Agent-based modelling of water balance in a social-ecological system: A multidisciplinary approach for mountain catchments

The increasing population, improving living standards, and expanding economic activities are responsible for rising water demands. In recent decades, the imbalance between water supply and demand has become increasingly prominent. Coupled with the unreasonable use of water resources, this has led to serious water scarcity problems that affect the sustainable development of modern society. Under this background, water scarcity has become an important environmental issue for our sustainable planet. Water scarcity is affected by regional water resource endowments and the ways in which water resources are developed and utilized by human beings. Quantity-induced water scarcity occurs when the quantity of water resources is insufficient. Meanwhile, pollution can cause water scarcity as the services provided by polluted water are not equivalent to that of clean water. Quality-induced water scarcity occurs when the pollutants exceed the environmental carrying capacity. Since Marlin Falkenmark first proposed the concept and assessment method of water scarcity in the 1980s, water scarcity assessment has developed for nearly 40 years. With the development of new theories, progress has been made, such as development of different approaches for assessing water scarcity, identification of influencing factors of water scarcity, revealing the formation mechanism, and exploring strategies to cope with water scarcity. As a result, water scarcity assessment has experienced an evolutionary pathway from a one-dimensional model emphasizing only quantity-induced water scarcity to a two-dimensional model (considering both quantity-induced and quality-induced water scarcity), toward a three-dimensional (3D) model (or 3D water scarcity theory) that considers water quantity, water quality, and environmental flows simultaneous. Based on a systematic review of water scarcity assessment in the literature, this study demonstrates the 3D water scarcity theory and elaborates on its basic concepts, principles, and core methods. The 3D water scarcity theory was first proposed by Chinese scholars and then accepted by international scholars. The quantity-induced water scarcity is assessed based on the ratio of water consumption to water availability, and the quality-induced water scarcity is quantified by comparing gray water footprint (the amount of water required to dilute pollutants in wastewater sufficiently to meet environmental water quality standards) with local available water resources. Environmental flow requirements are quantified based on the characteristics of climatic and hydrological conditions rather than the traditionally adopted simple approach by assuming that they are equal to 80% of water resources. Hence, the 3D water scarcity theory and assessment method can consider water quantity, water quality, and environmental flow requirements comprehensively. Finally, four main future research directions of water scarcity assessment are summarized: The further development and improvement of the theoretical framework of 3D water scarcity, applications of the assessment methods on multiple spatial scales, explicit consideration of the impacts of physical and virtual water flow, and mechanism of the evolution of water scarcity. 3D water scarcity is a key to achieving sustainable and harmonious development between humanity and water resources, and its assessment framework can break through the limitations of the traditional one- or two-dimensional models. Water scarcity theory provides methodological support for measuring the state of water resources on global, national, and regional scales, and it will help formulate appropriate water mitigation measures for integrated water resources management.

Triangle of Environment, Water and Energy: A Sociological Appraisal

Water scarcity assessments are crucial for sustainable water management in the future. Previously, such assessments were mainly conducted through simple statistical analyses and water resource models, which apply minimal observational data on socioeconomic factors. Here, we present a data-driven method to evaluate water scarcity, and reveal the interplaying mechanisms between its key driving factors. We selected Beijing as a case study for its complex human–water system representative of other megacities. We collected annual parameters from government’s statistics and yearbooks during 2000–2021 and designed a system dynamics (SD) framework to characterize the key human–water coupling components. The framework was then used to project water demand and supply changes under different shared socioeconomic pathways (SSPs) (2024–2050) using meteorological forcing from CMIP6. Results show that the SD framework performs reasonably in reproducing historical water supply and demand variability, showing continuously mitigated water scarcity severity due to government efforts to constrain agricultural and industrial water demand. Future projections indicate that water scarcity severity will be gradually mitigated until 2030, particularly under SSP1, SSP2, and SSP3 scenarios. After 2030, water scarcity is intensified across all scenarios except for SSP5. This is associated with increased (decreased) sectoral water demand, together with reduced (increased) total water supply under each scenario. Sensitivity analyses, by keeping key parameters constant, highlight the distinct roles of climatological and socioeconomic factors in shaping the timing and variability of water scarcity, and offer valuable policy implications. The SD framework in this study has unique strength in simulating the temporal evolutions of sectoral water demands and their complex interactions, and thus can improve the realism of water demand estimation, which is essential for water resource modeling and assessment studies.

