Modeling hydrothermal inputs to cold-water streams in urban watersheds.

Abstract

This research investigated the impact of urban development on the temperature of cold-water streams, which are crucial to maintaining viable populations of biota that are unable to survive in warmer waters. Since the temperature of these streams is typically maintained by significant amounts of groundwater inflow and riparian shading, the land cover conversion associated with urban development - replacement of crops or natural land with buildings, roads, lawns, and parking lots - has a negative impact, as these land-use changes tend to increase the amount of impervious surface area and reduce the amount of natural shading provided by vegetation. As a result, surface runoff rates and temperatures from rainfall events are amplified, watershed infiltration is reduced, and stream temperature increases. A primary goal of the project was to produce a tool to assess the impact of proposed urban development on stream temperatures in a particular watershed. The research procedure focused primarily on understanding and developing models for the hydrologic and heat transfer processes within a watershed, with particular focus on rainfall-runoff. Specifically, two process-based models were developed: one for estimation of runoff flow and temperature from urban surfaces, and a second for estimation of groundwater input to a stream from observations of water quality. The runoff temperature model demonstrated that heat export by rainfall-runoff from a paved surface is determined by antecedent pavement temperature and rainfall intensity/duration, and that stream-wise gradients in runoff temperature are negligible. The model contributed to the development of a more comprehensive stormwater modeling tool (MINUHET) by justifying the simpler solution technique used by MINUHET's runoff model. MINUHET was shown to accurately simulate runoff flow rate and temperature at the outlet of a small urban watershed, particularly when hydrologic data is available for calibration. The roof surface temperature analysis provided evidence that the contribution of heat from rooftops is negligible relative to that of paved surfaces. Lastly, the use of temperature as a groundwater tracer was shown to be an effective and inexpensive method for determining groundwater input to a stream, provided that the limitations of the approach are borne in mind when applying the method.