Donald C. Pierson

Examination of change factor methodologies for climate change impact assessment

[1] A variety of methods are available to estimate values of meteorological variables at future times and at spatial scales that are appropriate for local climate change impact assessment. One commonly used method is Change Factor Methodology (CFM), sometimes referred to as delta change factor methodology. Although more sophisticated methods exist, CFM is still widely applicable and used in impact analysis studies. While there are a number of different ways by which change factors (CFs) can be calculated and used to estimate future climate scenarios, there are no clear guidelines available in the literature to decide which methodologies are most suitable for different applications. In this study several categories of CFM (additive versus multiplicative and single versus multiple) for a number of climate variables are compared and contrasted. The study employs several theoretical case studies, as well as a real example from Cannonsville watershed, which supplies water to New York City, USA. Results show that in cases when the frequency distribution of Global Climate Model (GCM) baseline climate is close to the frequency distribution of observed climate, or when the frequency distribution of GCM future climate is close to the frequency distribution of GCM baseline climate, additive and multiplicative single CFMs provide comparable results. Two options to guide the choice of CFM are

High resolution measurements of sediment resuspension above an accumulation bottom in a stratified lake

A detailed record of suspended particulate matter (SPM) concentrations in the benthic boundary layer (BBL) 1.5 m above an accumulation bottom and 13.5 m below the surface was obtained from frequent (30 min interval) beam attenuation measurements made with a Sea Tech transmissometer in the main basin of Lake Erken, a moderately deep (mean depth 9 m, maximum depth 21 m) dimictic lake in central Sweden. Concentrations of SPM (g m−3) were not as strongly correlated to the beam attenuation coefficient (c, [m−1]), as were concentrations of the inorganic SPM fraction. Apparently, this was caused by large optically inactive organic particles which significantly affected the measurements of SPM, but had little effect on the attenuation of light.When the water column was thermally stratified, SPM concentrations in the BBL showed a seasonal increase which was related to an increase in the thermocline depth. As the epilimnion deepened, there was also a marked increase in the occurrence of rapid and large changes in SPM concentration. After the loss of stratification, the amount of SPM and the temporal variability in its concentration was reduced. Since surface waves could not influence sediment resuspension at the depth of measurement, these data show the importance of internal waves in promoting sediment resuspension in areas of sediment accumulation. The relatively short period in each summer, when the thermocline reaches a sufficient depth to allow for resuspension over accumulation bottoms, can have important consequences for both the redistribution of lake sediments and the internal loading of phosphorus.

Ecosystem Effects of a Tropical Cyclone on a Network of Lakes in Northeastern North America

Here we document the regional effects of Tropical Cyclone Irene on thermal structure and ecosystem metabolism in nine lakes and reservoirs in northeastern North America using a network of high-frequency, in situ, automated sensors. Thermal stability declined within hours in all systems following passage of Irene, and the magnitude of change was related to the volume of water falling on the lake and catchment relative to lake volume. Across systems, temperature change predicted the change in primary production, but changes in mixed-layer thickness did not affect metabolism. Instead, respiration became a driver of ecosystem metabolism that was decoupled from in-lake primary production, likely due to addition of terrestrially derived carbon. Regionally, energetic disturbance of thermal structure was shorter-lived than disturbance from inflows of terrestrial materials. Given predicted regional increases in intense rain events with climate change, the magnitude and longevity of ecological impacts of these storms will be greater in systems with large catchments relative to lake volume, particularly when significant material is available for transport from the catchment. This case illustrates the power of automated sensor networks and associated human networks in assessing both system response and the characteristics that mediate physical and ecological responses to extreme events.

