Integrated Surface Water-groundwater Model Research Articles

Multi-objective double layer water optimal allocation and scheduling framework combing the integrated surface water – groundwater model

Balancing the water consumption of agricultural and ecological is the key point of sustainable social and economic development in an inland river basin. The growth of desert riparian forests in inland river basins mainly depends on a certain phreatic water table depth (PWTD). The main object of this study was to allocate and schedule water resources to regulate the PWTD and satisfy agricultural water demand. Therefore, a multi-objective double layer optimal allocation and scheduling framework based on the computationally efficient integrated surface water-groundwater model (ISGWM), which can simulate the surface water processes, groundwater recharge and discharge processes, and PWTD changes, was constructed and applied to the mainstream of Tarim River Basin (TRB). The top layer model of the framework is an optimal ecological water allocation model, and its optimal allocation results are used as the initial solution of the bottom layer model. The results show that under 5 different inflow frequencies, the agricultural water shortage rate is 0, 17.38 %, 17.41 %, 14.06 %, and 19.94 %, respectively. The PWTD regulation has a great performance. After the optimal scheduling, the proportions of good growth of the control area behind the gate under different inflow frequencies were 98.18 %, 98.18 %, 98.18 %, 90.91 %, and 94.55 %. Agricultural water shortage is mainly due to the non-uniformity distribution of intra-annual inflow and the lack of controlling hydraulic engineering. The regulation of PWTD can guarantee the growth of desert riparian forests on both sides of the mainstream of TRB. Besides, we explored the feasibility of exploiting groundwater to supplement agricultural water consumption. The groundwater exploitation should be controlled within the scope of not causing excessive increase of PWTD (difference between PWTD and target depth <1 m), due to the groundwater exploitation to supplement agricultural water will lead to the increase of PWTD. Overall, this framework, which regulates the PWTD with the change of ecological water supply based on the ISGWM, provides a new idea for the allocation and scheduling of agricultural and ecological water resources in arid inland river basins. It also provides a new method for the coupled cooperative operation of surface water and groundwater.

Combined impacts of uncertainty in precipitation and air temperature on simulated mountain system recharge from an integrated hydrologic model

Abstract. Mountainous regions act as the water towers of the world by producing streamflow and groundwater recharge, a function that is particularly important in semiarid regions. Quantifying rates of mountain system recharge is difficult, and hydrologic models offer a method to estimate recharge over large scales. These recharge estimates are prone to uncertainty from various sources including model structure and parameters. The quality of meteorological forcing datasets, particularly in mountainous regions, is a large source of uncertainty that is often neglected in groundwater investigations. In this contribution, we quantify the impact of uncertainty in both precipitation and air temperature forcing datasets on the simulated groundwater recharge in the mountainous watershed of the Kaweah River in California, USA. We make use of the integrated surface water–groundwater model, ParFlow.CLM, and several gridded datasets commonly used in hydrologic studies, downscaled NLDAS-2, PRISM, Daymet, Gridmet, and TopoWx. Simulations indicate that, across all forcing datasets, mountain front recharge is an important component of the water budget in the mountainous watershed, accounting for 9 %–72 % of the annual precipitation and ∼90 % of the total mountain system recharge to the adjacent Central Valley aquifer. The uncertainty in gridded air temperature or precipitation datasets, when assessed individually, results in similar ranges of uncertainty in the simulated water budget. Variations in simulated recharge to changes in precipitation (elasticities) and air temperature (sensitivities) are larger than 1 % change in recharge per 1 % change in precipitation or 1 ∘C change in temperature. The total volume of snowmelt is the primary factor creating the high water budget sensitivity, and snowmelt volume is influenced by both precipitation and air temperature forcings. The combined effect of uncertainty in air temperature and precipitation on recharge is additive and results in uncertainty levels roughly equal to the sum of the individual uncertainties depending on the hydroclimatic condition of the watershed. Mountain system recharge pathways including mountain block recharge, mountain aquifer recharge, and mountain front recharge are less sensitive to changes in air temperature than changes in precipitation. Mountain front and mountain block recharge are more sensitive to changes in precipitation than other recharge pathways. The magnitude of uncertainty in the simulated water budget reflects the importance of developing high-quality meteorological forcing datasets in mountainous regions.

