Hydropower Relicensing and Climate Change1

Viers, Joshua H., 2011. Hydropower Relicensing and Climate Change. Journal of the American Water Resources Association (JAWRA) 47(4):655‐661. DOI: 10.1111/j.1752‐1688.2011.00531.x Abstract: Hydropower represents approximately 20% of the world’s energy supply, is viewed as both vulnerable to global climate warming and an asset to reduce climate‐altering emissions, and is increasingly the target of improved regulation to meet multiple ecosystem service benefits. It is within this context that the recent decision by the United States Federal Energy Regulatory Commission to reject studies of climate change in its consideration of reoperation of the Yuba‐Bear Drum‐Spaulding hydroelectric facilities in northern California is shown to be poorly reasoned and risky. Given the rapidity of climate warming, and its anticipated impacts to natural and human communities, future long‐term fixed licenses of hydropower operation will be ill prepared to adapt if science‐based approaches to incorporating reasonable and foreseeable hydrologic changes into study plans are not included. The licensing of hydroelectricity generation can no longer be issued in isolation due to downstream contingencies such as domestic water use, irrigated agricultural production, ecosystem maintenance, and general socioeconomic well‐being. At minimum, if the Federal Energy Regulatory Commission is to establish conditions of operation for 30‐50 years, licensees should be required to anticipate changing climatic and hydrologic conditions for a similar period of time.     

Scenario‐Based and Scenario‐Neutral Assessment of Climate Change Impacts on Operational Performance of a Multipurpose Reservoir

Scenario‐based and scenario‐neutral impacts assessment approaches provide complementary information about how climate change‐driven effects on streamflow may change the operational performance of multipurpose dams. Examining a case study of Cougar Dam in Oregon, United States, we simulated current reservoir operations under scenarios of plausible future hydrology. Streamflow projections from the CGCM3.1 general circulation model for the A1B emission scenario were used to generate stochastic reservoir inflows that were then further perturbed to simulate a potentially drier future. These were then used to drive a simple reservoir model. In the scenario‐based analysis, we found reservoir operations are vulnerable to climate change. Increases in fall and winter inflow could lead to more frequent flood storage, reducing flexibility to store incoming flood flows. Uncertainty in spring inflow volume complicates projection of future filling performance. The reservoir may fill more or less often, depending on whether springs are wetter or drier. In the summer, drawdown may occur earlier to meet conservation objectives. From the scenario‐neutral analysis, we identified thresholds of streamflow magnitude that can predict climate change impacts for a wide range of scenarios. Our results highlight projected operational challenges for Cougar Dam and provide an example of how scenario‐based and scenario‐neutral approaches may be applied concurrently to assess climate change impacts.     

Climate Change Impacts on Water Resources in the Upper Blue Nile River Basin,Ethiopia1

Kim, Ungtae and Jagath J. Kaluarachchi, 2009. Climate Change Impacts on Water Resources in the Upper Blue Nile River Basin, Ethiopia. Journal of the American Water Resources Association (JAWRA) 45(6):1361‐1378. Abstract: Climate change affects water resources availability of international river basins that are vulnerable to runoff variability of upstream countries especially with increasing water demands. The upper Blue Nile River Basin is a good example because its downstream countries, Sudan and Egypt, depend solely on Nile waters for their economic development. In this study, the impacts of climate change on both hydrology and water resources operations were analyzed using the outcomes of six different general circulation models (GCMs) for the 2050s. The outcomes of these six GCMs were weighted to provide average future changes. Hydrologic sensitivity, flow statistics, a drought index, and water resources assessment indices (reliability, resiliency, and vulnerability) were used as quantitative indicators. The changes in outflows from the two proposed dams (Karadobi and Border) to downstream countries were also assessed. Given the uncertainty of different GCMs, the simulation results of the weighted scenario suggested mild increases in hydrologic variables (precipitation, temperature, potential evapotranspiration, and runoff) across the study area. The weighted scenario also showed that low‐flow statistics and the reliability of streamflows are increased and severe drought events are decreased mainly due to increased precipitation. Joint dam operation performed better than single dam operation in terms of both hydropower generation and mean annual storage without affecting the runoff volume to downstream countries, but enhancing flow characteristics and the robustness of streamflows. This study provides useful information to decision makers for the planning and management of future water resources of the study area and downstream countries.     

