A sensitivity-based approach to evaluating future changes in Colorado River discharge

Projections of a drier, warmer climate in the U.S. Southwest would complicate management of the Colorado River system—yet these projections, often based on coarse resolution global climate models, are quite uncertain. We present an approach to understanding future Colorado River discharge based on land surface characterizations that map the Colorado River basin’s hydrologic sensitivities (e.g., changes in streamflow magnitude) to annual and seasonal temperature and precipitation changes. The approach uses a process-based macroscale land surface model (LSM; in this case, the Variable Infiltration Capacity hydrologic model, although methods are applicable to any LSM) to develop sensitivity maps (equivalent to a simple empirical model), and uses these maps to evaluate long-term annual streamflow responses to future precipitation and temperature change. We show that global climate model projections combined with estimates of hydrologic sensitivities, estimated for different seasons and at different change increments, can provide a basis for approximating cumulative distribution functions of streamflow changes similar to more common, computationally intensive full-simulation approaches that force the hydrologic model with downscaled future climate scenarios. For purposes of assessing risk, we argue that the sensitivity-based approach produces viable first-order estimates that can be easily applied to newly released climate information to assess underlying drivers of change and bound, at least approximately, the range of future streamflow uncertainties for water resource planners.     

Osmoregulation in the sheepshead minnow, Cyprinodon variegatus: Influence of a fluctuating salinity regime

Cyprinodon variegatus (sheepshead minnow), a common coastal resident of the western Atlantic Ocean and Gulf of Mexico, can live in ambient salinities ranging from 0‰ to greater than 140‰. Fish in this study were obtained from a Gulf of Mexico salt marsh near Cedar Key, Florida. This study examined the ability of individual C. variegatus to regulate plasma osmolality under the influence of a cycling salinity regime. Individuals of C. variegatus were exposed for 21 d to one of seven cyclical salinity regimes. Each cycle lasted for 2 d, with salinity varied between 10‰ and 30‰ each day. Plasma osmolality of fish from each group was determined on five dates during the course of the experiment. C. variegatus efficiently regulated plasma osmolality, even when fishes were exposed to large fluctuations in salinity. Fish previously exposed to large salinity fluctuations regulated plasma osmolality better than fish that previously had experienced no change or small changes in salinity. Increasing salinity had a greater impact on osmoregulation than did decreasing salinity.     

A numerical model of beach morphological evolution due to waves and currents in the vicinity of coastal structures

A numerical model was developed of beach morphological evolution in the vicinity of coastal structures. The model includes five sub-models for random wave transformation, surface roller development, nearshore wave-induced currents, sediment transport, and morphological evolution. The model was validated using high-quality data sets obtained during experiments with a T-head groin and a detached breakwater in the basin of the Large-scale Sediment Transport Facility at the Coastal and Hydraulics Laboratory in Vicksburg, Miss, USA. The simulations showed that the model reproduced well the wave conditions, wave-induced currents, and beach morphological evolution in the vicinity of coastal structures. Both salient and tombolo formation behind a T-head groin and a detached breakwater were simulated with good agreement compared to the measurements.     

Aeolian activity at a giant sandstone weathering pit in arid south‐central Utah

Aeolian abrasional, depositional and deflational features indicate exceptionally strong southwesterly winds in a giant sandstone weathering pit in Grand Staircase Escalante Monument, about 22 km southeast of Escalante, Utah. The 60 m wide, 5–20 m deep pit has developed near the summit of a broad, barren 160‐m‐high dome on the Lower Jurassic Navajo Sandstone. Unlike other giant weathering pits (10–30 m diameter) in the region, the bedrock floor of this pit is undulatory, and there is a cylindrical, 10‐m‐high rock pedestal in the centre of the pit. An active dune surrounds the central pedestal and at times has as much as 8 m of local relief. The dune shifts considerably over brief (<1 year) periods of time. Fine‐grained (<250 µm) dunal sand on the pit floor is periodically removed by deflation, leaving coarser sand (>250 µm) trapped in the pit. Dunal sand is typically derived from external sources (other than the pit walls and floor). Centimetre to metre‐scale abrasional features such as grooves, flutes and dedos occur on the bedrock walls and floor of the pit. These dedos and other streamlined aeolian sculpted host‐rock features occur in clusters and typically form in the lee of iron concretions. The dedos are similar to the controversial stalked blueberries on Mars. Above the western rim of the pit there is a 29‐m‐long, 5‐m‐wide aeolian groove with a fluted bedrock floor. A 1·2‐km‐long bedrock valley descends to the southwest from the pit and groove, amplifying southwesterly winds. Data from hand‐held anemometers suggest that southwesterly winds can be accelerated 200–300 per cent or more by local topography. Copyright © 2008 John Wiley and Sons, Ltd.     