Recent progress in coupled surface–ground water models and their potential in watershed hydro-biogeochemical studies: A review

Abstract Interactions between surface water (SW) and groundwater (GW) have been a focus of watershed hydrology research for a long time. A holistic perspective on integrated SW–GW modeling approach is necessary to understand the hydrological and biogeochemical processes of these two interconnected systems within the watershed. This paper reviewed the progress and coupling strategy of one important SW model (Soil and Water Assessment Tool, SWAT) and GW model (Modular Finite Difference Groundwater Flow, MODFLOW) since 1999. Three main stages of development of coupled SWAT–MODFLOW model are reflected by the high citation of publications by three pioneer studies, which are Sophocleous et al. (1999), Kim et al. (2008) and Bailey et al. (2016). Currently, the research scope of coupled SWAT–MODFLOW models is focused on hydrologic processes, solute transport and the effects of climate change and human activity on water resources. Major uncertainties of SWAT–MODFLOW from model structure, database and parameterization are discussed. In an era of big data, the coupled SWAT–MODFLOW model has great potential to improve understanding of hydro-biogeochemical processes and support sustainable water and ecological management in the watershed.

Flood inundation modeling and mapping by integrating surface and subsurface hydrology with river hydrodynamics

Modeling and mapping of floods using one (1D) and two-dimensional (2D) models has resulted in significant progress in our understanding of river hydraulics and hydrology. The existing modeling approach can be improved further through a better understanding of the interactions between river hydrodynamics and subsurface processes. Saturated conditions in subsurface and floodplains can lead to longer and deeper inundation from low intensity but continuous flood events. To understand the combined effect of surface-subsurface hydrology and hydrodynamics on streamflow generation and subsequent inundation during floods, this study uses an integrated surface water-groundwater (SW-GW) modeling approach for simulating flood conditions. Simulation of a high flooding event for the Upper Wabash River basin in Indiana, USA, shows that the river-floodplain hydrodynamics is simulated more accurately without the need for extensive calibration by using a two-dimensional integrated SW-GW model. Additionally, an integrated model provides a more realistic simulation of flood hydrodynamics for different antecedent soil conditions. Specifically, results show that the streamflow, flood stages and inundation area obtained for the dry scenario are significantly lower when compared to the saturated scenario. These findings are further validated by applying the integrated modeling approach for an entirely different watershed (Croton River) in the state of New York. Overall, the findings from this study suggest that the current practice of simulating floods which assumes an impervious surface may not be providing realistic estimates of flood inundation, and that an integrated approach incorporating all the hydrologic and hydraulic processes in the river system must be adopted.

A comprehensive graphical modeling platform designed for integrated hydrological simulation

This study developed a comprehensive graphical data processing and modeling system named Visual HEIFLOW (VHF) for integrated surface water-groundwater modeling. Its distinctive features include the following. First, VHF uses a generic multivariable-space-time data cube model, which enables the system to efficiently handle large time-series datasets over a large spatial domain. Second, VHF streamlines the entire modeling procedure from data preparation at the very beginning to visualization and analysis of modeling results in a uniform environment. Third, VHF allows updating the land use input at user-specified time points without manual intervention and therefore allows a direct simulation of the hydrological effects of changing land use. The applicability and versatility of VHF were demonstrated in a case study in the Heihe River Basin, the second largest inland river basin in China. The case study also demonstrated that VFH facilitates process understanding and supports management decision-making.

An integrated hydrological modeling approach for detection and attribution of climatic and human impacts on coastal water resources

Water resources in coastal areas can be profoundly influenced by both climate change and human activities. These climatic and human impacts are usually intertwined and difficult to isolate. This study developed an integrated model-based approach for detection and attribution of climatic and human impacts and applied this approach to the Luanhe Plain, a typical coastal area in northern China. An integrated surface water-groundwater model was developed for the study area using GSFLOW (coupled groundwater and surface-water flow). Model calibration and validation were performed for background years between 1975 and 2000. The variation in water resources between the 1980s and 1990s was then quantitatively attributed to climate variability, groundwater pumping and changes in upstream inflow. Climate scenarios for future years (2075–2100) were also developed by downscaling the projections in CMIP5. Potential water resource responses to climate change, as well as their uncertainty, were then investigated through integrated modeling. The study results demonstrated the feasibility and value of the integrated modeling-based analysis for water resource management in areas with complex surface water-groundwater interaction. Specific findings for the Luanhe Plain included the following: (1) During the historical period, upstream inflow had the most significant impact on river outflow to the sea, followed by climate variability, whereas groundwater pumping was the least influential. (2) The increase in groundwater pumping had a dominant influence on the decline in groundwater change, followed by climate variability. (3) Synergetic and counteractive effects among different impacting factors, while identified, were not significant, which implied that the interaction among different factors was not very strong in this case. (4) It is highly probable that future climate change will accelerate groundwater depletion in the study area, implying that strict regulations for groundwater pumping are imperative for adaptation.

