In the last few decades, there has been growing stress on surface-water and groundwater resources around the world (e.g., [1
]) due to climate change, population growth, and the degradation of historical water sources. There is a need to balance available future water supply with future increasing demand to provide reliable water supplies to various stakeholders and uses (domestic, agricultural, industrial, energy).
This can be performed using Integrated Water Resources Management (IWRM) at the catchment and watershed levels, which heavily depend on the use of computer model simulations that capture the underlying hydrological processes and surface-water/groundwater allocations. Examples of simulation models include HydroGeoSphere—Integrated Hydrologic Models [3
]; Integrated Water Flow Model IWFM [4
]; MIKE-SHE [5
]; and Penn State Integrated Hydrological Modelling System PIHM [6
Often-used hydrologic models for IWRM include: Soil and Water Assessment Tool (SWAT) [7
], a watershed modelling code that simulates the principal hydrologic fluxes at a daily time step; MODFLOW [8
], a groundwater modelling code that simulates groundwater head and associated flow rates in a heterogeneous aquifer system; and, to overcome the subsurface and surface process limitations in SWAT and MODFLOW, respectively, the coupled modelling code SWAT-MODFLOW [9
], which has been used in numerous studies for water supply estimation and water management (e.g., [10
]). However, these models represent the physical world (i.e., model spatial discretization and process simulation) differently, and each is limited to its simulation domain, with each having advantages and disadvantages when simulating biophysical processes and using computational resources [14
A key part of IWRM is the estimation of future climate (precipitation, climate) and its impact on water supply and water demand. The impact of climate change on water resources needs to be quantified from regional to basin scales with the purpose of facilitating water resources planning and management to cope with future challenges. Global climate models (GCMs) are frequently utilized to simulate future climate dynamics and project future temperatures and rates of precipitation for hydrologic assessment (e.g., [15
]). Additionally, many studies show that future climate may be more extreme, not only in the sense of more storms and flooding, but also with more severe droughts and water scarcity problems [17
Typically, computer models are utilized to access reservoir operation, water allocation, flood risk assessment, drought conditions, groundwater development, water quality, irrigation operation, and the forecasting and control of high water. There are large numbers of software available to simulate problems in water resources management, and they can be mainly divided into two groups: simulation models [20
] and allocation models (optimization or decision-making system models).
Simulation models address certain limitations of allocation models by solving physically based flow equations to offer spatially distributed water resources outputs for several parameters (runoff, water table elevation, etc.) [21
]. Allocation (optimization) models are frequently used in the applied problem of water resources management. These models optimize water allocation from various resources to meet a range of demands and to find which design and operating policy would best meet the identified objectives under a set of priorities and constraints [20
]. Allocation models are used in several applications of water resources, such as the development of a planning framework for short-term scenarios (land-use change) and long-term scenarios (climate change) [22
]; simulation crop evapotranspiration demand for agricultural land [23
]; and modelling current and future water demands [24
]. The main advantage of allocation models is their lower demand for computing resources and data. However, these models have a lack of ability to simulate feedbacks to the physical system and have limited ability to simulate connections within complex, heterogeneous, conjunctive management water systems.
In IWRM, ideally, a combined simulation–allocation approach can be used, with the simulation model accounting for all major hydrological processes, fluxes, and state variables within a river basin or watershed system. For example, a fully integrated surface–groundwater model would be able to assess spatially distributed hydrological variables such as river flow, groundwater table, and soil moisture content. More significantly, it would also be able to simulate the surface water and groundwater interaction in a heterogeneous and complex domain, as well as the land-use change impact on these variables.
The accuracy of these kinds of models is the central part of an IWRM system where water allocation models are driven by the outputs from the integrated surface–groundwater model to further simulate water management operations for defined operating policies and priorities under different scenarios of climate change. This integration framework is beneficial to grasp how the hydrological cycle is affected by management decisions, and vice versa.
