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.     

升钟水库富营养状况及生物群落调查评价

根据2008～2009年对升钟水库的逐月监测结果,采用综合营养状态指数法,对湖泊富营养化现状进行了评价,并对水库的浮游植物群落种类和密度进行了定性定量分析。结果表明,水库目前处于中营养状态,但在初夏和秋季由于温度和降水等气象条件的影响,可达到轻度富营养,其程度随季节变化明显：初夏和初秋形成高峰,冬、春季为低谷。从生物群落分析,浮游植物种类及密度在不同季节差别较大,初春及夏季的较其他季节多、冬季最低,优势种为蓝绿藻和硅藻。     

CHLOROPHYLL-BIOMASS-NUTRIENT RELATIONSHIPS FOR NATURAL ASSEMBLAGES OF FLORIDA PHYTOPLANKTON1

Parallel determination of phytoplankton biomass and chlorophyll a concentration were made on spring and summer phytoplankton samples collected from 165 Florida lakes. There was a significant correlation between chlorophyll a concentration and phytoplankton biomass (r=0.80; P < 0.01). Chlorophyll content per unit phytoplankton biomass ranged over two orders of magnitude. Nitrogen seemed to be a major factor influencing the chlorophyll content of Florida algae. Multiple regression analyses indicated that phytoplankton biomass was dependent on both the total phosphorus and total nitrogen concentration. Nutrient-phytoplankton and Secchi-phytoplankton relationships for the Florida lakes had higher coefficients of determination if chlorophyll a concentrations rather than phytoplankton biomass data were used in regression analyses.     

SUMMER LOW FLOWS IN NEW ENGLAND DURING THE 20TH CENTURY1

ABSTRACT: High springtime river flows came earlier by one to two weeks in large parts of northern New England during the 20th Century. In this study it was hypothesized that late spring/early summer recessional flows and late summer/early fall low flows could also be occurring earlier. This could result in a longer period of low flow recession and a decrease in the magnitude of low flows. To test this hypothesis, variations over time in the magnitude and timing of low flows were analyzed. To help understand the relation between low flows and climatic variables in New England, low flows were correlated with air temperatures and precipitation. Analysis of data from 23 rural, unregulated rivers across New England indicated little evidence of consistent changes in the timing or magnitude of late summer/early fall low flows during the 20th Century. The interannual variability in the timing and magnitude of the low flows in northern New England was explained much more by the interannual variability in precipitation than by the interannual variability of air temperatures. The highest correlation between the magnitude of the low flows and air temperatures was with May through November temperatures (r =?0.37, p= 0.0017), while the highest correlation with precipitation was with July through August precipitation (r = 0.67, p > 0.0001).     

Interactions of warming and altered nutrient load timing on the phenology of oxygen dynamics in Chesapeake Bay

The effects of nutrient loading on estuaries are well studied, given the multitude of negative water quality and ecosystem effects that have been attributed to excess nitrogen and phosphorus. A current gap in this knowledge involves the sensitivity of seasonal cycles of estuarine biogeochemical processes to direct (warming) and indirect influences (nutrient load timing) of climate change. We used a coupled hydrologic–biogeochemical model to investigate changes in the phenology of hypoxia and related biogeochemical processes in Chesapeake Bay under three different hydrologic regimes. Shifts to earlier nutrient load timing during idealized simulations reduced the overall annual hypoxic volume, resulting from discernable, but relatively small reductions in phytoplankton biomass and both sediment and water-column respiration. Simulated increases in water temperature caused an increase in spring/early summer hypoxic volume associated with elevated respiration rates, but an associated exhaustion of organic matter in the early summer caused a decrease in late summer/fall hypoxic volume due to lowered respiration. Warming effects on hypoxia were larger than nutrient timing effects in scenarios where warming was restricted to spring and when it was applied to all months of the year. These idealized simulations begin the process of understanding the potential impacts of future climatic changes in the seasonal timing of key biogeochemical processes associated with eutrophication.     

