Impacts of greenhouse warming on water temperature and water quality in the southern United States

The implications of possible increases in carbon dioxide and other greenhouse gases on water resources have attracted increasing attention. Well-established techniques have been used to relate air temperature scenarios produced by GISS, GFDL and OSU global climate models to surface water temperatures and, subsequently, to water quality through a waste load allocation model for streams across the southern United States. There is a consensus among the 3 GCM scenarios studied that surface water temperature increases of up to 7 'C can be expected. Results of the application of a Streeter-Phelps waste load allocation model indicate that even though existing advanced treatment technology would be sufficient to maintain desirable levels of instream dissolved oxygen under elevated water temperature conditions, the use of such technology would be necessary on a much wider scale than at present. The use of extensive riparian vegetative shading was considered as a supplement to advanced water treatment technology. Results show that under 2 of the 3 climate change scenarios and projected patterns of forest migration, shading could appreciably offset the greenhouse-induced water temperature increases east of longitude 97 "W. If present maximum levels of natural riparian assemblages were maintained but no species migration were to take place, water temperatures could still rise in some areas as much as 7 "C. If other environmental or anthropogenic stresses were to result in a major decrease in vegetative cover, such water temperature increases could easily be exceeded. INTRODUCTION AND BACKGROUND In recent years, the implications of global warming on water resources have attracted increasing attention. General overviews may be found in the 'EPA Journal' (United States Environmental Protection Agency [US EPA] 1989), in Jacoby (1989) and in Waggoner (1990), the latter report outlining the basic viewpoints of the American Academy for the Advancement of Science. In the main, these studies are of a broad, qualitative nature and emphasize the economic and social implications of water supply issues. Water supply considerations are critical to many aspects of greenhouse effect studies dealing with water quality issues. Miller & Brock (1989) explored connections between water quantity and water quality for ' N O M A R L on assignment to the Atmospheric Research and Exposure Assessment Laboratory, US Environmental Protection Agency, M.D. 80, Research Triangle Park, North Carolina 27711. USA O Inter-Research/Printed in F. R. Germany Tennessee Valley Authority reservoir systems. Cooter (1990) modeled greenhouse-induced changes in runoff-related pesticide transport patterns for corngrowing areas of the southern United States. These studies, as well as research by Crotch (1988), highlight the challenges of isolating clear-cut regional changes when comparing present, baseline temperature and precipitation patterns to hypothesized conditions associated with greenhouse warming effects. Regional patterns of change in precipitation are particularly difficult to establish. In comparison, determining regional patterns of temperature-dependent effects on water quality is far more straightforward. Wellestablished techniques are available to relate air temperatures to water temperatures, and water temperatures are a key variable in many available water quality models, especially in so-called waste load allocation models for streams (Zison et al. 1978, O'Connor 1984). For smaller stream systems, water temperatures and shading from riparian vegetation are strongly correClirn. Res. 1: 1-12, 1990 lated (US EPA 1973). These shading effects can be predicted and used as inputs for waste load allocation models. Since waste load allocation modeling usually focuses on typical summer low-flow, high-temperature conditions, this minimizes perturbations from precipitation-dependent variability. Focussing on summer critical conditions helps highlight changes tied to potential greenhouse effects on surface air temperature regimes. Attention to the impacts of different degrees of stream shading also focuses attention on a policy variable subject to human control. Temperature-dependent waste load allocation models can predict changes in such important water quality features as in-stream dissolved oxygen levels. Much of the thrust of such public laws as the US Clean Water Act aims at controlling the effluents of point source discharges, to avoid depletion of dissolved oxygen levels deleterious to fish or other aquatic life. While tradtional approaches for controlling in-stream dissolved oxygen levels have stressed the technologybased treatment of waste waters, attention is now turning to innovative uses of wetlands and riparian vegetation to achieve comparable water quality benefits (Wetland Training Institute 1989). As will be seen, the preservation, or restoration, of timber cover in riparian corridors could become an important policy option under future scenarios of greenhouse warming. The estimated shading benefits would help damp-out potential increases in water temperatures. Reducing stream temperatures reduces oxygen demands from discharged effluents, thus serving to protect water quality conditions using available technological measures for wastewater treatment. In reaching these conclusions, data for current climatological conditions were combined with predictions from general circulation models (GCMs), to derive regional estimates of high summer (July) air temperatures. Results of 3 such models were used to establish a range of possible water temperature scenarios for the southern United States (Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, Oklahoma, Tennessee, Texas, South Carolina and Virginia). Although each GCM sets out to simulate similar physical processes, the manner and scale in time and space by which these models operate vary widely. As a result, the estimation of air temperature changes associated with a doubling of CO2 in the atmosphere varies as well. Yet, throughout the southern United States, there is general agreement among the 3 models on the direction of temperature change. Finally, regional estimates of July air temperatures were used to determine water temperature and dissolved oxygen as well as the implications of these results for water treatment facilities. METHODOLOGY FOR USE OF GCM OUTPUTS This study considered outputs from 3 widely used GCM models. The models are: the GISS model from the Goddard Institute of Space Studies (Hansen et al. 1984); the GFDL model from the Geophysical Fluid Dynamics Laboratory (Manabe & Wetherald 1987); and the OSU model from Oregon State University (Schlesinger & Zhao 1989). Table 1 summarizes the major features of these models. From each of these models, data are available from a 'base' 10 yr steadystate run assuming current levels of atmospheric COn and a 10 yr steady-state run assuming a doubling of atmospheric CO2. For both the 'base' and future runs, a variety of estimated climatological statistics pertinent to this study are available (e.g. mean monthly air temperatures, humidity, surface incident solar radiation, total cloud cover, and average surface wind speeds). The primary use of such model-generated values is to provide scenarios with a significant degree of spatial climate change detail. It is not the aim of this study to assess the accuracy of such model scenarios. A major criterion for such assessments would be the degree to which the 'base' (or 'current' condition) simulations correspond to climatological averages using actual historical data. Grotch (1988) offers a comparison of the Table 1. Select GCM model attributes