.|  Baltimore Ecosystem Study
Meteorological Research Projects

Urban Influences on Soil and Stream Temperature Regimes
  • Gordon Heisler
  • Kenneth Belt
  • Richard Pouyat
  • Wayne Zipperer
  • Richard H. Grant
  • John Hom
  • Sue Grimmond
Research on soil and stream temperatures is being planned.

Research Problem and Justification:

Considerable attention has been given to urban temperature excess compared to nearby rural areas, a phenomenon commonly called an urban heat island. The temperature excesses have generally been measured in air or as temperatures of uppermost surfaces (landscape "skin temperature") using remote sensing with infrared sensors from aerial platforms. Relatively little attention has been paid to urban effects on temperatures in soil and water. It has often been recommended that tree cover be increased in urban areas to moderate urban heat islands. Tree cover will also influence temperatures of soil, storm-water runoff, and streams. More knowledge is needed of this effect in urban areas where tree canopies are incomplete, and where buildings and paved areas also interact to modify soil-, runoff-, and stream-temperature ranges and regimes. Because of the special problems of sampling in the disturbed soils and modified stream courses in urban areas, part of this research problem is to develop methodologies for making temperature measurements and characterizing the relevant soil, stream, and vegetation properties.

Soil temperatures are important because organic-matter decomposition, and thus carbon storage, and other soil biological processes generally increase with temperature. In one study about 25% of the difference in decomposition rate along an urban-rural gradient was explained by changes in soil temperature. Soil temperature also influences the length of the growing or "decomposing" season. Earthworms play a large role in formation of soil structure and in organic-matter decomposition. Earthworm productivity is influenced by soil temperature. Activity in the soil during the dormant season in temperate climates is important because as the soil warms in spring, microorganism activity increases before root uptake activity, and therefore there is a period of high nutrient release, which can cause high nutrient levels in streams, especially for nitrate. In urban watersheds this affect may be either less pronounced because soils are less likely to freeze during the dormant season, or greater because soil warming happens very early in spring, thus extending the period of microbial activity before root uptake begins.

Urban stream ecosystem structure and function is constrained by heat excess related to shifts in diurnal and seasonal temperature patterns due to the urban heat island effect, the lack of riparian and upland forests, and the intermittent direct transfer of thermal loads from impervious surfaces (roofing, parking lots, roads) via storm-water runoff. Thermal excesses cause direct mortality to aquatic organisms, reduce dissolved oxygen concentrations, interfere with life cycle patterns by increasing growth rates of aquatic organisms, and exacerbate the toxic effects of various water quality constituents. This results in reduced populations of many fauna and flora, lower diversity in biological communities, and excessive growth of benthic nuisance algae, which further lowers minimum oxygen levels and reduces habitat and food for invertebrates and fish. Moreover, high temperatures contribute to the degradation of the quality of a recreational resource that is often especially important for urban residents who do not have ready access to similar resources in rural areas.

Research Objectives:

  1. To measure soil temperatures on a continuous basis at several depths in the active rooting zone of a range of urban soil types (including both disturbed and undisturbed, upland and riparian soils), with varying amounts of on-site and neighborhood tree cover, with varying relationships to nearby buildings, and with varying amounts of neighborhood buildings.
  2. To measure stream temperatures continuously at locations where stream water and urban runoff originates from watersheds with differing amounts of tree cover, impervious and built surfaces, and in reaches with differing shade from tree cover.
  3. To derive semi-empirical models of soil temperature for different soil types as a function of depth in the soil, air temperature, antecedent precipitation, and estimated net all-wave radiation input as influenced by tree cover.
  4. To derive semi-empirical models of stream temperature as a function of estimated stream energy budget, which is largely a function of stream flow rate and the radiation budget of the stream and runoff-producing impervious surfaces. This will require adaptation of methods to sample canopy cover density of riparian vegetation and urban trees.
  5. Relate measured stream temperatures to those levels required for benthic macroinvertebrates and fish of the Gwynns Falls and surrounding region.

Products:

  1. Reports of measurements of soil and stream temperatures in the Baltimore area, particularly in the Gwynns Falls Watershed.
  2. Published models of soil temperature means and extremes as functions of soil type, soil depth, moisture, and urban tree cover; along with evaluations of the models and proposals for required research to improve the initial soil temperature models.
  3. Published models of stream temperature as a function of watershed land use/cover, drainage structure, stream-base and storm-water flow rates, and weather conditions (air temperature regime, cloudiness, precipitation), along with evaluations of the models and proposals for required research to improve the initial stream temperature models.
  4. Applications of soil and stream temperature models to the range of conditions within the Gwynns Falls Watershed of Baltimore County, Maryland and a discussion of the probable impacts and constraints posed by temperature-related problems to aquatic life in the various subwatersheds of the Gwynns Falls.
A request for funding has been submitted to the USDA Forest Service for a pilot study that would develop methods as well as useful preliminary information to serve as the basis for future funding requests from other agencies. This research would benefit from data that are being collected in Baltimore as part of long-term continuous measurements or special surveys supported by BES, NSF, and Forest Service. These include sampling of soil properties in an urban soil survey, land cover information from on-the-ground and remote-sensing surveys, continuous measurements of all relevant climate variables at a reference meteorology station, measurements of solar radiation at three locations, measurements of rainfall at six sites, measurements of latent and sensible heat flux from a tall tower, and continuous stream gauging at nine sites.