USE OF CLIMATE PREDICTIONS TO DECIDE A WATER MANAGEMENT PROBLEM1

ABSTRACT: Seasonal precipitation predictions were utilized in a water management decision with major economic, societal, and political ramifications. A summer (1984) drought had created a situation calling for possible fall season use of state waters from two major multipurpose reservoirs with an ensuing effect on water price negotiations. Choices facing management and use of water from the reservoirs were to invoke expensive water restrictions with a 33 percent chance of being right, do nothing (66 percent chance of wrong outcome), or use the precipitation predictors (for above normal fall rain) having a 50 percent chance of error. Hydrologists chose to follow the precipitation predictions, which proved to be accurate for the fall of 1984, helping to reveal the long-term value of using well understood climate predictions in water management.     

Environmental Control and Limnological Impacts of a Large Recurrent Spring Bloom in Lake Washington,USA

A series of statistical analyses were used to identify temporal and spatial patterns in the phytoplankton and nutrient dynamics of Lake Washington, an mesotrophic lake in Washington State (USA). These analyses were based on fortnightly or monthly samples of water temperature, Secchi transparency, ammonium (NH₄), nitrate (NO₃), inorganic phosphorus (IP), total nitrogen (TN), total phosphorus (TP), dissolved oxygen (DO), pH and chlorophyll a (chl a) collected during 1995–2000 from 12 stations. Lake Washington has a very consistent and pronounced annual spring diatom bloom which occurs from March to May. During this bloom, epilimnetic chl a concentrations peak on average at 10 μg/L, which is 3 times higher than chl a concentrations typically seen during summer stratified conditions. The spring bloom on average comprised 62% diatoms, 21% chlorophytes and 8% cyanobacteria. During summer stratification, diatoms comprised 26% of the phytoplankton community, chlorophytes 37% and cyanobacteria 25%. Cryptophytes comprised approximately 8% of the community throughout the year. Overall, 6 phytoplankton genera (i.e., Aulacoseira, Fragilaria, Cryptomonas, Asterionella, Stephanodiscus, and Ankistrodesmus) cumulatively accounted for over 50% of the community. These analyses also suggest that the phytoplankton community strongly influences the seasonality of NO₃, IP, DO, pH and water clarity. According to a MANOVA, seasonal fluctuations explained 40% of the total variability for the major parameters, spatial heterogeneity explained 10% of variability, and the seasonal-spatial interaction explained 10% of variability. Distinctive patterns were identified between offshore and inshore sampling stations. The results of our analyses also suggest that spatial variability was substantial, but much smaller than temporal variability.     

EFFECTS OF CLIMATE CHANGE ON HYDROLOGY AND WATER RESOURCES IN THE COLUMBIA RIVER BASIN1

ABSTRACT: As part of the National Assessment of Climate Change, the implications of future climate predictions derived from four global climate models (GCMs) were used to evaluate possible future changes to Pacific Northwest climate, the surface water response of the Columbia River basin, and the ability of the Columbia River reservoir system to meet regional water resources objectives. Two representative GCM simulations from the Hadley Centre (HC) and Max Planck Institute (MPI) were selected from a group of GCM simulations made available via the National Assessment for climate change. From these simulations, quasi-stationary, decadal mean temperature and precipitation changes were used to perturb historical records of precipitation and temperature data to create inferred conditions for 2025, 2045, and 2095. These perturbed records, which represent future climate in the experiments, were used to drive a macro-scale hydrology model of the Columbia River at 1/8 degree resolution. The altered streamflows simulated for each scenario were, in turn, used to drive a reservoir model, from which the ability of the system to meet water resources objectives was determined relative to a simulated hydrologic base case (current climate). Although the two GCM simulations showed somewhat different seasonal patterns for temperature change, in general the simulations show reasonably consistent basin average increases in temperature of about 1.8–2.1°C for 2025, and about 2.3–2.9°C for 2045. The HC simulations predict an annual average temperature increase of about 4.5°C for 2095. Changes in basin averaged winter precipitation range from -1 percent to + 20 percent for the HC and MPI scenarios, and summer precipitation is also variously affected. These changes in climate result in significant increases in winter runoff volumes due to increased winter precipitation and warmer winter temperatures, with resulting reductions in snowpack. Average March 1 basin average snow water equivalents are 75 to 85 percent of the base case for 2025, and 55 to 65 percent of the base case by 2045. By 2045 the reduced snowpack and earlier snow melt, coupled with higher evapotranspiration in early summer, would lead to earlier spring peak flows and reduced runoff volumes from April-September ranging from about 75 percent to 90 percent of the base case. Annual runoff volumes range from 85 percent to 110 percent of the base case in the simulations for 2045. These changes in streamflow create increased competition for water during the spring, summer, and early fall between non-firm energy production, irrigation, instream flow, and recreation. Flood control effectiveness is moderately reduced for most of the scenarios examined, and desirable navigation conditions on the Snake are generally enhanced or unchanged. Current levels of winter-dominated firm energy production are only significantly impacted for the MPI 2045 simulations.     

