H13J-01 INVITED
Impacts of Climate Variability and Change on Water Quality
Water availability is a critical issue in semiarid regions and is impacted by diminution of water quantity and degradation of water quality. The objective of this study was to evaluate impacts of climate and land use change on soil water and groundwater quality in the Southern High Plains, Texas and the Amargosa Desert, Nevada. Boreholes were drilled in natural and agricultural ecosystems and soil samples were analyzed for chloride, perchlorate, sulfate, fluoride, and nitrate. In addition, the water balance was monitored over an eight-year period with weighing lysimeters in the Mojave Desert about 60 km from the Amargosa Desert site. Large salt inventories have accumulated in the unsaturated zone as a result of Pleistocene-to-Holocene climate change and associated mesic-to-xeric vegetational change. Native vegetation is highly adapted to utilize virtually all infiltrated water as shown by lysimeter and subsurface pressure monitoring, as well as satellite-derived NDVI data. Unusually strong ENSO conditions brought 2.3–2.5 times normal precipitation to the Mojave Desert region during the monitoring period. Somewhat surprisingly, the increased precipitation did not produce subsurface drainage or recharge because of negative feedbacks brought about by rapid response of xeric vegetation to increased moisture availability. On decadal-to-centennial timescales, land use change from natural to agricultural ecosystems has had large-scale impacts on soil and groundwater quantity and quality. Conversion of native vegetation to rainfed (non-irrigated) and irrigated cropland has increased drainage below the root zone, mobilizing salt reservoirs that have been accumulating since the Pleistocene. Land use change has modified nutrient cycles by adding nitrogen to the system, mobilizing pre- existing nitrate reservoirs (as documented in the Amargosa Desert), and creating new nitrate reservoirs through mineralization of soil organic nitrogen (as documented in the southern High Plains). The effect of increased groundwater quantity beneath rainfed agriculture in the southern High Plains far outweighs the degradation of groundwater quality from salt mobilization. Similarly, high quality irrigation water in the Amargosa Desert is predicted to have negligible impacts on groundwater quality in this region. Impacts of climate and land use change need to be considered together in assessing future water resources for semiarid regions.
H13J-02
Effects of Climate Variability on Groundwater and Streamwater Quality Determined by Non-linear Statistics
Climate change is usually associated with a more or less continuous shift of temperature, precipitation, etc. For water quality issues, however, single extreme events are of more concern. An increase of frequencies or intensities of extreme rain storms, droughts, or sequences of frost and thawing is likely to have substantial impact on groundwater and streamwater quality in vulnerable systems. Quantifying these effects, however, is challenging due to a multitude of biogeochemical processes that determine water quality at the catchment scale. Water quality data from some thousand groundwater and streamwater samples from the forested Lehstenbach and Lysina catchments were available since the end of the 1980s. The catchments are 60 km apart from each other and are very similar with respect to bedrock, soil, land use, altitude and climate. The data sets were investigated using a non-linear principal component analysis approach. The dominant components could be ascribed to biogeochemical processes. Time series of component scores were analysed as quantitative measures of these processes. Relating these time series to those of nitrate and sulfate helped considerably to better understand some intriguing patterns of the latters. Redox processes had a major effect on water quality, especially on the nitrogen and sulfur dynamics. Clear seasonal patterns could be identified as well as the impact of single extreme drought periods and heavy rainstorms. Long-term trends were detected both in groundwater and in streamwater. Besides, the impact of preferential flow on groundwater quality could be determined. Again, this component exhibited a long-term trend at some sites which was related to climate variability. In addition, the impact of the long-lasting acidifying deposition of the preceding decades was identified. The chosen approach allowed to differentiate between anthropogenic and climate variability effects.
