kind of natural disaster has flooded the lower area, which greatly negative impacts on the health, lifestyle and socioeconomic life of the affected population in coastal cities. In the following decades, tidal floods will damage more than a hundred major coastal cities worldwide. This paper aims to examine the causes, effects and impacts of tidal floods as well as methods that have been taken globally by reviewing papers published in global indexed journals concerning the handling of tidal floods around the world. This study reveals three methods; the first is the hard solution method involving infrastructures such as dikes, sea walls, water storage and pumps (polder system). Then the second is the soft solution method related to social, cultural, and habitual methods, such as adaptation, encouraging local wisdom and collective action. While the third is the policy and management system solution method that implements integrated policies and flood management.

Improvement of technological processes of cultivation of vegetable crops in the Far East. Review

The results of the selection work of the Primorsky vegetable experimental station, work in the field of seed production of vegetable crops are presented, the characteristics of the negative processes occurring in the arable soils of the region and their removal by the developed ridgeridge technology of cultivation by technical means with a wide base are given. A comparative characteristic of the former and the wide-row version in terms of energy consumption and fuel consumption is givenAuthors´ Contribution: All authors contributed to the planning and setting up the experiment, aswell as in the analysis of experimental data and writing of the article.Based on the results of the selection work of the Primorsky Vegetable Experimental Station for the period 1993-2023, the station bred and released 56 varieties and hybrids of 19 main vegetable crops. The experience of the Primorsky branch of the Federal scientific vegetable center allows organizing production at a modern level, taking into account the rich experience of other regions of Russia. In conditions of monsoon climate and soils of heavy granulometric composition, with a general lack of heat, their fertility is negatively affected by a large number of external factors. To grow vegetables, a set of measures is needed for serious drainage and irrigation reclamation (creation of polder systems with the laying of systematic plastic drainage) against the backdrop of ridge-ridge cultivation technology, fine irrigation system). The developed wide-cut technology and a set of machines make it possible to introduce a resource-saving technology that saves energy costs and fuel.

Effectiveness analysis of East Sunter polder system for flood control management using HEC-HMS & HEC-RAS software

DKI Jakarta is the capital city of Indonesia well known as a city with the most rapid development and growth in the country. This indicates that with the increase in the population of Jakarta, the need for a considerable change of land use cannot be denied. Environmental problems will get worse over time and have resulted in flooding in Jakarta which has been the concern of the local government every year. A polder system is stated to be effective in resolving flood problems especially in Jakarta that cannot drain water by gravity. The goal is to give out recommendation that is based on the existing inundation map of East Sunter II Petukangan polder system using HEC-RAS-Ras Mapper by adding pump system and storage area. The result will indicate the effectivity of the East Sunter II Petukangan polder system, as well as to give out recommendation effectiveness to the flood control management using HEC-RAS.

A raster-based estimation of watershed phosphorus load and its impacts on surrounding rivers based on process-based modelling

Quantifying phosphorus (P) load from watersheds at a fine scale is crucial for studying P sources in lake or river ecosystems; however, it is particularly challenging for mountain–lowland mixed watersheds. To address this challenge, we proposed a framework to estimate the P load at the grid scale and assessed its risk to surrounding rivers in a typical mountain–lowland mixed watershed (Huxi Region in Lake Taihu Basin, China). The framework coupled three models: the Phosphorus Dynamic model for lowland Polder systems (PDP), the Soil and Water Assessment Tool (SWAT), and the Export Coefficient Model (ECM). The coupled model performed satisfactory for both hydrological and water quality variables (Nash–Sutcliffe efficiency >0.5). Our modelling practice revealed that polder, non-polder, and mountainous areas had P load of 211.4, 437.2, and 149.9 t yr−1, respectively. P load intensity in lowlands and mountains was 1.75 and 0.60 kg ha−1 yr−1, respectively. A higher P load intensity (>3 kg ha−1 yr−1) was mainly observed in the non-polder area. In lowland areas, irrigated cropland, aquaculture ponds and impervious surfaces contributed 36.7%, 24.8%, and 25.8% of the P load, respectively. In mountainous areas, irrigated croplands, aquaculture ponds, and impervious surfaces contributed 28.6%, 27.0%, and 16.4% of the P load, respectively. Rivers with relatively high P load risks were mainly observed around big cities during rice season, owing to a large contribution of P load from the non-point source pollution of urban and agricultural activities. This study demonstrated a raster-based estimation of watershed P load and their impacts on surrounding rivers using coupled process-based models. It would be useful to identify the hotspots and hot moments of P load at the grid scale.