LIMNOLOGICAL PROPERTIES OF A ROCKY MOUNTAIN HEADWATER RESERVOIR1

ABSTRACT: Hyalite Reservoir, Montana, was studied to determine properties of this small, montane, headwater, deep-release reservoir relative to reservoirs at lower elevations. While retention times for waters were as brief as 12 d, the mean residency of 40 d from mid-March to mid-December was within the range reported for other reservoirs. No significant through-reservoir gradients for suspended sediments were observed, contrasting to observations for most reservoirs. Thermal stratification, evident during the first part of the summer, was disrupted in August by cool, dense tributary inflows and strong wind-induced mixing. Dissolved oxygen concentrations paralleled temperature patterns in the reservoir; lowest average values for both occurred in waters sampled nearest the outlet. Total phosphorus averaged greater than twice the total nitrogen concentrations; greatest average concentrations for both were found in the near-bottom waters nearest the outlet. Enrichment of nitrogen concentrations in outflow over inflow waters is hypothesized to occur through nitrogen fixation by Aphanizonwnon flos-aquae. Despite the relatively high quality of waters from tributary inflows, an algal bloom, chlorophyll a concentrations, and primary productivity estimates suggested that the reservoir was mesotrophic. Circulation of waters within the reservoir was primarily influenced by wind-induced mixing, thermal gradients, and currents produced by the deep-water outlet.     

Bacteriological quality of spring water in Bahrain

Summary The bacteriological quality of untreated spring waters in Bahrain were evaluated. Samples were collected from 12 natural springs and analysed for total plate count, total coliform, total faecal coliform, total faecal streptococci and occurrence ofShigella andSalmonella. The total bacterial counts ranged from 2.4×10¹ to 1.6×10⁴ CFU mL^–1 with an average of 2.8×10³.The coliform organisms were present in all springs. Faecal coliforms were present in 11 out of 12 springs studied, and three springs were heavily contaminated. Based on criteria which used the ratio of faecal coliform to faecal streptococci, all springs studied may be contaminated with human wastes, except one spring which may be contaminated with animal wastes. NeitherSalmonella norShigella were detected in any of the spring waters. It is concluded that the spring waters studied are unsuitable for human consumption unless disinfected.Dr Abdulrahman O. Musaiger is the senior author, to whom correspondence should be addressed. He is Head of the Nutrition Unit at the Bahrain Public Health Directorate. Mr Zakaria A. Khunji is a qualified microbiologist and the nutritionist at the Unit. Mr Hussain A. Mohana is a graduate and technician in charge of the food and water microbiology section at the Public Health Laboratory, Ministry of Health, Bahrain.     