Integrated Modeling of Water Supply and Demand under Management Options and Climate Change Scenarios in Chifeng City,China

Water resource management is becoming increasingly challenging in northern China because of the rapid increase in water demand and decline in water supply due to climate change. We provide a case study demonstrating the importance of integrated watershed management in sustaining water resources in Chifeng City, northern China. We examine the consequences of various climate change scenarios and adaptive management options on water supply by integrating the Soil and Water Assessment Tool and Water Evaluation and Planning models. We show how integrated modeling is useful in projecting the likely effects of management options using limited information. Our study indicates that constructing more reservoirs can alleviate the current water shortage and groundwater depletion problems. However, this option is not necessarily the most effective measure to solve water supply problems; instead, improving irrigation efficiency and changing cropping structure may be more effective. Furthermore, measures to increase water supply have limited effects on water availability under a continuous drought and a dry‐and‐warm climate scenario. We conclude that the combined measure of reducing water demand and increasing supply is the most effective and practical solution for the water shortage problems in the study area.     

Impacts of Multiple Stresses on Water Demand and Supply Across the Southeastern United States1

Abstract: Assessment of long‐term impacts of projected changes in climate, population, and land use and land cover on regional water resource is critical to the sustainable development of the southeastern United States. The objective of this study was to fully budget annual water availability for water supply (precipitation ? evapotranspiration + groundwater supply + return flow) and demand from commercial, domestic, industrial, irrigation, livestock, mining, and thermoelectric uses. The Water Supply Stress Index and Water Supply Stress Index Ratio were developed to evaluate water stress conditions over time and across the 666 eight‐digit Hydrologic Unit Code basins in the 13 southeastern states. Predictions from two Global Circulation Models (CGC1 and HadCM2Sul), one land use change model, and one human population model, were integrated to project future water supply stress in 2020. We found that population increase greatly stressed water supply in metropolitan areas located in the Piedmont region and Florida. Predicted land use and land cover changes will have little effect on water quantity and water supply‐water demand relationship. In contrast, climate changes had the most pronounced effects on regional water supply and demand, especially in western Texas where water stress was historically highest in the study region. The simulation system developed by this study is useful for water resource planners to address water shortage problems such as those experienced during 2007 in the study region. Future studies should focus on refining the water supply term to include flow exchanges between watersheds and constraints of water quality and environmental flows to water availability for human use.     

Incorporating Climate Change Into Water Resources Planning in England and Wales1

Arnell, Nigel W., 2011. Incorporating Climate Change Into Water Resources Planning in England and Wales. Journal of the American Water Resources Association (JAWRA) 47(3):541‐549. DOI: 10.1111/j.1752‐1688.2011.00548.x Abstract: Public water supplies in England and Wales are provided by around 25 private‐sector companies, regulated by an economic regulator (Ofwat) and environmental regulator (Environment Agency). As part of the regulatory process, companies are required periodically to review their investment needs to maintain safe and secure supplies, and this involves an assessment of the future balance between water supply and demand. The water industry and regulators have developed an agreed set of procedures for this assessment. Climate change has been incorporated into these procedures since the late 1990s, although has been included increasingly seriously over time and it has been an effective legal requirement to consider climate change since the 2003 Water Act. In the most recent assessment in 2009, companies were required explicitly to plan for a defined amount of climate change, taking into account climate change uncertainty. A “medium” climate change scenario was defined, together with “wet” and “dry” extremes, based on scenarios developed from a number of climate models. The water industry and its regulators are now gearing up to exploit the new UKCP09 probabilistic climate change projections – but these pose significant practical and conceptual challenges. This paper outlines how the procedures for incorporating climate change information into water resources planning have evolved, and explores the issues currently facing the industry in adapting to climate change.     

Estimating Potential Climate Change Effects on the Upper Neuse Watershed Water Balance Using the SWAT Model

Climate change poses water resource challenges for many already water stressed watersheds throughout the world. One such watershed is the Upper Neuse Watershed in North Carolina, which serves as a water source for the large and growing Research Triangle Park region. The aim of this study was to quantify possible changes in the watershed’s water balance due to climate change. To do this, we used the Soil and Water Assessment Tool (SWAT) model forced with different climate scenarios for baseline, mid‐century, and end‐century time periods using five different downscaled General Circulation Models. Before running these scenarios, the SWAT model was calibrated and validated using daily streamflow records within the watershed. The study results suggest that, even under a mitigation scenario, precipitation will increase by 7.7% from the baseline to mid‐century time period and by 9.8% between the baseline and end‐century time period. Over the same periods, evapotranspiration (ET) would decrease by 5.5 and 7.6%, water yield would increase by 25.1% and 33.2%, and soil water would increase by 1.4% and 1.9%. Perhaps most importantly, the model results show, under a high emission scenario, large seasonal differences with ET estimated to decrease by up to 42% and water yield to increase by up to 157% in late summer and fall. Planning for the wetter predicted future and corresponding seasonal changes will be critical for mitigating the impacts of climate change on water resources.     