Potential of model predictive control of a polder water system including pumps, weirs and gates

This work presents an assessment of the potential of model predictive control (MPC) of a Dutch polder system. The system drains to the Linge river and includes 13 weirs, 4 hydraulic gates and 4 large pumping stations each equipped with multiple pumps, managed by the Water Board Rivierenland. The management of the system must comply with several goals: keep the water levels within the bounds of safety, pump out the excess water at minimum cost or CO2 emission, but always have enough water for irrigation and shipping. To achieve these goals there are weirs regulating the water level in different pools, pumping stations to pump water in and out and gates to let water in and out by free flow when possible. These pumping stations consume large amounts of energy. We propose multi-objective mixed-integer optimization by using goal programming to prioritize different operational objectives. For the control of the pumps mixed-integer optimization is used, which makes it possible to not only model the energy consumption of the pumps while in operation, but also to model if the pumps are turned on or off. The control system is implemented using RTC-Tools, an open-source software tool to implement MPC. It is demonstrated that the proposed control system implementation can comply with the operational goals of the water board: keeping the water levels within the bounds while reducing the operational costs. The proposed control system has been tested numerically on data from the year 2013, and it is shown that it highly outperforms the current operation.

Coupling mountain and lowland watershed models to characterize nutrient loading: An eight-year investigation in Lake Chaohu Basin

Quantifying nitrogen (N) and phosphorus (P) sources and their contributions at a watershed scale is a primary step to design best management practices. This task is challenging for a mixed watershed with both mountain and lowland areas, because most existing watershed models were developed for mountain areas with clear water flow directions. To solve this challenge, we coupled the Soil and Water Assessment Tool (SWAT) for mountain watersheds, Phosphorus Dynamic model for lowland Polder systems (PDP) and Nitrogen Dynamic model for lowland Polder systems (NDP) for lowland artificial watersheds (i.e., polders) to characterize N and P loading for a typical mountain-lowland mixed watershed, Lake Chaohu Basin in China. The coupled model was calibrated based on eight-year (2011–2018) meteorological, hydrological and water quality datasets, and had a satisfactory performance for both hydrological and water quality variables (R2 > 0.6, NS > 0.6). Our eight-year investigation in the five watersheds of Lake Chaohu Basin revealed that 1) the estimated N and P loading had an annual averaged amount of 10,832 and 781 t/yr, and an intensity of 15.72 and 1.33 kg/ha/yr, respectively. Three watersheds (Nanfei, Pai and Baishitian River Watersheds) had relatively high loading intensity for N (35.16, 18.34 and 13.34 kg/ha/yr) and P (2.49, 1.25 and 2.72 kg/ha/yr), and contributed 67.6% (N) and 68.5% (P) to the loading of the five investigated watersheds. 2) Polders in Lake Chaohu Basin can be N and P sources or sinks for their surrounding rivers, depending on the polder-river interaction. Within Nanfei, Pai and Baishitian River Watersheds, N and P retention ability was higher in polders than that in mountain watersheds. 3) Population density, precipitation intensity and soil erosion were critical drivers of N and P loss and showed positive correlation with N and P loss intensity. This study demonstrated the high value model coupling to characterize N and P loading in a mountain-lowland mixed watershed system, and thus to improve our understanding on its N and P cycling.

