Runoff Temperature Model for Paved Surfaces

Abstract

Interest in thermal pollution due to storm-water runoff has risen significantly since it was recognized that fish habitat in coldwater streams may deteriorate or even disappear following urban development or logging. The need to project changes in both runoff temperature and volume in response to land use changes has been recognized. Surface runoff hydrographs can be predicted or simulated using a variety of existing models. Few tools exist to predict or simulate the thermograph of that runoff, i.e., the water flow rate and temperature as a function of time. To simulate runoff temperature for small parcels of land of uniform cover such as parking lots, a new hydrothermal runoff model was developed. The runoff portion of the model is semianalytical and spatially integrated. The runoff model is discrete in time, so that it may be used to analyze events with observed rainfall intensity variations at a resolution of 15 min or less. The runoff model closely approximates the simulation results of a one-dimensional kinematic wave model. For runoff temperature simulations, a heat transfer model was linked to the runoff model. The heat transfer model includes heat conduction to the ground and heat exchange with the atmosphere. Weather data at short time resolution (e.g., 15 min) are a model input requirement. To test and illustrate the usefulness of the hydrothermal runoff model, the response of a 24-acre parking lot to three midsummer rainfall events was simulated. The model was found to predict the average runoff temperatures within 0.3–0.5°C of observed values, and total runoff volumes within 1–15%. The root-mean-square error of simulated 2 min runoff temperatures was 1.1°C. The proposed model is to be included in a simulation tool to assess the hydrothermal impact of proposed land development on coldwater streams. Additional features are being added to the hydrothermal model for applications to pervious, vegetated land. Areas targeted for development are divided into small subwatersheds which are simulated individually. The change in total runoff temperature and volume due to development will be obtained by routing of subwatershed outputs.