Impacts of Climate Change on Water Resources: Assessment and Modeling—Second Edition

1. Introduction to This Special Issue

Water resources are an essential strategic asset for sustainable development and a determinant of human life and socioeconomic progress [1]. However, water resources are increasingly vulnerable due to the effects of climate change, which influences both their availability and quality [2]. Many aspects of the environment, economy, and wider society are dependent upon water resources, and changes in the hydrological resource base have the potential to severely affect environmental quality, economic development, and social well-being [3]. The IPCC Sixth Assessment Report also concludes that the intensification of the hydrological cycle due to anthropogenic climate change has multifaceted and severe effects on cultural, economic, social, and political pathways [4]. While scientists across the world are devoting considerable attention to assessing and modeling water resources in the context of climate change, many uncertainties persist. The wider problem of uncertainty pertaining to the effects of climate change on water resources has been discussed in a number of studies, including [5,6,7]. However, the majority of research typically concludes with an assessment of the impacts of climate change; studies typically do not proceed to identify possible adaptation measures. Modeling the effect of climate change on hydrology and water resources using climate models is hindered by the high uncertainty of outputs, which increases with every additional step [8].

This Special Issue was announced to advance research in this field. Researchers were invited to present their results related to the assessment and modeling of hydrological processes and water resources under climate change conditions, to regularities in spatiotemporal variability with respect to water management, and to related threats.

In our opinion, the intended goal of this Special Issue was successfully reached: ten original papers have been published. Authors from Croatia, Greece, India, Iran, Kazakhstan, Mexico, Pakistan, South Korea, Taiwan, and the United States presented their research, and their valuable contributions represent diversified analyses in terms of the investigated topics and applied methodologies, encompassing both measurements and mathematical modeling.

To help readers familiarize themselves with the contents of this Special Issue, we provide a brief overview of the articles published below. The articles are arranged in the order in which they were published.

2. Overview of Contributions to This Special Issue

Dalai et al. investigated the impact of future climate change on irrigation water requirements. The coastal districts of Odisha, located in Eastern India, specifically within the Phulnakhara distributary’s command area of the main Puri canal system, were taken as a case study. Climatic parameters such as rainfall, minimum and maximum temperature, and daily solar radiation on the Earth’s surface up to 2051, as well as land use and land cover maps of the study area, were analyzed. Based on field investigations conducted during the wet (kharif) and dry (rabi) seasons of 2019–2020 and 2020–2021, the optimal future irrigation water needs for these two seasons under changing climatic conditions were assessed. Emphasis was placed on the combined use of surface and groundwater resources. The authors concluded that peak irrigation demand would occur in the kharif season of 2042–2043 and the rabi season of 2044–2045. Moreover, a significant decline in groundwater levels was anticipated—ranging from 1.23 to 1.42 m below ground level during the kharif season and from 1.46 to 1.64 m during the rabi season—over the next 30 years (2021–2022 to 2050–2051), with the most pronounced groundwater table decline being projected for the years 2042–2043 (kharif) and 2044–2045 (rabi), respectively. The findings of this study underline an urgent need for sustainable water resource management strategies in the region [contribution 1].

Morales Martínez et al. analyzed the spatiotemporal evolution of precipitation in the State of Guanajuato, Mexico, by examining monthly series from 65 meteorological stations recorded in the multi-annual period 1981–2016. Only stations with more than 30 years of continuous data and less than 10% missing values were investigated. Multiple Imputation by Chained Equations (MICE) with predictive mean matching was applied to handle missing data, preserving the statistical properties of the time series as validated by Kolmogorov–Smirnov tests (p = 1.000 for all stations). Homogeneity was assessed using Pettitt, SNHT, Buishand, and von Neumann tests, classifying 60 stations (93.8%) as useful, 3 (4.7%) as doubtful, and 2 (3.1%) as suspicious for monthly analysis. Breakpoints were predominantly clustered around periods of instrumental changes (2000–2003 and 2011–2014), underscoring the necessity of homogenization prior to trend analysis. The Trend-Free Pre-Whitening Mann–Kendall (TFPW-MK) test was applied to account for significant first-order autocorrelation (ρ1 > 0.3) present in all series.

The analysis revealed no statistically significant monotonic trends in monthly precipitation at any of the 65 stations (α = 0.05). A total of 75.4% of the stations showed slight non-significant increasing tendencies, and 24.6% showed non-significant decreasing tendencies, with negligible and statistically indistinguishable from zero Sen’s slope estimates. No discernible spatial patterns or correlation between trend magnitude and altitude (ρ = −0.022, p > 0.05) were found, indicating region-wide precipitation stability during the study period. Based on these findings, the authors suggested that the severe water crisis in the study area cannot be attributed to declining precipitation; rather, it is to be attributed to anthropogenic factors, primarily unsustainable groundwater extraction for agriculture [contribution 2].