H13J-03
An ENSO-Based Multivariate Wavelet-Decomposition Time Series Model for Nitrate Loads in the Little River Watershed
The El-Niño/Southern Oscillation (ENSO) is a periodic global climate phenomenon with strong effects on the weather patterns of the southeast United States. ENSO has been shown to have predictable effects on stream flow, rainfall, crop yield, and nutrient loads in runoff. In monitoring and research efforts during the last century, ENSO indices have emerged as one of the most consistent for describing low-frequency climate variability on both global and regional scales. To better understand the relationship between Sea Surface Temperature (SST) anomalies in the equatorial Pacific Ocean and hydrology and climate in the southeast United States, we have done an analysis in the frequency domain on 30 years of SST, precipitation, flow, and nutrient load data from an agricultural coastal plain watershed in Tifton, Georgia. To specifically understand the low-frequency oscillations and inter-annual or decadal variability inherent in these hydrological time series as a non-stationary process, wavelet analysis was used. We found that the 3-7 year mode of variability known in ENSO cycles exists in the Little River Watershed's precipitation, flow, nitrate and total phosphorus load time series. SST's and both nutrient loads, stream flow and precipitation time series also demonstrated shared periodicity and high covariance and correlation in the 3-7 year period in cross and coherence wavelet analysis. This indicates that the ENSO signal could be used as a predictor for nutrient loads in the southeast United States. Reconstructed Components (RC) from the 3-7-year period of each time series were extracted and used to create an optimized local monthly multivariate time series nutrient load model based on SST's and the most relevant hydrological variables investigated. This predictor of nutrient loads can then be compared to a process-based hydrological model simulation of the same system. This model could be useful for land and water managers in the southeast United States, as high risk months for greater than average nutrient loading could be identified and managed in advance, based on predictions of ENSO phase.
H13J-04
Simulating the Effect of Alternative Climate Change Scenarios on Pollutant Loading Reduction Requirements for Meeting Water Quality Standards Under USEPA's Total Maximum Daily Load Program
The United States Environmental Protection Agency (USEPA) total maximum daily load (TMDL) program requires that individual states assess the condition of surface waters and identify those which fail to meet ambient water quality standards. Waters failing to meet those standards must have a TMDL assessment conducted to determine the maximum allowable pollutant load which can enter the water without violating water quality standards. While most of the nearly 30,000 TMDL assessments completed since 1995 use mechanistic or empirical water quality models to forecast water quality conditions under alternative pollutant loading reduction scenarios, few, if any, also simulate water quality conditions under alternative climate change scenarios. As a result, model-based loading reduction requirements (which serve as the cornerstone for implementing water resource management plans, and initiating environmental management infrastructure projects), believed to improve water quality in impaired waters and reinstate their designated use, may misrepresent the actual required reduction when future climate change scenarios are considered. For example, recent research indicates a potential long term future increase in both the number of days between, and the intensity of, individual precipitation events. In coastal terrestrial and aquatic ecosystems, such climate conditions could lead to an increased accumulation of pollutants on the landscape between precipitation events, followed by a washoff event with a relatively high pollutant load. On the other hand, anticipated increases in average temperature and evaporation rate might not only reduce effective rainfall rates (resulting in less energy for transporting pollutants from the landscape) but also reduce the tidal exchange ratio in shallow estuaries (many of which are valuable recreational, commercial, and aesthetic natural resources). Here, we develop and apply a comprehensive watershed-scale model for simulating water quality in coastal shellfish harvesting waters of Eastern North Carolina. Our model endpoints include fecal indicator bacteria concentrations in surface water (in accordance with national shellfish harvesting area water quality standards), and associated water quality-based management decisions (such as restricting, or allowing, shellfish harvesting in a particular growing area). Our model also includes two innovative features which allow us to explore the effects of alternative climate change scenarios on coastal water quality dynamics. First, we model precipitation dynamics using the Tweedie family of probability distributions, an approach which (unlike the common approach of using historic precipitation time series data) allows us to simulate alternative precipitation scenarios by varying parameters of the Tweedie distribution. Second, we encode a probabilistic data-based mechanistic rainfall-runoff model as part of our comprehensive watershed model which propagates not only precipitation variability, but future temperature and pan evaporation rate changes as well, into watershed runoff rates and, subsequently, into estuarine flushing rates and water quality concentration dynamics. Our model therefore indicates not only how climate change scenarios might affect water quality, but also how those changes might lead to a change in water resource management decisions.