Latitudinal dependence of signal-to-noise patterns from two general circulation models with CO2 forcing

 Detection of an enhanced greenhouse effect on climate depends on recognition of a signal of change amidst the combined noise of climatic variability and uncertainty in the nature of the signal (functional response to changing CO₂). Using two different GCMs (one with a coupled dynamic upper ocean) and an ensemble of 20 equilibrium experiments with CO₂ ranging from 100 to 3500 ppm, we find that that two measures of signal-to-noise (S/N) for the response of surface temperature to CO₂ forcing are larger over tropical and subtropical oceans than over low-latitude landmasses and larger than at higher latitudes generally. One S/N measure has the noise based solely on inherent model variability, while the other S/N measure includes both this variability and a measure of the uncertainty in the functional nature of the signal. Although the experiments were not for transient forcing and sulphate aerosols and other potentially important forcings (e.g., ozone or solar variability) were not considered, the results suggest that the effects of enhanced greenhouse climate may be detected more readily in surface temperatures from low-latitude oceanic regions than from global or zonal temperature averages. Received: 27 June 1995/Accepted: 28 October 1996     

Use of a particle-tracking model for predicting entrainment at power plants on the Hudson River

A major assumption of the Empirical Transport Model (ETM), widely adopted by both electric utilities and regulatory agencies for estimating the effects of entrainment mortality on fish populations in estuaries, is that the fraction of ichthyoplankton entrained varies only in response to changes in water withdrawals, not to changes in freshwater flow. We evaluated this assumption using a particle-tracking model to estimmate the probability of entrainment at power plants on the Hudson River during low and high freshwater flow periods and comparing those probabilities with estimates calculated from the ETM. We found that freshwater flow had a profound effect on the probability of entrainment. Both the number of river regions from which particles were entrained and the probabilities of entrainment for particles in those river regions differed between low-flow and high-flow periods. During high flow, particles spent less time in the grid box next to the intakes, reducing the probability of entrainment for particles released in the river region of each power plant and the average probability of entrainment across all regions at three power plants. The reduced probability of entrainment for particles released in the river regions of two power plants was offset by higher entrainment for particles upriver of these power plants. Although the average probabilities of entrainment across all river regions estimated with the particle-tracking model and the ETM were relatively similar for some power plants at high flow, low flow, or both, the probabilities for each river region differed considerably between the models. The number of river regions from which particles were entrained using the ETM was consistently undersestimated, resulting in probabilities for regions where entrainment occurred that were biased high compared with the particle-tracking model.     

Interactions among some dominant estuarine nekton species

Interactions between pairs of numerically dominant species collected at inlet and creek shorezone and channel habitats within a high salinity estuary in northeastern South Carolina were examined using two-way contingency tables and binomial tests. Of the significant species interactions, over 71% were positive and these primarily occurred within shorezone habitats. The strongest positive interactions were between young-of-the-year spot (Leiostomus xanthurus) and blue crab (Callinectes sapidus) juveniles in both shorezone habitats, and between striped killifish (Fundulus majalis), white mullet (Mugil curema), and striped anchovies (Anchoa hepsetus) in the inlet shorezone habitat. One of the most positive species associations in channel habitats was between the bay anchovy (Anchoa mitchilli) and the Atlantic brief squid (Lolliguncula brevis). These positive relationships between species may be explained by one species enhancing the habitat for another, both species responding to similar environmental conditions, cooperative social interactions such as mixed schooling, or the attraction of predators to prey. Negative interactions were found between schools of Atlantic silversides (Menidia menidia) and striped killifish in the inlet shorezone and between schools of Atlantic silversides and bay anchovies in the creek shorezone. Schools of Atlantic silversides may either displace or compete with other common shorezone species. Positive and negative interactions suggest that relationships between some species pairs did not occur randomly within certain habitats and may have contributed to the organization of the estuarine nekton community. Differences in the strengths and direction of interactions of certain species pairs among habitats and seasons were probably related to the differences in the physical characteristics of those habitats and/or changes in the relative abundance of dominant species and life stages over time.     