Bachmann investigated changes in selected meteorological variables, including mean, minimum, and maximum air temperatures, dew points, and precipitation over 1033 lakes in the coterminous United States over the summer months in the multi-annual period 1981–2024. Near-surface water temperatures in the same lakes were calculated with equations using 8-day mean daily air temperatures, latitude, elevation, and the year of sampling. It was concluded that over the past 43 years, there have been changes in air temperatures over many lakes in the United States, with generally increasing trends for minimum air temperatures and mean air temperatures from June to September. The author indicated that the greatest increases had been in daily minimum air temperatures, followed by the mean daily air temperatures. Maximum daily air temperatures did not show a statistically significant increase for the summer season but did show a significant increase for September. It was determined that along with the changes in the climate, the near-surface water temperatures of the lakes of the United States, on average, showed increases of 0.33 °C per decade for the four summer months and increases for each of the summer months [contribution 3].

Bonacci et al. examined variations in the hydrological regime of the Krapina River in Croatia. Analyses of hydrological data were collected from the Kupljenovo gauge in the multi-annual period 1964–2023. Based on minimum mean daily discharges, a statistically insignificant increasing trend was detected, while the mean annual and maximum annual mean daily discharges exhibited statistically insignificant declines. While a non-significant decreasing trend in annual precipitation totals was determined, the mean annual air temperatures demonstrated a statistically significant increasing trend, with a pronounced intensification from 1986. The annual runoff coefficients series exhibited a statistically insignificant downward trend. Furthermore, application of the New Drought Index (NDI) revealed a marked increase in the frequency of strong and extreme droughts in the study area since 2000 [contribution 4].

Stamou et al. assessed the potentially significant climate hazards of the Almopeos Dam and reservoir (D&R) systems in Greece and—employing a methodology consistent with the technical guidelines of the European Commission on the climate proofing of infrastructure—analyzed the vulnerability of these systems to climate change. In accordance with these guidelines, the authors (1) described the Almopeos D&R system, which is in the design stage; (2) assessed the effects of climate change; and (3) carried out a vulnerability assessment using literature surveys, expert opinions, and climate models. On these grounds, they determined that temperature increase, extreme heat, precipitation decrease, aridity, droughts, and extreme precipitation and flooding were potentially significant climate hazards. It was also concluded that the vulnerability assessment allowed for the identification of climate indicators, the most important effects, the most vulnerable components of the D&R system that can be used in the risk assessment, and the identification of significant climate hazards. These provide a basis for proposing targeted adaptation strategies aiming to reduce these risks to an acceptable level [contribution 5].

Kim et al. attempted to evaluate the prospective alterations in rainfall quantiles in South Korea due to ongoing climate change. In their study, a multi-model ensemble (MME) derived from 23 Global Climate Models (GCMs) associated with the Coupled Model Intercomparison Project Phase 6 (CMIP6) under four Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5) was employed. With the use of historical rainfall data from simulations (1985–2014), future projections (2015–2044, 2043–2072, and 2071–2100) were analyzed across 615 sites in South Korea.

The results obtained by the authors proved substantial temporal and spatial changes, including increased precipitation in central inland and eastern coastal regions of the country, with peak monthly increases exceeding 40 mm under high-emission scenarios.

Under the SSP2-4.5 and SSP5-8.5 scenarios, the 100-year rainfall quantile was forecasted to increase by over 40% across the country. These findings underline growing challenges for water resource management, infrastructure planning, disaster mitigation, and climate adaptation strategies in South Korea [contribution 6].

Chen et al. analyzed hydrological data from 65 gauges in Taiwan, with over 30 years of records between 1960 and 2022, to characterize the annual mean number of low-flow days, flow variability, and the seasonality of low-flow occurrences. Indices such as the intermittency ratio, the Richards–Baker flashiness index, and six-month seasonality of the dry period (SD6) were applied, and trends in these indices were evaluated using the Mann–Kendall test. This study revealed that nearly 70% of the stations had an intermittency ratio of less than 0.1, although the number of low-flow days had significantly increased over time. Additionally, gauges located in the southwestern rivers exhibited higher flow variability. However, the flow variability trends were not statistically significant. Low-flow events were predominantly recorded during the dry season, with 68% of the gauges experiencing such events between January and March. The findings on flow characteristics and their long-term trends can provide references for future river management and water resource planning [contribution 7].

Asif et al. analyzed the projections of five general circulation models (GCMs) from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to evaluate variations in the seasonal and annual patterns of future precipitation over the northern highlands of Pakistan. The analysis focused on precipitation variations projected for the near future (2021–2050), in comparison to the historical climate records (1985–2014), utilizing two combined scenarios from the Shared Socioeconomic Pathways and the Representative Concentration Pathways (SSP2-4.5 and SSP5-8.5).