Climate Change Impacts and Uncertainties on Spring Flooding of Lake Champlain and the Richelieu River

The source of the Richelieu River is Lake Champlain, located between the states of New York, Vermont, and Québec. In 2011, the lake and the Richelieu River reached historical flood levels, raising questions about the influence of climate change on the watershed. The objectives of this work are to model the hydrology of the watershed, construct a reservoir model for the lake and to analyze flooding trends using climate simulations. The basin was modeled using the HSAMI lumped conceptual model from Hydro‐Québec with a semi‐distributed approach in order to estimate the inflows into Lake Champlain. The discharge at the Richelieu River was computed by using a mass balance equation between the inputs and outputs of Lake Champlain. Future trends were estimated over the 2041‐2070 and 2071‐2100 periods using a large number of outputs from general circulation models and regional climate models downscaled with constant scaling and daily translation methods. While there is a certain amount of uncertainty as to future trends, there is a decreasing tendency in the magnitude of the mean spring flood. A flood frequency analysis showed most climate projections indicate the severity of most extreme spring floods may be reduced over the two future periods although results are subject to a much larger uncertainty than for the mean spring flood. On the other hand, results indicate summer‐fall extreme events such as caused by hurricane Irene in August 2011 may become more frequent in the future.     

Impacts of Dams on Flow Regimes in Three Headwater Subbasins of the Columbia River Basin,United States1

Moore, Johnnie N., Alicia S. Arrigoni, and Andrew C. Wilcox, 2012. Impacts of Dams on Flow Regimes in Three Headwater Subbasins of the Columbia River Basin, United States. Journal of the American Water Resources Association (JAWRA) 48(5): 925‐938. DOI: 10.1111/j.1752‐1688.2012.00660.x Abstract: We compared long‐term changes in flow regimes resulting from climate change with those resulting from dams in three matched pairs of natural and modified headwater subbasins of the Columbia River. Based on the analysis of 12 flow‐regime metrics, we found that damming had minimal effect on most quantity of flow metrics, but major effect on timing of flow metrics, especially those representing “spring runoff.” In all modified subbasins, “spring runoff” metrics occurred much earlier than natural flow (up to ～44 days earlier for April‐July flows). Storage capacity modulated the magnitude of timing of flow‐metric changes, with the largest storage capacity leading to the most change. However, even in subbasins with low storage capacity, we found significant change in most timing of flow metrics. We also found that damming, especially in subbasins with higher storage capacity, overwhelmed climate variability in all basins for most flow metrics. This shows that reservoir operations need to be modified to more closely match the natural timing of flow regimes to promote positive ecologic response in modified rivers, even in basins where quantity of flow metrics have not changed substantially as a result of damming.     

Predicting the Thermal Effects of Dam Removal on the Klamath River

The Klamath River once supported large runs of anadromous salmonids. Water temperature associated with multiple mainstem hydropower facilities might be one of many factors responsible for depressing Klamath salmon stocks. We combined a water quantity model and a water quality model to predict how removing the series of dams below Upper Klamath Lake might affect water temperatures, and ultimately fish survival, in the spawning and rearing portions of the mainstem Klamath. We calibrated the water quantity and quality models and applied them for the hydrometeorological conditions during a 40-year postdam period. Then, we hypothetically removed the dams and their impoundments from the models and reestimated the rivers water temperatures. The principal thermal effect of dam and reservoir removal would be to restore the timing (phase) of the rivers seasonal thermal signature by shifting it approximately 18 days earlier in the year, resulting in river temperatures that more rapidly track ambient air temperatures. Such a shift would likely cool thermal habitat conditions for adult fall chinook (Oncorhynchus tshawytscha) during upstream migration and benefit mainstem spawning. By contrast, spring and early summer temperatures could be warmer without dams, potentially harming chinook rearing and outmigration in the mainstem. Dam removal might affect the rivers thermal regime during certain conditions for over 200 km of the mainstem.     