Assessment of Future Climate Change Impact on Water Quality of Chungju Lake,South Korea,Using WASP Coupled with SWAT

This study is to evaluate the future potential impact of climate change on the water quality of Chungju Lake using the Water Quality Analysis Simulation Program (WASP). The lake has a storage capacity of 2.75 Gm³, maximum water surface of 65.7 km², and forest‐dominant watershed of 6,642 km². The impact on the lake from the watershed was evaluated by the Soil and Water Assessment Tool (SWAT). The WASP and SWAT were calibrated and validated using the monthly water temperatures from 1998 to 2003, lake water quality data (dissolved oxygen, total nitrogen [T‐N], total phosphorus [T‐P], and chlorophyll‐a [chl‐a]) and daily dam inflow, and monthly stream water quality (sediment, T‐N, and T‐P) data. For the future climate change scenario, the MIROC3.2 HiRes A1B was downscaled for 2020s, 2050s, and 2080s using the Change Factor statistical method. The 2080s temperature and precipitation showed an increase of +4.8°C and +34.4%, respectively, based on a 2000 baseline. For the 2080s watershed T‐N and T‐P loads of up to +87.3 and +19.6%, the 2080s lake T‐N and T‐P concentrations were projected to be 4.00 and 0.030 mg/l from 2.60 and 0.016 mg/l in 2000, respectively. The 2080s chl‐a concentration in the epilimnion and the maximum were 13.97 and 52.45 μg/l compared to 8.64 and 33.48 μg/l in 2000, respectively. The results show that the Chungju Lake will change from its mesotrophic state of 2000 to a eutrophic state by T‐P in the 2020s and by chl‐a in the 2080s. Editor's note: This paper is part of a featured series on Korean Hydrology. The series addresses the need for a new paradigm of river and watershed management for Korea due to climate and land use changes.     

Impacts of Climate Change on Hydrology and Water Resources in the Boise and Spokane River Basins1

Jin, Xin and Venkataramana Sridhar, 2012. Impacts of Climate Change on Hydrology and Water Resources in the Boise and Spokane River Basins. Journal of the American Water Resources Association (JAWRA) 48(2): 197‐220. DOI: 10.1111/j.1752‐1688.2011.00605.x Abstract: In the Pacific Northwest, warming climate has resulted in a lengthened growing season, declining snowpack, and earlier timing of spring runoff. This study characterizes the impact of climate change in two basins in Idaho, the Spokane River and the Boise River basins. We simulated the basin‐scale hydrology by coupling the downscaled precipitation and temperature outputs from a suite of global climate models and the Soil and Water Assessment Tool (SWAT), between 2010 and 2060 and assess the impacts of climate change on water resources in the region. For the Boise River basin, changes in precipitation ranged from ?3.8 to 36%. Changes in temperature were expected to be between 0.02 and 3.9°C. In the Spokane River region, changes in precipitation were expected to be between ?6.7 and 17.9%. Changes in temperature appeared between 0.1 and 3.5°C over a period of the next five decades between 2010 and 2060. Without bias‐correcting the simulated streamflow, in the Boise River basin, change in peak flows (March through June) was projected to range from ?58 to +106 m³/s and, for the Spokane River basin, the range was expected to be from ?198 to +88 m³/s. Both the basins exhibited substantial variability in precipitation, evapotranspiration, and recharge estimates, and this knowledge of possible hydrologic impacts at the watershed scale can help the stakeholders with possible options in their decision‐making process.     