Development of a visualization tool for integrated surface water–groundwater modeling

Physically-based, fully integrated surface water (SW)-groundwater (GW) models have been increasingly used in water resources research and management. The integrated modeling involves a large amount of scientific data. The use of three-dimensional (3D) visualization software to integrate all the scientific data into a comprehensive system can facilitate the interpretation and validation of modeling results. Nevertheless, at present few software tools can efficiently perform data visualization for integrated SW–GW modeling. In this study, a visualization tool named IHM3D was designed and developed specifically for integrated SW–GW modeling. In IHM3D, spatially distributed model inputs/outputs and geo-referenced data sets are visualized in a virtual globe-based 3D environment. End users can conveniently explore and validate modeling results within the 3D environment. A GSLFOW (an integrated SW–GW model developed by USGS) modeling case in the Heihe River Basin (Northwest China) was used to demonstrate the applicability of IHM3D at a large basin scale. The visualization of the modeling results significantly improved the understanding of the complex hydrologic cycle in this water-limited area, and provided insights into the regional water resources management. This study shows that visualization tools like IHM3D can promote data and model sharing in the water resources research community, and make it more practical to perform complex hydrological modeling in real-world water resources management.

Optimizing conjunctive use of surface water and groundwater for irrigation to address human-nature water conflicts: A surrogate modeling approach

In arid and semi-arid areas where agriculture competes keenly with ecosystem for water, integrated management of both surface water (SW) and groundwater (GW) resources at a basin scale is crucial, but often lacks scientific support. This study implemented physically-based, fully integrated SW–GW modeling in optimizing water management, and performed surrogate modeling to replace the computationally expensive model with simple response surfaces. Water use conflicts between agriculture and ecosystem in Heihe River Basin (HRB), the second largest inland river basin in China, were investigated. Based on the integrated model GSFLOW (Coupled Ground-Water and Surface-Water Flow Model), the conjunctive use of SW and GW for irrigation in the study area was optimized using a surrogate-based approach named DYCORS (DYnamic COordinate search using Response Surface models). Overall, the study demonstrated that, with the surrogate modeling approach, an expensive integrated model could be efficiently incorporated into an optimization analysis, and the integrated modeling would make feasible a physically based interpretation of the optimization results. In the HRB case study, the surrogate-based optimization suggested a very different time schedule of water diversion in opposite to the existing one, indicating the critical role of SW–GW interactions in the water cycle. With the temporal optimization, a basin-scale water saving could be achieved by reducing non-beneficial evapotranspiration. In addition, the current flow regulation in HRB may not be sustainable, because the ecosystem recovery in the lower HRB would be at the cost of the ecosystem degradation in the middle HRB.

Impacts of model initialization on an integrated surface water–groundwater model

AbstractIntegrated hydrologic models characterize catchment responses by coupling the subsurface flow with land surface processes. One of the major areas of uncertainty in such models is the specification of the initial condition and its influence on subsequent simulations. A key challenge in model initialization is that it requires spatially distributed information on model states, groundwater levels and soil moisture, even when such data are not routinely available. Here, the impact of uncertainty in initial condition was explored across a 208 km2 catchment in Denmark using the ParFlow.CLM model. The initialization impact was assessed under two meteorological conditions (wet vs dry) using five depth to water table and soil moisture distributions obtained from various equilibrium states (thermal, root zone, discharge, saturated and unsaturated zone equilibrium) during the model spin‐up. Each of these equilibrium states correspond to varying computation times to achieve stability in a particular aspect of the system state.Results identified particular sensitivity in modelled recharge and stream flow to the different initializations, but reduced sensitivity in modelled energy fluxes. Analysis also suggests that to simulate a year that is wetter than the spin‐up period, an initialization based on discharge equilibrium is adequate to capture the direction and magnitude of surface water–groundwater exchanges. For a drier or hydrologically similar year to the spin‐up period, an initialization based on groundwater equilibrium is required. Variability of monthly subsurface storage changes and discharge bias at the scale of a hydrological event show that the initialization impacts do not diminish as the simulations progress, highlighting the importance of robust and accurate initialization in capturing surface water–groundwater dynamics. Copyright © 2015 John Wiley &amp; Sons, Ltd.