Estimating Nitrogen Loading to Ground Water and Assessing Vulnerability to Nitrate Contamination in a Large Karstic Springs Basin,Florida1

Abstract: A nitrogen (N) mass‐balance budget was developed to assess the sources of N affecting increasing ground‐water nitrate concentrations in the 960‐km² karstic Ichetucknee Springs basin. This budget included direct measurements of N species in rainfall, ground water, and spring waters, along with estimates of N loading from fertilizers, septic tanks, animal wastes, and the land application of treated municipal wastewater and residual solids. Based on a range of N leaching estimates, N loads to ground water ranged from 262,000 to 1.3 million kg/year; and were similar to N export from the basin in spring waters (266,000 kg/year) when 80‐90% N losses were assumed. Fertilizers applied to cropland, lawns, and pine stands contributed about 51% of the estimated total annual N load to ground water in the basin. Other sources contributed the following percentages of total N load to ground water: animal wastes, 27%; septic tanks, 12%; atmospheric deposition, 8%; and the land application of treated wastewater and biosolids, 2%. Due to below normal rainfall (97.3 cm) during the 12‐month rainfall collection period, N inputs from rainfall likely were about 30% lower than estimates for normal annual rainfall (136 cm). Low N‐isotope values for six spring waters (δ¹⁵N‐NO₃ = 3.3 to 6.3‰) and elevated potassium concentrations in ground water and spring waters were consistent with the large N contribution from fertilizers. Given ground‐water residence times on the order of decades for spring waters, possible sinks for excess N inputs to the basin include N storage in the unsaturated zone and parts of the aquifer with relatively sluggish ground‐water movement and denitrification. A geographical‐based model of spatial loading from fertilizers indicated that areas most vulnerable to nitrate contamination were located in closed depressions containing sinkholes and other dissolution features in the southern half of the basin.     

THERMAL AND DISSOLVED OXYGEN CHARACTERISTICS OF A SOUTH CAROLINA COOLING RESERVOIR1

ABSTRACT: Temperature and dissolved oxygen concentrations were measured monthly from January 1971 to December 1982 at 1-m depth intervals at 13 stations in Keowee Reservoir in order to characterize spatial and temporal changes associated with operation of the Oconee Nuclear Station. The reservoir water column was i to 4°C warmer in operational than in non-operational years. The thermo-dine was at depths of 5 to 15 m before the operation of Oconee Nuclear Station, but was always below the upper level of the intake (20 m) after the station was in full operation; this suggests that pumping by the Oconee Nuclear Station had depleted all available cool hypolimnetic water to this depth. As a result summer water temperatures at depths greater than 10 m were usually 10°C higher after plant operation began than before. By fall the reservoir was nearly homothemious to a depth of 27 m, where a thermocine developed. Seasonal temperature profiles varied with distance from the plant; a cool water plume was evident in spring and a warm water plume was present in the summer, fall, and winter. A cold water plume also developed in the northern section of the reservoir due to the operation of Jocassee Pumped Storage Station. Increases in the mean water temperature of the reservoir during operational periods were correlated with the generating output of the power plant. The annual heat load to the reservoir increased by one-third after plant operations began. The alteration of the thermal stratification of the receiving water during the summer also caused the dissolved oxygen to mix to greater depths.     

A modeling assessment of the thermal regime for an urban sport fishery

Water temperature is almost certainly a limiting factor in the maintenance of a self-sustaining rainbow trout (Oncorhynchus mykiss, formerlySalmo gairdneri) and brown trout (Salmo trutta) fishery in the lower reaches of the Cache la Poudre River near Fort Collins, Colorado, USA. Irrigation diversions dewater portions of the river, but cold reservoir releases moderate water temperatures during some periods. The US Fish and Wildlife Service’s Stream Network Temperature Model (SNTEMP) was applied to a 31-km segment of the river using readily available stream geometry and hydrological and meteorological data. The calibrated model produced satisfactory water temperature predictions (R ²=0.88,P<0.001, N=49) for a 62-day summer period. It was used to evaluate a variety of flow and nonflow alternatives to keep water temperatures below 23.3°C for the trout. Supplemental flows or reduced diversions of 3 m³/sec would be needed to maintain suitable summer temperatures throughout most of the study area. Such flows would be especially beneficial during weekends when current irrigation patterns reduce flows. The model indicated that increasing the riparian shade would result in little improvement in water temperatures but that decreasing the stream width would result in significant temperature reductions. Introduction of a more thermally tolerant redband trout (Oncorhynchus sp.), or smallmouth bass (Micropterus dolomieui) might prove beneficial to the fishery. Construction of deep pools for thermal refugia might also be helpful.     