The Sensitivity of California Water Resources to Climate Change Scenarios1

Abstract: Using the latest available General Circulation Model (GCM) results we present an assessment of climate change impacts on California hydrology and water resources. The approach considers the output of two GCMs, the PCM and the HadCM3, run under two different greenhouse gas (GHG) emission scenarios: the high emission A1fi and the low emission B1. The GCM output was statistically downscaled and used in the Variable Infiltration Capacity (VIC) macroscale distributed hydrologic model to derive inflows to major reservoirs in the California Central Valley. Historical inflows used as inputs to the water resources model CalSim II were modified to represent the climate change perturbed conditions for water supply deliveries, reliability, reservoir storage and changes to variables of environmental concern. Our results show greater negative impacts to California hydrology and water resources than previous assessments of climate change impacts in the region. These impacts, which translate into smaller streamflows, lower reservoir storage and decreased water supply deliveries and reliability, will be especially pronounced later in the 21st Century and south of the San Francisco bay Delta. The importance of considering how climate change impacts vary for different temporal, spatial, and institutional conditions in addition to the average impacts is also demonstrated.     

Climate Change Impacts on Irrigation Water Needs in the jaguaribe River Basin1

Gondim, Rubens S., Marco A.H. de Castro, Aline de H.N. Maia, Sílvio R.M. Evangelista, and Sérgio C. de F. Fuck, Jr., 2012. Climate Change Impacts on Irrigation Water Needs in the Jaguaribe River Basin. Journal of the American Water Resources Association (JAWRA) 48(2): 355‐365. DOI: 10.1111/j.1752‐1688.2011.00620.x Abstract: Climate change is conceptually referred to as a modification to the average of climate variables and their natural variability, due to both natural and anthropogenic driving forces, such as greenhouse gas emissions. Climate change potentially impacts rainfall, temperature, and air humidity, which have relationship with plant evapotranspiration and consequently to irrigation water needs (IWN). The purpose of this research is to assess climate change impacts on irrigation water demand, based on climatic impacts stemming from future greenhouse gas emission scenarios. The study area includes eight municipalities in the Jaguaribe River Basin, located in the Ceará State of semiarid northeast Brazil. The FAO Penman‐Monteith method is used for the calculation of a reference evapotranspiration with limited climatic data. IWN projections are calculated using bias‐corrected climate projections for monthly rainfall and surface temperature derived from the United Kingdom’s Hadley Centre Regional Climate Model simulations. The increase in the average IWN is projected to be 7.9 and 9.1% over the period 2025‐2055 for the A2 and B2 scenarios, respectively with respect to 1961‐1990 baseline.     

Relative Value of Water for Hydropower and Municipal Supply in Southeastern Reservoirs

Population growth in the Southeast has driven withdrawals for municipal water beyond the limits of local supplies. With few options left for development of virgin sources, a number of urban areas are looking toward demand management and additional supplies by reallocating storage in reservoirs that were built primarily or in part for hydropower. Hydropower has become a lesser part of the mix of energy sources, and the question arises as to value of water for that purpose relative to its value for municipal use. Three cases are used to examine the issue. Effects of withdrawal for municipal water supply on output of electric energy are estimated. Benefits of foregone energy are evaluated using the least cost alternative for replacement, and benefits for municipal water are estimated using costs for development of new sources. Benefits for use as municipal water are found to be considerably higher than benefits for hydroelectric energy at existing prices, even higher than the least cost alternative for replacement. Given the spatial distribution of the cases, that finding would appear to hold in general across the region.     

Water Supply Reliability Tradeoffs between Removing Reservoir Storage and Improving Water Conveyance in California

Population growth, climate change, aging infrastructure, and changing societal values alter how water must be managed in the 21st Century. O'Shaughnessy Dam, located in Yosemite National Park, has been identified as a possible candidate for dam removal. It is a component of San Francisco's Hetch Hetchy System and is operated for water supply and hydropower. This article describes a spatially scaled approach to analyze water reliability without O'Shaughnessy Dam, but with improved water conveyance between the Hetch Hetchy System and existing reservoirs and aqueducts at the watershed, regional Bay Area, and statewide scales. It broadens previous research to highlight larger scale implications of removing O'Shaughnessy Dam and evaluates the role of improved water conveyance for water management. CALifornia Value Integrated Network, a large‐scale hydro‐economic model evaluates intertied water management using estimated urban and agricultural water demands for year 2050 with 72‐year historical and warm, dry hydrologic conditions. Results suggest that O'Shaughnessy Dam can be removed with additional conveyance at any spatial scale while maintaining water reliability. With a warm, dry climate, water reliability, and storage decline, indicating removing O'Shaughnessy Dam may have less effect on water management than climate change when conveyance is improved between the Hetch Hetchy System and nearby systems. Improving water conveyance can sometimes substitute for water storage in storage‐rich watersheds.     