Driving effects of ecosystems and social systems on water supply and demand in semiarid areas

Book Reviews

Scale matters: Effects of temporal and spatial data resolution on water scarcity assessments

Lower reaches of the Amu Darya River Basin (LADB) is one of the typical regions which is facing the problem of water shortage in Central Asia. During the past decades, water resources demand far exceeds that supplied by the mainstream of the Amu Darya River, and has resulted in a continuous decrease in the amount of water flowing into the Aral Sea. Clarifying the dynamic relationship between the water supply and demand is important for the optimal allocation and sustainable management of regional water resources. In this study, the relationship and its variations between the water supply and demand in the LADB from the 1970s to 2010s were analyzed by detailed calculation of multi-users water demand and multi-sources water supply, and the water scarcity indices were used for evaluating the status of water resources utilization. The results indicated that (1) during the past 50 years, the average total water supply (TWS) was 271.88 × 108 m3/y, and the average total water demand (TWD) was 467.85 × 108 m3/y; both the volume of water supply and demand was decreased in the LADB, with rates of −1.87 × 108 m3/y and −15.59 × 108 m3/y. (2) percentages of the rainfall in TWS were increased due to the decrease of inflow from the Amu Darya River; percentage of agriculture water demand was increased obviously, from 11.04% in the 1970s to 44.34% in 2010s, and the water demand from ecological sector reduced because of the Aral Sea shrinking. (3) the supply and demand of water resources of the LADB were generally in an unbalanced state, and water demand exceeded water supply except in the 2010s; the water scarcity index decreased from 2.69 to 0.94, indicating the status changed from awful to serious water scarcity. A vulnerable balanced state has been reached in the region, and that water shortages remain serious in the future, which requires special attention to the decision-makers of the authority.

Rising agricultural water scarcity in China is driven by expansion of irrigated cropland in water scarce regions

Climate change will affect the water resources system, on global and regional levels. Over the past thirty years, the High Atlas Mountains in Morocco have experienced severe droughts, which causes a decrease in water supply that affects both agriculture and the urban water system. In this paper, we assess the impact of climate change and socio-economic activities on water supply and demand in the Ourika watershed (High Atlas of Morocco), then we evaluate the efficiency and sustainability of regional adaptation strategies for water supply management. For this, we simulate and analyze the future water situation using the statistical downscaling model (SDSM) and the water assessment and planning tool (WEAP). After the model’s calibration and validation, the precipitation, minimum (Tmin) and maximum (Tmax) temperatures, water demand and unmet water demand were projected for 2100 horizon, using different climate change scenarios. The results revealed that the model’s performance, calibration and validation were found to be satisfactory. The analysis shows that the mean precipitation will decrease by 49.25% and 34.61% by 2100, under A2 and B2 emission scenarios of the Intergovernmental Panel on Climate Change (IPCC). The projected mean Tmax and Tmin will be warmer than the baseline period, with Tmax increasing by 4.2 °C (A2) and 3.6 °C (B2), and Tmin by 3.5 °C (A2) and 2.9 °C (B2) by 2100. The results also show that water demand and the unmet water demand will increase in all scenarios, the pressure on water resources will increase, leading to water scarcity. The results reveal that, under the influence of climate change, future unmet water demand is expected to reach 64 million cubic meters (MCM) by 2100. The results demonstrate that the assessments of the proposed adaptation strategies are effective, but not sufficient to ensure water sustainability for the Ourika watershed.

Assessing future water scarcity in China under climate and socioeconomic change: Can inter-basin transfers mitigate rising stress?