셀기반 지하수 개발가능량 산정기법

현재 우리나라의 지하수 개발가능량은 10년빈도 갈수시 함양량으로 결정되며 일정면적 이상의 수문학적 유역 또는 행정구역별로 넓게 구분되어 있어 소규모 유역의 지하수 영향평가에 적용하기가 어려운 면이 있다. 이에 본 연구에서는 셀기반의 지하수 개발가능량을 추정하는 새로운 기법을 경주지역에 대해 제시하였다. 이를 위해 지표수-지하수 통합 수문해석 모형인 SWAT-MODFLOW를 이용하여 셀기반 지하수 함양량을 산정하였다. 지하수 개발가능량을 결정하는 기준은 10년빈도 갈수시 강수량에 함양계수를 곱하여 추정하는 기존방식을 준용했다. 격자별 개발가능량을 산정한 후에는 목적에 따라 경주지역에서 143개로 세분화된 소유역별, 읍면리등 행정구역별로 개발가능량을 제시할 수 있었다. 지하수 이용량과 개발가능량을 연계하여 분석한 결과 경주시 전체 이용율은 평균 24%의 비교적 양호한 이용률을 보였으나 지역적으로는 7.1~108.8%까지 편차가 큰 이용현황을 보이는 것을 쉽게 확인할 수 있었다. 또한 임의의 시군구에서 지역별 추가 개발가능량도 평가할 수 있어 그 활용성이 기대된다. Sustaiable development yield of groundwater in Korea has been determined according to 10 year drought frequency of groundwater recharge in the standard mid-sized watershed or relatively large area of district. Therefore, the evaluation of groundwater impact in a small watershed is hard to apply. Fot this purpose, a novel approach to estimate cell based sustainable development yield of groundwater (SDYG) is suggested and applied to Gyeongju region. Cell based groundwater recharge is computed using hydrological component analysis using the SWAT-MODFLOW which is an integrated surface water-groundwater model. To estimate the potential amount of groundwater development, the existing method which uses 10 year drought frequency rainfall multiplied by recharge coefficient is adopted. Cell based SDYGs are computed and summed for 143 sub-watersheds and administrative districts. When these SDYGs are combined with groundwater usage data, the groundwater usage rate (total usage / SDYG) shows wide local variations (7.1~108.8%) which are unseen when average rate (24%) is only evaluated. Also, it is expected that additional SDYGs in any small district could be estimated.

Modeling surface water-groundwater interaction in arid and semi-arid regions with intensive agriculture

In semi-arid and arid areas with intensive agriculture, surface water-groundwater (SW-GW) interaction and agricultural water use are two critical and closely interrelated hydrological processes. However, the impact of agricultural water use on the hydrologic cycle has been rarely explored by integrated SW-GW modeling, especially in large basins. This study coupled the Storm Water Management Model (SWMM), which is able to simulate highly engineered flow systems, with the Coupled Ground-Water and Surface-Water Flow Model (GSFLOW). The new model was applied to study the hydrologic cycle of the Zhangye Basin, northwest China, a typical arid to semi-arid area with significant irrigation. After the successful calibration, the model produced a holistic view of the hydrological cycle impact by the agricultural water use, and generated insights into the spatial and temporal patterns of the SW-GW interaction in the study area. Different water resources management scenarios were also evaluated via the modeling. The results showed that if the irrigation demand continuous to increase, the current management strategy would lead to acceleration of the groundwater depletion, and therefore introduce ecological problems to this basin. Overall, this study demonstrated the applicability of the new model and its value to the water resources management in arid and semi-arid areas.

Estimating exploitable amount of groundwater abstraction using an integrated surface water–groundwater model: Mihocheon watershed, South Korea

A holistic approach to groundwater sustainability considers the hydrological, ecological, socio-economic and technological aspects of groundwater utilization. Exploitable groundwater should be determined by physical analysis, as well as by social compromises within a community. In this work, an integrated surface water–groundwater model, SWAT-MODFLOW, is used to examine changes in hydrological components due to various groundwater pumping scenarios in the Mihocheon watershed of South Korea. Under a total groundwater abstraction of 104 mm, which is slightly less than the amount currently permissible by law, a change in storage levels of –16 mm occurred. This abstraction value, which corresponds to 33% of the annual recharge, may be an exploitable groundwater reserve available to Cheongju City, Korea.Editor D. Koutsoyiannis; Guest editor V. Krysanova