The applications of coupled simulation–allocation models in water resources are abundant, for instance, the evaluation of the reliability between water supply and demand in climate change scenarios using coupled SWAT- RIBASIM [26
]. Within the study, the SWAT model was utilized to simulate the basin hydrology for the baseline and future climate periods, while the RIBASIM model was used for water allocation based on the current supply–demand chain and predefined governmental water allocation rules for different water uses. In addition, Dehghanipour et al. [27
] used a linked MODFLOW-WEAP model to investigate management alternatives that maintain agricultural production. Another application included exploring the future climate change impact on the agricultural water supply capacity of irrigation facilities using an integrated modelling framework that included a water balance network model (MODSIM) and SWAT model [28
]. Ashraf Vaghefi et al. [29
] examined the water productivity of irrigated wheat and maize yields in semi-arid regions with coupled SWAT-MODSIM considering dynamic irrigation requirements instead of constant-demand time series. A linked SWAT-WEAP model was used to study the climate change impact on the abstraction capability of key water sectors [30
]. The SWAT model was used to simulate future streamflow; then, streamflow outputs were used to estimate total unmet demands. The WEAP model was utilized for the evaluation of the unmet demands system under static, increasing, and decreasing surface-water demand scenarios. No study, however, included integrated hydrologic modelling using coupled surface–subsurface modelling, such as SWAT-MODFLOW [9
], into an allocation modelling scheme, which may have important implications for the overall movement of water within a watershed system.
The objective of this article is to evaluate the impacts of likely future water use for the public water supply on the water resources in a heavily managed watershed system, under the stress of future climate change. This objective is achieved by linking the surface/subsurface hydrologic model SWAT-MODFLOW to the Water Evaluation and Planning (WEAP) software program to simulate hydrologic fluxes and different scenarios of water use for urban water supply in future climate scenarios. The linked simulation/allocation modelling system is demonstrated for the highly managed River Dee watershed in Wales, United Kingdom, wherein surface water is abstracted for public water supply in the city of Chester, in the downstream area of the watershed. The SWAT-MODFLOW model is constructed and then tested against streamflow and groundwater head; then, it is used with climate projections from the UKCP18 project (NCAR-CCSM4 model) for a future scenario of RCP85 from 2020 to 2040. The simulated streamflow of the SWAT-MODFLOW model with UKCP18 model data is used as the input to the WEAP model to create different scenarios of water use in Chester. We consider four scenarios of water use rate: maximum licensed water abstraction (constant value), 50% of maximum licensed abstraction (constant value), time series of maximum water withdraw per year based on historical records repeated every year with 1% increases in water use, and time series with 1% annual increases in water use. We expect that the methodology proposed herein can be utilized for other managed watersheds to aid with future water management.
5. Summary and Conclusions
In this article, we present a coupled SWAT-MODFLOW-WEAP system to estimate the impact of future climate conditions on water supply and demand in a complex watershed system, the Dee River watershed in Wales, United Kingdom. The SWAT model is created and coupled with the groundwater flow model MODFLOW to simulate streamflow and baseflow for the Dee River basin, with the latter being a key component of minimum flows and overall water resources management in the region.
The coupled SWAT-MODFLOW-WEAP model is constructed, calibrated, and tested against historical streamflow, baseflow, and groundwater head and then used with climate change data of the UKCP18 project to evaluate the unmet demand for water for public water supply in the downstream region (city of Chester) as well as to check the unmet streamflow requirement in Chester weir. The high-resolution future climate data of the UKCP18 model (precipitation, maximum and minimum air temperature) for the future scenario of RCP85 are used as inputs in the calibrated SWAT-MODFLOW model to simulate the catchment hydrology for the period of 2021–2039 with a 2-year warm-up period. The simulated discharge from the SWAT-MODFLOW model is used as the input to the modelled reach in the WEAP model at a monthly time step.
Four scenarios of the water use rates at four selected locations of public water supplies downstream of the Dee River basin with a considerable amount of water abstraction is utilized to evaluate the likely unmet demands. The results agree that there is expected unmet demand in large quantities, especially in the summer season (June, July, and August). Actions and measurements for mitigating the effects of unmet water demands and uncertainties about how the climate will change and how it will affect water resources are the challenges that planners and designers will have to cope with. How water resources management will have to adapt to climate changes is the pressing question to be answered. The possible mitigations for the unmet water demands are:
Augmenting streamflow from a deep well source;
Using reclaimed water;
Storing and recovering surface water or groundwater;
Transferring water into basins;
Adjusting reservoir regulation rules.
Overall, the study shows a promising direction for using coupled surface–groundwater models and allocation models (WEAP) in IWRM. We expect that this same system can be used in other regions of the world to estimate future water supplies and aid in water management decisions.