一种能实现单频PPP-RTK的GNSS局域参考网数据处理算法

全球范围内大量布设的GNSS(Global Navigation Satellite System)参考网为精密定位、导航和授时等应用提供了丰富的数据资源.基于局域参考网,先后发展了若干侧重实现双频精密定位的技术,如NRTK(Network Real Time Kinematic),PPP(Precise Point Positioning)和PPP-RTK等.其中,PPP-RTK融合了NRTK和PPP的技术优势,是目前相关研究的热点.本文改进了利用局域参考网提取各类改正信息的算法,以便于实现单频PPP-RTK,具体步骤包括:1) 逐参考站实施非组合PPP,并固定已知站星距和卫星钟差,预估电离层延迟、浮点模糊度等参数;2)联合所有参考站的PPP模糊度预估值,通过重新参数化,形成一组双差整周模糊度和接收机、卫星相位偏差;3) 固定双差整周模糊度,精化求解卫星相位偏差和各参考站PPP电离层延迟.基于网解中用到的卫星轨道和钟差,以及网解所提供的卫星相位偏差和(内插的)电离层延迟,参考网内的单频流动站即可实施PPP-RTK.基于澳大利亚某连续运行参考站网和流动站的实测数据,考察了:1) 参考网数据处理中,双差模糊度的固定成功率(98.89%)和卫星相位偏差估值的时间稳定性(各连续弧段优于0.2周);2)流动站处电离层延迟的内插精度(优于10 cm);3)单天内任一历元起算,固定静态(动态)单频PPP整周模糊度所需时长(均不超过10 min);4) 模糊度固定前后,单频动态PPP的定位精度(模糊度固定后,平面和天顶RMS分别优于5 cm和10 cm;模糊度固定前,相应RMS仅为28~53 cm).     

Evaluation of Hydrologically Relevant PCM Climate Variables and Large-Scale Variability over the Continental U.S.

The ability of the Parallel Climate Model (PCM) to reproduce the mean and variability of hydrologically relevant climate variables was evaluated by comparing PCM historical climate runs with observations over temporal scales from sub-daily to annual. The domain was the continental U.S, and the model spatial resolution was T42 (about 2.8 degrees latitude by longitude). The climate variables evaluated include precipitation, surface air temperature, net surface solar radiation, soil moisture, and snow water equivalent. The results show that PCM has a winter dry bias in the Pacific Northwest and a summer wet bias in the central plains. The diurnal precipitation variation in summer is much stronger than observed, with an afternoon maximum in summer precipitation over much of the U.S. interior, in contrast with an observed nocturnal maximum in parts of the interior. PCM has a cold bias in annual mean temperature over most of the U.S., with deviations as large as ?8 K. The PCM daily temperature range is lower than observed, especiallyin the central U.S. PCM generally overestimates the net solar radiation over most of the U.S, although the diurnal cycle is simulated well in spring, summer and winter. In autumn PCM has a pronounced noontime peak in solar radiation that differs by 5–10% from observations. PCM'ssimulated soil moisture is less variable than that of a sophisticated land-surface hydrology model, especially in the interior of the country. PCM simulates the wetter conditions over the southeastern U.S. and California during warm (El Niño) events, but shifts the drier conditions in the PacificNorthwest northward and underestimates their magnitude. The temperature response to the North Pacific Oscillation is generally captured by PCM, but the amplitude of this response is overestimated by a factor of about two.     

Evaluating the influence of lake morphology,trophic status and diagenesis on geochemical profiles in lake sediments

Recent geochemical studies provide evidence that changes in vertical distributions of nutrients in lake sediments are driven by anthropogenic activities, based primarily on trends of increasing concentrations in upper sediment layers. However, the present study shows that vertical concentration profiles of C, N and P in lake sediments can be higher in the upper, most recently deposited sediment strata, driven largely by natural diagenetic processes and not eutrophication alone. Sediment cores from 14 different lakes in New Zealand and China were examined ranging from oligotrophic to highly eutrophic and shallow to deep, and it was found that the shape of vertical profiles of total P, a key nutrient for lake productivity, can be similar in sediments across gradients of widely differing trophic status. Empirical and mechanistic diagenesis steady state profile models were derived and applied to describe the vertical distribution of C, N and P in the sediments. These models, which focus on large scale temporal (decades) and spatial (up to 35 cm in the vertical) processes, revealed that density-differentiated burial and biodiffusive mixing, were strongly correlated with vertical concentration gradients of sediment C, N and P content, whereas lake trophic status was not. A sensitivity analysis of parameters included in the diagenetic model further showed that the processes including flux of organic matter to the sediment–water interface, burial (net sedimentation), breakdown of organic matter and biodiffusion all can significantly influence the vertical distribution of sediment P content. It was concluded that geochemical studies attempting to evaluate drivers of the vertical distribution of sediment C, N and P content in lake sediments should also account for the natural diagenetic drivers of vertical concentration gradients, assisted with application of similar models to those presented in this study. This would include quantification of key sediment diagenesis model parameters to separate out the influence of anthropogenic activities.