Application of an integrated surface water-groundwater model to multi-aquifers modeling in Choushui River alluvial fan, Taiwan

This research proposes a combination of SWAT and MODFLOW, MD-SWAT-MODFLOW, to address the multi-aquifers condition in Choushui River alluvial fan, Taiwan. The natural recharge and unidentified pumping/recharge are separately estimated. The model identifies the monthly pumping/recharge rates in multi-aquifers so that the daily streamflow can be simulated correctly. A multi-aquifers condition means a subsurface formation composed of at least the unconfined aquifer, the confined aquifer, and an in-between aquitard. In such a case, the variation of groundwater level is related to pumping/recharge activities in vertically adjacent aquifer and the river-aquifer interaction. Both factors in turn affect the streamflow performance. Results show that MD-SWAT-MODFLOW performs better than SWAT alone in terms of simulated streamflow, especially during low flow period, when pumping/recharge rates are properly estimated. A sensitivity analysis of individual parameter suggests that the vertical leakance may be the most sensitive among all investigated MODFLOW parameters in terms of the estimated pumping/recharge among aquifers, and the Latin-Hypercube-One-factor-At-a-Time sensitivity analysis indicates that the hydraulic conductivity of channel is the most sensitive to the model performance. It also points out the necessity to simultaneously estimate pumping/recharge rates in multi-aquifers. The estimated net pumping rate can be treated as a lower bound of the actual local pumping rate. As a whole, the model provides the spatio-temporal groundwater use, which gives the authorities insights to manage groundwater resources. Copyright © 2012 John Wiley & Sons, Ltd.

Analysis of coupling errors in a physically-based integrated surface water–groundwater model

Several physically-based models that couple 1D or 2D surface and 3D subsurface flow have recently been developed, but few studies have evaluated the errors directly associated with the different coupling schemes. In this paper we analyze the causes of mass balance error for a conventional and a modified sequential coupling scheme in worst-case scenario simulations of Hortonian runoff generation on a sloping plane catchment. The conventional scheme is noniterative, whereas for the modified scheme the surface–subsurface exchange fluxes are determined via a boundary condition switching procedure that is performed iteratively during resolution of the nonlinear subsurface flow equation. It is shown that the modified scheme generates much lower coupling mass balance errors than the conventional sequential scheme. While both coupling schemes are sensitive to time discretization, the iterative control of infiltration in the modified scheme greatly limits its sensitivity to temporal resolution. Little sensitivity to spatial discretization is observed for both schemes. For the modified scheme the different factors contributing to coupling error are isolated, and the error is observed to be highly correlated to the flood recession duration. More testing, under broader hydrologic contexts and including other coupling schemes, is recommended so that the findings from this first analysis of coupling errors can be extended to other surface water–groundwater models.

지표수지하수 통합모형을 이용한 무심천 유역의 지하수 개발가능량 산정

우리나라의 지하수관리는 연간 지하수 함양량을 기반으로 수행되어 왔으나 지히수 함양량과 지표수-지하수 상호작용은 시공간적으로 변동하는 양이므로 지속적인 모니터링과 동적인 분석을 통해 지하수자원의 지속가능성이 평가되어야 할 것이다. 본 연구에서는 무심천 유역을 대상으로 지표수-지하수 통합해석 모형인 SWAT-MODFLOW를 이용하여 다양한 지하수 양수 시나리오(무양수 및 현 이용량의 1~3배)에 대한 수문성분 해석을 수행하였다. 현 이용량으로 양수하는 경우가 비교적 지속가능한 개발량으로 판단되었는데 이때 연간 기저유출량은 16%, 대수층 저류량은 27 mm 감소하는 것으로 나타났다. 지하수의 지속가능성을 평가하기 위해서는 지하수 이용에 따른 수문학적, 생태학적, 사회 경제적, 기술적 영향을 평가하는 총체적 접근법이 도입되어야 하며 이를 위해서는 물리적인 분석과 함께 지역사회의 합의 또한 중요한 의사결정 방식이라고 판단되었다. In Korea, groundwater management has been conducted based on the estimation of annual average of groundwater recharge. Since groundwater recharge and surface water-groundwater interactions show spatiotemporal variation, continuous monitoring and dynamic analysis must be carried out to evaluate the sustainability of groundwater resources. In this study, SWAT-MODFLOW, an integrated surface water-groundwater model was used to analyze surface-groundwater interactions for various groundwater pumping scenarios in Musimcheon watershed. When current usage is applied, the baseflow reduction is about 16%, and annual averaged storage reduction is about 27 mm for whole watershed. As a holistic approach to groundwater sustainability considers the hydrological, ecological, socioeconomic, technological aspects of groundwater utilization, the exploitable groundwater should be determined by physical analysis as well as social compromise in a community.