As water scarcity becomes the new norm in the Western United States, states such as California have increased their efforts to improve water resilience. Achieving water security under climate change and population growth requires an integrated multi-sectoral approach, where adaptation strategies combine water supply and demand management interventions. Yet, most studies consider supply-side and demand-side water management strategies separately. Further, publicly available data to assess the effectiveness of these strategies and their dependency on individual and collective human behavior is often hard to find and unstructured. Water conservation efforts are driven by water scarcity and policy requirements, with conservation targets and water use restrictions often designed assuming a degree of rationality of human behavior and based on cost-effective options and ease of implementation. In this work, we develop a data-driven analysis aimed at evaluating historical synergies and possible trade-offs between water supply and demand management strategies in California. Our analysis is based on CaRDS – the statewide California Residential water Demand and Supply open dataset, which contains monthly values of water supply and residential water demand for 404 water suppliers in California from 2013 to 2021. In this time span, Californian water agencies had to adapt and mitigate the effects of two droughts (in 2012-2016 and 2020-2022) through residential water demand reductions, as well as address rapid changes in demand associated with the global COVID-19 pandemic (2020). Our trade-off analysis integrates the following three sequential steps: (i) trend analysis – we use Random Forest regression to control for seasonal factors (i.e., temperature and precipitation) that affect water supply and demand at the utility scale; (ii) multi-criteria trade-off analysis – we examine the temporal relationship between water supply and demand by utilizing Dynamic Time Warping to identify trade-offs and management patterns. Next, we cluster water suppliers in 6 groups based on their combined management patterns; (iii) and driver analysis – we utilize explainable Machine Learning by combining SHAP (Shapley values) with LGBM (Light Gradient Boosting Method) to identify the drivers of each cluster. Potential drivers include climatic region, water supply portfolio, indoor vs. outdoor water use, local and state policies,  population, supplier size, and income. We finally validate the results of our analysis by comparing our findings with responses from water supplier interviews carried out in 2017 and reveal differences between intended and actual water management outcomes. This research contributes insights into the combined effects of policies on water supply and demand at a statewide level. Further it facilitates the formulation of adaptive resilience strategies for human actors in water management and decision makers alike to address vulnerability of small and large water systems to a rapidly changing climate and a society with non-linear changes in human behavior.

Abstract. Increasing population and socio-economic development have put great pressure on water resources of the Yellow River (YR) basin. The anticipated climate and socio-economic changes may further increase water stress. Many studies have investigated the changes in renewable water resources under various climate change scenarios, but few have considered the joint pressure from both climate change and socio-economic development. In this study, we assess water scarcity under various socio-economic pathways with emphasis on the impact of water scarcity on food production. The water demands in the 21st century are estimated based on the newly developed shared socio-economic pathways (SSPs) and renewable water supply is estimated using the climate projections under the Representative Concentration Pathway (RCP) 8.5 scenario. The assessment predicts that the renewable water resources would decrease slightly then increase. The domestic and industrial water withdrawals are projected to increase in the next a few decades and then remain at the high level or decrease slightly during the 21st century. The increase in water withdrawals will put the middle and lower reaches in a condition of severe water scarcity beginning in the next a few decades. If 40 % of the renewable water resources were used to sustain ecosystems, a portion of irrigated land would have to be converted to rain-fed agriculture, which would lead to a 2–11 % reduction in food production. This study highlights the links between water, food and ecosystems in a changing environment and suggests that trade-offs should be considered when developing regional adaptation strategies.