Contrasting Lumped and Distributed Hydrology Models for Estimating Climate Change Impacts on California Watersheds1

Maurer, Edwin P., Levi D. Brekke, and Tom Pruitt, 2010. Contrasting Lumped and Distributed Hydrology Models for Estimating Climate Change Impacts on California Watersheds. Journal of the American Water Resources Association (JAWRA) 46(5):1024–1035. DOI: 10.1111/j.1752-1688.2010.00473.x Abstract: We compare the projected changes to streamflows for three Sierra Nevada rivers using statistically downscaled output from 22 global climate projections. The downscaled meteorological data are used to drive two hydrology models: the Sacramento Soil Moisture Accounting model and the variable infiltration capacity model. These two models differ in their spatial resolution, computational time step, and degree and objective of calibration, thus producing significantly different simulations of current and future streamflow. However, the projected percentage changes in monthly streamflows through mid-21st Century generally did not differ, with the exceptions of streamflow during low flow months, and extreme low flows. These findings suggest that for physically based hydrology models applied to snow-dominated basins in Mediterranean climate regimes like the Sierra Nevada, California, model formulation, resolution, and calibration are secondary factors for estimating projected changes in extreme flows (seasonal or daily). For low flows, hydrology model selection and calibration can be significant factors in assessing impacts of projected climate change.     

The Impact of Dynamic Environmental Flow Releases on Hydropower Production in the Zambezi River Basin

Incorporation of environmental flow releases from reservoirs has proven to be challenging due to fear of losses to existing water uses. Moreover environmental flow requirements (EFR) have not often been operationalized. This study compares the possibility of implementing dynamic EFR based on natural flows lagged against an upstream unregulated gauging point with static EFR. It simulates different scenarios with a high flow release in the wet season and analyses its impacts on hydropower production. This method accounts fully for the natural variability of environmental flows, implying less pressure on existing water uses during relatively dry years. Joint operation of two cascading dams vs. individual operation for EFR was also explored. These approaches were tested for the Zambezi River basin in Southern Africa using a water resources model, WAFLEX. Historic data on reservoir water levels, releases and power generation of the hydropower schemes were synthesized. Combining these yielded a validated series of monthly flow data for a 28 year period (1982‐2010). The results show that Kariba and Cahora Bassa reservoirs face a reduction in power produced when they would annually release an environmental flow. However, the dynamic EFR method entails smaller hydropower losses. Joint environmental flow operations will reduce overall basin power production more than if Cahora Bassa alone would release an environmental flow. However, such joint operation would be more beneficial to the ecosystem.     

气候变化对中国旅游业持续发展的影响及应对措施

气候变化已不同程度地威胁到中国旅游业持续发展所依赖的主要生态系统,使旅游业遭受到巨大的经济损失,并且使旅游业面临着改变其产品和形象的威胁,可能进一步加剧旅游业与自然保护之间的冲突、凸显社会公平问题。尽管中国现行立法已为保障旅游业持续发展提供了相应的制度支撑,但远不足以应对气候变化对中国旅游业持续发展带来的严峻挑战。当前迫切需要开展旅游业的可持续管理与教育、培育旅游市场和发展循环经济等联合行动。     

Modeling Climate Change Impacts on Hydrology and Nutrient Loading in the Upper Assiniboine Catchment1

Shrestha, Rajesh R., Yonas B. Dibike, and Terry D. Prowse, 2011. Modeling Climate Change Impacts on Hydrology and Nutrient Loading in the Upper Assiniboine Catchment. Journal of the American Water Resources Association (JAWRA) 48(1): 74‐89. DOI: 10.1111/j.1752‐1688.2011.00592.x Abstract: This paper presents a modeling study on climate‐induced changes in hydrologic and nutrient fluxes in the Upper Assiniboine catchment, located in the Lake Winnipeg watershed. The hydrologic and agricultural chemical yield model, Soil and Water Assessment Tool (SWAT) was employed to model a 21‐year baseline (1980‐2000) and future (2042‐2062) periods with model forcings for future climates derived from three regional climate models (RCMs) and their ensemble means. The modeled future scenarios reveal that potential future changes in the climatic regime are likely to modify considerably hydrologic and nutrient fluxes. The effects of future changes in climatic variables, especially precipitation and temperature, are clearly evident in the resulting snowmelt and runoff regimes. The future hydrologic scenarios consistently show earlier onsets of spring snowmelt and discharge peaks, and higher total runoff volumes. The simulated nutrient loads closely match the dynamics of the future runoff for both nitrogen and phosphorus, in terms of earlier timing of peak loads and higher total loads. However, nutrient concentrations could decrease due to the higher rate of runoff increase. Overall, the effects of these changes on the nutrient transport regime need to be considered together with possible future changes in land use, crop type, fertilizer application, and transformation processes in the receiving water bodies.     