ZOOPLANKTON COMMUNITIES OF A NEW PUMPED STORAGE RESERVOIR1

ABSTRACT: Zooplankton colonization was followed for 16 months in Lake Oconee, Georgia, a new pumped storage reservoir. Data were interpreted to identify differences among stations and seasons, as a function of the reservoir's early stage of development and of pumped storage operations. Colonization was rapid, and the zooplankton community was characterized by a high species diversity; approximately 40 rotifer species and 14 cladoceran genera were recorded. Zooplankton density varied along an environmental gradient from riverine to lentic conditions. Rotifer abundance varied from 10⁴-10⁶ individuals/m³, with maxima in the summers Copepod and cladoceran densities ranged from 10³ to nearly 10⁵ individuals/m³; maxima for stations other than the dam were observed in the summer and early fall, but high values at the dam station occurred throughout winter 1980. When pumped storage operations began in December 1979, zooplankton densities increased at the dam station. Pumpback decreased the intensity of the environmental gradient from riverine to lentic conditions, and led to a more similar zooplankton community structure throughout the reservoir.     

THE LIMNOLOGY OF LAKE WABUKAYNE,A STORM-WATER IMPOUNDMENT1

ABSTRACT The limnology of a 1.9-ha storm-water detention pond is described. The eutrophic nature of this impoundment is attributed to the nutrients in runoff from the surrounding residential area. During the summer, photosynthetic activity of the phytoplankton caused surface waters to become super-saturated with oxygen, while decomposing organic material greatly reduced dissolved oxygen concentrations in the deeper water. Sediment derived from construction activity within the drainage basin caused the impoundment to be turbid. The use of road deicing salts within the drainage basin produced high chloride concentrations and a temporary meromixis during the winter and early spring. The benthic fauna consisted primarily of oligochaetes, chironomids, and chaoborids. High densities of oligochaetes were present in the settling basin. Chaoborid larvae were abundant in the deep basin where low oxygen concentrations reduced the numbers of other benthic macroinvertebrates.     

PHYSICOCHEMICAL FACTORS AND THEIR EFFECTS ON ALGAL GROWTH IN A NEW SOUTHERN CALIFORNIA RESERVOIR1

ABSTRACT: Some physical and chemical characteristics of Lake Perris, a new southern California reservoir, were investigated with regard to their influence on phytoplankton biomass and community structure. The concentration of three major nutrients – nitrogen, phosphorus, and iron – was approximately equivalent to the demand ratio of fresh water plants. Large increases in iron and phosphorus concentrations in late summer due to releases from sediments, however, tended to shift the balance toward a nitrogen-limited situation. Nitrogen limitation favored nitrogen-fixing blue-green algae, and after a decline of competing algae during the summer, the blue-green population bloomed in September. Series of measurements taken over one-day periods during summer stratification showed that some iron, phosphorus, and manganese from the hypolimnion could move upward, corresponding to diel shifts in the thermocline depth. Vertical transport of nutrients could thus occur long before complete lake mixing and could support summer/fall algal blooms.     

THE POTENTIAL EFFECTS OF GLOBAL WARMING ON THE PRIMARY PRODUCTIVITY OF A SUBALPINE LAKE1

ABSTRACT Atmospheric scientists have predicted that large-scale climatic changes will result from increasing levels of tropospheric CO₂ We have investigated the potential effects of climate change on the primary productivity of Castle Lake, a mountain lake in Northern California. Annual algal productivity was modeled empirically using 25 years of limnological data in order to establish predictive relationships between productivity and the climatic variables of accumulated snow depth and precipitation. The outputs of monthly temperature and precipitation from three general circulation models (GCMs) of doubled atmospheric CO₂ were then used in the regression model to predict annual algal productivity. In all cases, the GCM scenarios predicted increased algal productivity for Castle Lake under cenditions of doubled atmospheric CO₂The primary cause of enhanced productivity was the increased length of the growing season resulting from earlier spring ice-out.