Impacts of climate change on phosphorus loading from a grassland catchment: Implications for future management

Dynamic modelling was used to quantify the impact of projected climate change, and potential changes in population and land use, on phosphorus (P) export from a sub-catchment in SW Ireland using the Generalised Watershed Loading Functions (GWLF) model. Overall the results indicated that the increase in annual total phosphorus loads attributable to climate change was greater than that from either population or land use change, and therefore that future climate variability will pose an increasingly significant threat to the successful long-term implementation of catchment management initiatives. The seasonal pattern in projected P export mirrored changes in streamflow, with higher rates between January and April and lower rates in summer. The potential reduction in export in summer was, however, negated when increases in population were included in simulations. A change in the slurry spreading period from that stipulated in national regulations to the months between April and September could potentially mitigate against future increases in dissolved P export in spring. The results indicate that projected changes in climate should be included when undertaking modelling exercises in support of decision making for catchment management plans.

Effects of warmer world scenarios on hydrologic inputs to Lake Mälaren, Sweden and implications for nutrient loads

A simple, rapid, and flexible modelling approach was applied to explore the impacts of climate change on hydrologic inputs and consequent implications for nutrient loading to Lake Mälaren, Sweden using a loading function model (GWLF). The first step in the process was to adapt the model for use in a large and complex Swedish catchment. We focused on the Galten basin with four rivers draining into the western region of Mälaren. The catchment model was calibrated and tested using long-term historical data for river discharge and dissolved nutrients (N, P). Then multiple regional climate model simulation results were downscaled to the local catchment level, and used to simulate possible hydrological and nutrient loading responses to warmer world scenarios. Climate change projections for the rivers of Galten basin show profound changes in the timing of discharge and nutrient delivery due to increased winter precipitation and earlier snow melt. Impacts on total annual discharge and load are minimal, but the alteration in river flow regime and the timing of nutrient delivery for future climate scenarios is strikingly different from historical conditions.

Investigating the impact of climate change on New York City’s primary water supply

Future climate scenarios projected by three different General Circulation Models and a delta-change methodology are used as input to the Generalized Watershed Loading Functions – Variable Source Area (GWLF-VSA) watershed model to simulate future inflows to reservoirs that are part of the New York City water supply system (NYCWSS). These inflows are in turn used as part of the NYC OASIS model designed to simulate operations for the NYCWSS. In this study future demands and operation rules are assumed stationary and future climate variability is based on historical data to which change factors were applied in order to develop the future scenarios. Our results for the West of Hudson portion of the NYCWSS suggest that future climate change will impact regional hydrology on a seasonal basis. The combined effect of projected increases in winter air temperatures, increased winter rain, and earlier snowmelt results in more runoff occurring during winter and slightly less runoff in early spring, increased spring and summer evapotranspiration, and reduction in number of days the system is under drought conditions. At subsystem level reservoir storages, water releases and spills appear to be higher and less variable during the winter months and are slightly reduced during summer. Under the projected future climate and assumptions in this study the NYC reservoir system continues to show high resilience, high annual reliability and relatively low vulnerability.

Light-scattering features of turbidity-causing particles in interconnected reservoir basins and a connecting stream

Light-scattering features of minerogenic particles in interconnected reservoir basins and a connecting stream in the watershed of New York Citys water supply system, where these particles dominate scattering, were characterized by scanning electron microscopy interfaced with automated X-ray microanalysis and image analysis (SAX). SAX provided information on composition (in terms of elemental X-rays), shapes, number concentration, size distribution, and projected area concentration (PAV(m)) of particle populations. Mie theory calculations based on SAX results were used to estimate the scattering coefficient and the mean scattering efficiency at a wavelength of 660 nm [b(m)(660) and ]. Throughout the study system, nonspherical clay mineral particles in the 1-10 microm size range dominated PAV(m), light scattering and its surrogate, nephelometric turbidity (T(n)). Patterns of particle size contributions to b(m)(660) (and T(n)) remained relatively invariant over a wide range of T(n) (more than 200-fold difference). The median size for these contributions was most often approximately 2.5 microm. The credibility of the SAX characterizations of the light-scattering features of the minerogenic particles and the calculations based on Mie theory for the study system was supported by (1) the strength of the T(n)-PAV(m) relationship, (2) the reasonable closure between T(n) measurements and calculated values of b(m)(660), and (3) the closeness of to the limiting value of 2 for polydispersed particle populations. Upstream sources of turbidity-causing particles within the study system were demonstrated to have highly similar light-scattering features. This indicates similar potencies for the particle populations from these sources for turbidity impacts in downstream waters and supports the direct incorporation of T(n) measurements into loading calculations to evaluate relative contributions of these inputs with respect to such impacts.