Algorithm for Flow Direction Enforcement Using Subgrid-Scale Stream Location Data

Producing realistic surface water flow patterns can be difficult for hydrologic models when there is insufficient grid resolution as a result of computational constraints or when available digital elevation model (DEM) data are relatively coarse. This technical note describes an algorithm that allows for more realistic overland flow by incorporating subgrid-scale location data without increasing grid resolution. The algorithm takes location data from the National Hydrography Dataset (NHD), maps it to the hydrologic model horizontal grid coordinates, and produces a list of ordered points along the stream. Because the algorithm uses indexes to efficiently order points along the stream, large-scale meanders require special treatment, whereas small (grid) scale meanders are explicitly included in the algorithm logic. Slopes are ensured to be continuous along the stream's path as defined on the model grid, distinguishing this approach from traditional stream burning algorithms. Stream coordinates on the model grid are calculated along with corresponding elevation and slope values so that the can then be integrated into the DEM if desired. The algorithm's flow routing capabilities are demonstrated by using an integrated surface water-groundwater model, ParFlow, under rain and recession conditions. This case study is performed by using the real topography of Owens Valley, California, and NHD flowline data for the Owens River. An approach using a DEM that has undergone standard processing to fill sinks and a typical stream burning approach [similar to those often used in geographic information system (GIS) applications] fail to route flow out of the flood plain in ParFlow's overland flow model that partitions and routes topography-driven flow along adjacent cells in one of four cardinal directions. In contrast, this approach, with the river elevations and continuous slopes integrated into the DEM, routes water to the river and out of the catchment, creating more realistic surface flow patterns in the region. Although the method is applied here to address problems associated with a flat flood plain, it may also be applied to any area in which flow is discontinuous because of insufficient resolution of topography on a model grid. DOI: 10.1061/(ASCE)HE.1943-5584.0000340. (C) 2011 American Society of Civil Engineers.

Assessing distributed groundwater recharge rate using integrated surface water-groundwater modelling: application to Mihocheon watershed, South Korea

A method of estimating groundwater recharge, based on water-balance components using the SWAT-MODFLOW model (an integrated surface water-groundwater model), is described. A multi-reservoir storage routing module is suggested instead of a single storage routing module in SWAT; this represents a more realistic delay in the travel of water through the vadose zone. By using this module, the parameter related to the delay time can be optimized by checking the correlation between simulated recharge and observed groundwater levels. The final step of this procedure is to compare simulated groundwater levels as well as the simulated watershed stream flow with the observed groundwater levels and watershed stream flow. This method is applied to the Mihocheon watershed in South Korea to estimate spatio-temporal groundwater recharge distribution. The computed annual recharge rate is compared with the independently estimated recharge rate using BFLOW. The hydrologic modelling results show that the annual average recharge rate should be estimated by a long-term continuous simulation with a distributed hydrologic modelling technique.

Development and verification of a 3-D integrated surface water–groundwater model

Summary Coupled modelling of surface and subsurface systems is a valuable tool for quantifying surface water–groundwater interactions. In the present paper, the 3-D non-steady state Navier–Stokes equations, after Reynolds averaging and with the assumption of a hydrostatic pressure distribution, are for the first time coupled to the 3-D saturated groundwater flow equations in an Integrated suRface watEr–grouNdwater modEl (IRENE). A finite-difference method is used for the solution of the governing equations of IRENE. A semi-implicit scheme is used for the discretisation of the surface water flow equations and a fully implicit scheme for the discretisation of the groundwater flow equations. The two sets of equations are coupled at the common interface of the surface water and groundwater bodies, where water exchange takes place, using Darcy’s law. A new approach is proposed for the solution of the coupled surface water and groundwater equations in a simultaneous manner, in such a fashion that gives computational efficiency at low computational cost. IRENE is verified against three analytical solutions of surface water–groundwater interaction, which are chosen so that different components of the model can be tested. The model closely reproduces the results of the analytical solutions and can therefore be used for analysing and predicting surface water–groundwater interactions in real-world cases.