Combining Flood Risk Mitigation and Carbon Sequestration to Optimize Sustainable Land Management Schemes: Experiences from the Middle-Section of Hungary’s Tisza River

The record floods experienced along the Tisza River between 1998 and 2001 brought a paradigm shift in infrastructural solutions for flood protection. A flood peak polder system was built for transient water storage without any substantial change in land use in the polders, despite the potential to do so under the new scheme. The recent improvement of quantified flood risk assessment methodologies and stronger foundations for the valuation of carbon sequestration benefits now provide more information on the magnitude of missed opportunities and the potential for comprehensive land use and flood risk management solutions. This paper evaluates and combines the results of three cost-benefit type analyses on the conflicting relations of pursuing flood risk mitigation and land management goals. Although the studies were conducted at different locations of the same river stretch, they are all inspected using the same flood waves. Results assert that as EU-CAP agricultural subsidies stabilize individual benefits from arable land use in the short-run, public benefits and long-term individual benefits fail to reach their potential value. The combined analysis of flood risk change and CO2 sequestration provides the economic rationale for the ecological revitalization along rivers with flood peak polders, helping to solve the conflict between hydrological and ecological objectives in floodplains. Capitalizing the value of the community benefits of forests in terms of CO2 sequestration is limited by the unresolved property rights allocation of this natural capacity between landowners and the state, the latter being responsible for fulfilling international CO2 reduction agreements; this uncertain legal background is an obstacle to the creation of sustainable economic conditions for the development and expansion of beneficial land management processes along rivers.

Расчет параметров реконструируемых польдерных систем сельскохозяйственного назначения

Analysis of field data on the operation of existing polder systems and the results of numerical experiments shows that re-construction of existing polder systems based on experimentally determined values of the drainage flow modulus has a significant potential to increase the efficiency of their operation.

Analisis Hidrologi untuk Perencanaan Sistem Polder di DKI Jakarta

The application of the polder system for flood control in DKI Jakarta has become a must for coastal areas, especially with the construction of sea dikes along the coast of Jakarta as a solution to anticipate tidal flooding. One thing to consider when using a polder system in flood control is how much pump capacity and reservoir are needed. To answer this quetions, a hydrological analysis has been carried out with several method approaches, starting from determining the rainfall design in the form of a depth duration frequency curve in 1 hour to 48 hours, and then applied area reduction factor (ARF) to corrected rainfall design. Natural Resources Conservation Service (NRCS) method are uses to calculated runoff or effective rainfall and then with the unit hydrograph by time area method to produce a runoff hydrograph. Based on this approach, the Sentiong Polder plan has been carried out with the result that the required pump capacity for a 25-year return period is 32 m3/s and 50 m3/s for a 100-year return period with a storage capacity used of 1,507,500 m3. If you want to reduce the pump capacity, it is necessary to increase the storage capacity.

Statistical and Hydrological Evaluations of Water Dynamics in the Lower Sai Gon-Dong Nai River, Vietnam

The water levels downstream of the Sai Gon and Dong Nai river in Southern Vietnam have been significantly changed over the last three decades, leading to severe impacts on urban flooding and salinity intrusion and threating the socio-economic development of the region and lives of many local people. In this study, the Mann-Kendall (MK) and trend-free prewhitening (TFPW) tests were applied to detect the water level trends and changepoints based on a water level time series at six gauging stations that were located along the main rivers to the sea over 1980–2019. The results indicated that the water level has rapidly increased by about 0.17 to 1.8 cm/year at most gauge stations surrounding Ho Chi Minh City, strongly relating to urbanization and the dike polder system’s impacts that eliminates the water storage space. In addition, the operation of upstream reservoirs has contributed to water level changes with significant consequences since the high-water level at Tri An station on the Dong Nai river occurs 1000–1500 times compared to 300–500 times before the operation. Although the effects of the flows from the sea are less than the two other factors, the local government should seriously consider water level changes, especially in the coastal regions. Our study contributes empirical evidence to evaluate the water level trends in the planning and development of infrastructure, which is necessary to adapt to future changes in Southern Vietnam’s hydrologic system.