Turbidity Model for Ashokan Reservoir, New York: Case Study

Terrigenous inorganic particles delivered during runoff events cause problems of high turbidity in many lakes and reservoirs. A turbidity model, composed of a two-dimensional hydrothermal/transport submodel and a turbidity submodel, is developed and tested for Ashokan Reservoir, New York, that experiences elevated turbidity levels following runoff events. A robotic monitoring network, rapid profiling instrumentation, and individual particle analyses are used to support the modeling, by specifying turbidity loads and in-reservoir patterns and features of the particles that guided representation of settling. The turbidity-causing particles are clay minerals, 1–10 μm in diameter. The hydrothermal/transport submodel that serves as the physical framework for the overall model, was separately validated for a 13-year period. The turbidity submodel considered three particle-size/settling velocity classes of turbidity, consistent with the independent individual particle characterizations. Robust performance is dem...

Optical properties and light climate in Lake Verevi.

The optical properties and light climate during the ice-free period in the highly stratified Lake Verevi (Estonia) have been studied together with other lakes in same region since 1994. The upper water layer above the thermocline belongs to class “moderate” by optical classification of Estonian lakes but can turn “turbid” (concentration of chlorophyll a up to 73 mg m−3 and total suspended matter up to 13.2 g m−3) during late summer blooms. In the blue part of the spectrum, light is mainly attenuated by dissolved organic matter and in red part notably scattering but also absorption by phytoplanktonic pigments effect the spectral distribution of underwater light. Consequently, the underwater light is of greenish-yellow color (550–650 nm). Rapid change in optical properties occurs with an increase of all optically active substances close to thermocline (2.5–6 m). Optical measurements are often hampered beneath this layer so that modeling of the depth distribution of the diffuse attenuation coefficient is an useful compliment to field measurements. Kd,PAR ranges from 0.8 to 2.9 m−1 in the surface layer, and model results suggest that it may be up to 5.8 m−1 in the optically dense layer. This forms a barrier for light penetration into the hypolimnion.

Characterizations and modeling of turbidity in a water supply reservoir following an extreme runoff event

Abstract The findings from an integrated program of short- and long-term monitoring, individual particle analyses (IPA), and mechanistic modeling to characterize and simulate the turbidity (Tn) effects of an extreme runoff event (2011) on a water supply reservoir were documented. A robotic profiling platform and rapid profiling instrumentation resolved turbidity and temperature (T) patterns in time and space in the reservoir. Metalimnetic enrichment in Tn following the event was reported and attributed to the entry of turbid stream water as density currents, or plunging inflows. The diminishment of high Tn levels following the event was well represented by a first-order loss rate of about 0.023 d−1. The highest Tn levels were avoided in water withdrawn for the water supply following the event by selection of vertical intake alternatives, although Tn values in the withdrawal remained distinctly above typical baseline conditions for nearly 2 months. Based on IPA, the Tn-causing particles were mostly clay minerals in the 1–20 μm size range. The operation of sorting processes determining settling losses from the minerogenic particle population, according to their size and shape, following the runoff event was resolved. The set-up and testing of a mechanistic Tn model, composed of 2 submodels, a 2-dimensional hydrothermal/transport submodel, and a Tn kinetics submodel, is described. The hydrothermal/transport submodel was tested separately and performed well in simulating the dynamics of the reservoir’s stratification regime and the entry of the dense streams as plunging inflows during the extreme runoff event. The overall Tn model needed to represent the loss processes of both settling and coagulation to perform well in simulating the in-reservoir and withdrawal Tn patterns following the runoff event.