Assessment of Flood Vulnerability Based on CMIP5 Climate Projections in South Korea

The objective of this article was to assess flood vulnerability based on the representative concentration pathways (RCP) scenarios at city and county levels. A quantile mapping method was adopted to correct bias that is inherent in climate change scenarios. A series of proxy variables related to climate exposure, sensitivity, and adaptive capacity were chosen to assess flood vulnerability. Proxy variables were standardized using the Z‐score method. Principal component analysis was carried out to calculate the weighting of proxy variables. The study area was the Korean peninsula. The spatial resolution was on a city and county basis and the temporal resolution was 1990s, 2025s, 2055s, and 2085s (divided into 1976‐2005, 2011‐2040, 2041‐2070, and 2071‐2100). In the spatial comparison, we found that the areas with high‐level flood vulnerability increased over time in the central region, including metropolitan areas, and near the southern coast. In the temporal comparison, we found that the RCP4.5 scenario showed a tendency to increase steadily and the RCP8.5 scenario showed a tendency to decrease in the 2055s slightly and increase again in the 2085s. The study findings may provide useful data for the determination of priority for countermeasure development, though robustness of these findings with additional future projections should be established.     

Effects of Climate and Land Cover on Hydrology in the Southeastern U.S.: Potential Impacts on Watershed Planning

The hydrologic response to statistically downscaled general circulation model simulations of daily surface climate and land cover through 2099 was assessed for the Apalachicola‐Chattahoochee‐Flint River Basin located in the southeastern United States. Projections of climate, urbanization, vegetation, and surface‐depression storage capacity were used as inputs to the Precipitation‐Runoff Modeling System to simulate projected impacts on hydrologic response. Surface runoff substantially increased when land cover change was applied. However, once the surface depression storage was added to mitigate the land cover change and increases of surface runoff (due to urbanization), the groundwater flow component then increased. For hydrologic studies that include projections of land cover change (urbanization in particular), any analysis of runoff beyond the change in total runoff should include effects of stormwater management practices as these features affect flow timing and magnitude and may be useful in mitigating land cover change impacts on streamflow. Potential changes in water availability and how biota may respond to changes in flow regime in response to climate and land cover change may prove challenging for managers attempting to balance the needs of future development and the environment. However, these models are still useful for assessing the relative impacts of climate and land cover change and for evaluating tradeoffs when managing to mitigate different stressors.     

Impacts of Changes in Precipitation Amount and Distribution on Water Resources Studied Using a Model Rainwater Harvesting System

Water supply reliability is expected to be affected by both precipitation amount and distribution changes under recent and future climate change. We compare historical (1951‐2010) changes in annual‐mean and annual‐maximum daily precipitation in the global set of station observations from Global Historical Climatology Network and climate models from the Inter‐Sectoral Impact Model Intercomparison Project (ISI‐MIP), and develop the study to 2011‐2099 for model projections under high radiative forcing scenario (RCP8.5). We develop a simple rainwater harvesting system (RWHS) model and drive it with observational and modeled precipitation. We study the changes in mean and maximum precipitation along with changes in the reliability of the model RWHS as tools to assess the impact of changes in precipitation amount and distribution on reliability of precipitation‐fed water supplies. Results show faster increase in observed maximum precipitation (10.14% per K global warming) than mean precipitation (7.64% per K), and increased reliability of the model RWHS driven by observed precipitation by an average of 0.2% per decade. The ISI‐MIP models show even faster increase in maximum precipitation compared to mean precipitation. However, they imply decreases in mean reliability, for an average 0.15% per decade. Compared to observations, climate models underestimate the increasing trends in mean and maximum precipitation and show the opposite direction of change in reliability of a model water supply system.