Arid and semi-arid regions such as the Middle East and North Africa are increasingly suffering from water scarcity, exacerbated by climate change and population growth. This trend calls for new strategies for managing water demand and supply to face global changes in social-economic development, water system expansions, and cross-border differences.In this work, we explore the potential to mitigate the existing conflicts over the Nile River Basin, interconnecting water demand and supply using novel technological solutions, such as desalination and aquaponics, combined with traditional uses (i.e., groundwater extraction and water reuse). We analyse the complex dynamics and tradeoffs between energy production and irrigation water supply in Ethiopia, Sudan, and Egypt. We propose innovative portfolios of interventions that combine the coordinated operation of large water dams (i.e., the Grand Ethiopian Renaissance, Merowe, and High Aswan) and the main irrigation diversions with smart water demand management options. Desalination involves the process of removing salt and other minerals from seawater, making it suitable for irrigation and other domestic uses. Aquaponics involves the cultivation of fish and plants in a symbiotic environment, with the waste produced by the fish providing nutrients for the plants and the plants purifying the water for the fish. This technology can be an efficient and sustainable way to produce food with very low water consumption.Our approach is used to study current and future tradeoffs, generating solutions that are efficient and resilient to future hydroclimatic and demographic scenarios. We first quantified the impacts of dynamically downscaled and bias-adjusted climate projections for three Representative Concentrated Pathways (i.e., RCP2.6, RCP4.5 and RCP8.5) on the runoffs of the main tributaries of the Nile. We also considered stochastic projections of water demand based on Shared Socioeconomic Pathways (SSPs), and a strategic model that reallocates crops according to future climatic and demographic scenarios, according to a balanced diet and agricultural intensification strategy to generate a positive impact on food self-sufficiency.Our results show that the Nile River Basin features both strong tradeoffs and synergies across riparian countries, with the irrigation supply in Sudan playing a major role in allocating water between competing sectors. The results show a decrease of up to 20% of the Nile River's runoff and a doubling of the Egyptian municipal demand in the most severe scenario that leads to exacerbating tensions between the three countries. Notably, the potential reduction of the Egyptian water demand through different combinations of aquaponics, desalination, reuse, and groundwater pumping in the Nile Delta, along with a substantial decrease in Sudan irrigation demand through crop reallocation, can contribute to mitigating existing and future conflicts. Further technological improvements are needed for attaining large water demand reductions via soilless agriculture and desalination, which today cannot completely substitute reuse and groundwater contributions, whose high exploitation can induce relevant environmental risks.

Trajectory in water scarcity and potential water savings benefits in the Yellow River basin

The Syr Darya River Basin is a transboundary glacier-fed river system, supporting the livelihoods of millions of people across Central Asia. The sustainable allocation of water resources in this basin has become a pressing concern due to the increasing demands coupled with environmental degradation and climate uncertainty. Consequently, developing robust water allocation mechanisms that acknowledge the Water-Energy-Food-Environment-Nexus (WEFE) is vital for sustaining human and ecosystem needs. This study scrutinizes the relationship between upstream and downstream water users in the upper Syr Darya Basin, which encompasses the Uzbek and Kyrgyz Republics, including the Fergana Valley, Central Asia's "breadbasket”.Whereas the individual effect of climate change on either water demand or supply is widely studied, the interaction between these two, considering local nexus-related systemic dependencies, requires a better understanding to improve sustainable water allocation in the region. For example, climate change may reduce upstream hydropower demands in winter, favouring water supplies in summer elsewhere. Recognizing the intricate relationships among water, energy, food, and the environment, especially in regions with geopolitical complexities like Central Asia, we aim to uncover the feedback mechanisms shaping the WEFE nexus by defining and assessing storylines representing climate and socio-economic change in a coupled cyrospheric-hydrological and water allocation model (SPHY-WEAP).First, we assess the influence of climate change on reservoir inflows of Toktogul and Andijan, key reservoirs regulating water availability within Ferghana Valley. We force the model with CMIP6 climate simulations to assess changes in reservoir storage and inflows for multiple future time horizons, thereby focussing on potential storage gaps as glaciers shrink and its effect on existing reservoir release patterns. Secondly, we assess the future evolution of water, energy, food, and environmental demands under the combined influence of climate and related socio-economic changes. Hereto, we define representative storylines, integrating insights from policy documents and local stakeholder consultations to depict plausible future pathways. Finally, forcing the coupled SPHY-WEAP allocation model with quantitative storylines, we explore local feedbacks in the intricate relationship between climate change and water availability, supply, and demands. Specific focus will be on how the equilibrium between water supply and demand shifts for varying storylines, thereby pinpointing tipping points where water demands can no longer be met for a given season or throughout the year.The results of this study are expected to provide a systematic assessment of water-energy-food-environment storylines, revealing how these storylines either facilitate or impede sustainable water management practices in the basin. This study aligns with SDG 6 and lays the groundwork for promoting efficient water allocation strategies and decision-making under climate change to promote transboundary cooperation and long-term water security for all.