Estimation of Runoff Temperatures and Heat Export from Different Land and Water Surfaces

Abstract

This report describes work to analyze runoff temperatures and runoff heat export rates for a variety of terrestrial land covers and aquatic surfaces. Surface runoff temperatures and heat export have been simulated for ten terrestrial covers, an unshaded wet detention pond, a lake/reservoir, and a vegetated pond. A continuous simulation was run from April 1 to October 31, yielding a total of about 280 precipitation events for six years (1998-2000, 2003-2005). Six years of 15-minute climate data from the weather station at the MnROAD facility in Albertville, MN, were used as model input. In general, the variation in average runoff temperatures from terrestrial land covers and open water surfaces was moderate, from 24.9 °C for concrete to 21.5 °C for a forest. Pavements, commercial rooftops, bare soil, wet detention ponds, and lakes/reservoirs were all found to give runoff temperatures high enough to significantly impact stream temperature. Vegetated surfaces gave substantially lower runoff temperature and heat export than paved surfaces. Runoff temperatures from bare soils were consistently higher than from vegetated surfaces, but lower than from pavements. Residential roofs gave, on average, low runoff temperatures, due to very low thermal mass, while commercial roofs gave high runoff temperatures in some cases. Large water bodies (lakes and reservoirs) generally give very high runoff temperatures, but the quantity of runoff is highly dependent on the water level prior to the storm event. Analysis of a vegetated pond indicates that shading from emergent vegetation can reduce runoff temperature up to 6°C compared to an unshaded pond.