This research revealed noticeable seasonal and annual variations in precipitation across the study area. The average annual precipitation is expected to decrease in both scenarios, with SSP2-4.5 expecting a reduction of −21.42% and SSP5-8.5 expecting a decrease of −22.43% compared to the historical average precipitation. While the seasonal precipitation patterns were similar in both scenarios, the changes were more prominent in spring and summer. Both SSPs predict a 15% decrease in summer precipitation, while SSP2-4.5 and SSP5-8.5 predict a 5% and 4% decrease in spring precipitation, respectively. It is expected that changes can result in more frequent and intense periods of drought, which might have adverse effects on agriculture, human health, the environment, and hydropower generation. The authors pointed out that understanding these changes in precipitation is of key importance in developing strategies for adapting to the climate, assuring water security, and promoting sustainable agricultural practices in the northern highlands of Pakistan [contribution 8].

Birimbayeva et al. investigated the impacts of climate change on drought conditions in the Zhaiyk–Caspian, Tobyl–Torgai, Yesil, and Nura–Sarysu water management basins in Kazakhstan. Using hydro-meteorological data recorded in 45 hydrological gauges and 46 meteorological stations up to 2021, the standardized precipitation index (SPI) and streamflow drought index (SDI) were calculated to assess the drought intensity and duration in the respective basins. The results indicated an increase in the intensity and frequency of drought periods in the analyzed areas, which the authors associated with climate variability. These unfavorable changes could have serious implications for agriculture, ecological balance, and water resources of Kazakhstan. The findings of this study could prove be useful in the development of climate change adaptation strategies and the sustainable management of natural resources in the region [contribution 9].

Afsari et al. explored the impacts of climate change on the number of dry days and very heavy precipitation days within Iran’s metropolises, namely, Tehran, Mashhad, Isfahan, Karaj, Shiraz, and Tabriz, aiming to comprehensively understand how climate change would affect precipitation patterns in these cities. The sixth phase of the Coupled Model Intercomparison Project (CMIP6) Global Circulation Models (GCMs) was applied to predict future precipitation conditions under various Shared Socioeconomic Pathways (SSPs) from 2025 to 2100. It was found that the SSP126 scenario typically resulted in the highest number of dry days, suggesting that under lower emission scenarios, precipitation events would become less frequent but more intense. Conversely, SSP585 generally led to the lowest number of dry days. Higher emission scenarios (SSP370, SSP585) consistently revealed an increase in the number of very heavy precipitation days across all analyzed cities, indicating a trend towards more extreme weather events along with the rising emissions. The authors emphasized that these insights were crucial for urban planners, policymakers, and stakeholders in developing effective adaptation and mitigation strategies to address anticipated climatic changes [contribution 10].

3. Conclusions

The articles published in this Special Issue focus on a wide variety of topics pertaining to the assessment and modeling of the effects of climate change on elements of the hydrological cycle and water resources in relation to both the natural environment and socio-economic development. A variety of methods have been applied to assess and model the projected changes, and these papers can be divided into three main groups.

The largest group consists of six articles dealing with changes in selected meteorological variables, notably precipitation and temperature. It includes the contribution from India, in which climate projections considering various climate scenarios were utilized to determine the future irrigation demands in a command area located in the coastal districts of Odisha, as well as the contribution from Iran, where the impacts of climate change on the number of dry days and very heavy precipitation days within Iran’s metropolises were explored. Moreover, the study from Mexico, which analyzed the spatial and temporal evolution of precipitation in the State of Guanajuato, is included in this group, along with the study from Pakistan, which examined the variations in the spatiotemporal patterns of the projected annual and seasonal precipitation in the Northern Highlands under the SSP2-4.5 and SSP5-8.5 scenarios. Also included within this first group are the paper from South Korea, evaluating the prospective alterations in rainfall quantiles across that country, and the paper from the United States, focusing on the effects of selected meteorological variables on lakes in the USA.

The second group includes three articles focusing on the projected alterations in river flow under the conditions of climate change. These include the contribution from Croatia, demonstrating a declining streamflow trend in the Krapina River near Kupljenovo in the northern part of the country; the paper from Kazakhstan, analyzing the spatial and temporal variability of hydrological drought regimes in the lowland rivers of that country; and the article from Taiwan, which studied the intermittency of Taiwanese rivers in relation to projected climate change scenarios.

In the third group we have the contribution from Greece, in which the Almopeos Dam in Northern Greece was used as a case study by which to assess the vulnerability of dams and reservoir systems to climate change in the Mediterranean region.

Naturally, the papers published in this Special Issue do not exhaust the topics related to the effects of climate change on water resources. However, they all shed new light on this problem and confirm that climate change has profound effects on every aspect of the global water cycle and various domains of human life, as concluded by [4,9,10] and other studies. Furthermore, these contributions indicate future research directions in the assessment and modeling of water resources at different scales. We strongly believe that these findings will be of interest to both scientists and practitioners dealing with alterations in the hydrological cycle under the conditions of climate change.