H13J-05
Future Stream Temperature Projections for the US Pacific Northwest: Potential Implications for Salmon Habitat
Climatic changes contribute to changes in hydrologic and thermal regimes of rivers and streams. Water temperature is a crucial driver of aquatic ecosystems. Both high and low temperatures can be lethal for stream biota. Cold-water organisms (e.g., salmonids), in particular, depend on favorable stream temperatures for a variety of biologic functions, including feeding and metabolic activities, osmoregulatory functions, reproduction, migrations, disease resistance, and predator avoidance. A one-dimensional mechanistic model is developed and evaluated to simulate stream temperature in the Pacific Northwest region of U.S.A. Future scenarios for stream temperature are developed by incorporating potential climate change scenarios reported by the Intergovernmental Panel on Climate Change (IPCC). The variable infiltration capacity (VIC) hydrologic model and a stream and river routing algorithm are used to develop hydrological and meteorological data that feed the temperature model as inputs. The heat budget equations are solved by a mixed Eulerian-Lagrangian method, which is robust, and scalable both in space and time. The simulated stream temperature data will be used to map potential geographic range and quality changes in salmon habitat within the fresh water domain.
H13J-06 INVITED
Climate variability effects on landscape hydrologic connectivity, riparian buffering potential, and solute fluxes
Differences in the amount and timing of precipitation and snowmelt can have significant impacts on the redistribution of water in the landscape, altering flowpaths, residence times, streamflow magnitudes, and solute concentrations and loads. Watershed outlet dynamics represent the distribution, frequency, and integration of local scale processes. However, landscape characteristics such as vegetation distributions, soil characteristics, topography/topology, and solute loading patterns all affect potential response to perturbation and climate variability. We seek to investigate these space-time relationships with the following overarching questions: What can the intersection of temporal climatic variability and organized heterogeneity in landscape characteristics mean for watershed scale hydrologic connectivity and solute export? How might increases or decreases in the frequency and duration of hydrologic connectivity affect riparian buffering potential and stream solute dynamics and loading? We investigated these broad questions in both natural experimental watersheds across contrasting climate years and in rapidly developing watersheds experiencing increased nutrient loading. Our findings suggest that feedbacks between the hydrological and biogeochemical aspects of watersheds can lead to enhanced response to variability.
H13J-07
Influence of Hydrology and Seasonality on DOC Exports from 3 Contrasting Upland Catchments
How climate variability influences soil processes, production and export of DOC are important in understanding hydrologically mediated carbon losses from soils and its affect on water quality. This necessitates understanding factors controlling the quantity and timing of carbon availability for export from soils to the drainage network. Long-term records (1987-2006) of DOC concentrations at 6 catchments (0.44- 10.0 km2) across a climatic transect in Scotland were investigated for intra-annual relationships to evaluate potential long-term seasonal as well as inter-annual patterns. Catchments in wetter Central Scotland with high rainfall-runoff ratios, short transit times and well-connected responsive soils show a distinct annual periodicity in DOC concentrations throughout the long-term datasets. Increased DOC concentrations occurred between June and November with correspondingly lower DOC concentrations from December to May. This appears unrelated to discharge, and is dependent mainly on higher temperatures driving biological activity, increasing decomposition of available organic matter and solubility of DOC. The drier eastern catchments have lower rainfall-runoff ratios, longer transit times and annual drying-wetting regimes linked to changing connectivity of soils. These are characterised by seasonal DOC concentration- discharge relationships with an autumnal flush of DOC. Temperature influences the availability of organic matter for subsequent DOC transport producing a high DOC concentration-discharge relationship in summer/autumn and low DOC concentration-discharge relationship in winter/spring. These two distinct modes of seasonal DOC transport have important implications for understanding changes in DOC concentrations and export brought about by climate changes (temperature and precipitation) and modeling of aquatic carbon losses from soil-types under different hydrological regimes.
H13J-08
Annual-scale Variations in Runoff and Streamwater Chemistry
Climate variables, such as precipitation amount and temperature, may exert a strong influence on solute concentrations and fluxes. The relation between solute concentrations, fluxes, and hydroclimate variables were examined for eight catchments in the western U.S ranging in size from 0.04 to 469 km2. Although runoff increased in nearly direct proportion to annual precipitation, annual volume-weighted mean concentrations of most solutes exhibited much less variation. As a result, net fluxes of solutes, including weathering products, exhibited a strong, positive relation to annual precipitation amount. The positive relation between precipitation and fluxes of weathering products has implications for carbon cycling and sequestration. Carbonic acid weathering of silicate minerals is an important type of weathering reaction, and it consumes CO2. If precipitation amount increases, more CO2 will be consumed via carbonic acid weathering reactions, and more carbon will be transported to the ocean as bicarbonate where it may be incorporated into marine sediments.