MINUHET (Minnesota Urban Heat Export Tool): A software tool for the analysis of stream thermal loading by urban stormwater runoff

Abstract

Urbanization affects the temperature of cold water resources, coldwater streams in particular. In Minnesota such streams are typically found in well-shaded watersheds and receive large groundwater inputs. They are ecologically significant because they support coldwater fisheries and other wildlife that would be unable to survive in warmer streams. The conversion of land from existing agricultural use or natural conditions poses a threat to these streams. Urban expansion usually requires removing crops and trees and replacing them with buildings, parking lots, roads, lawns, and parks. These changes affect shading, heat transfer, and hydrology within the watershed. Currently, there are few tools available to project to what extent stream temperatures are influenced by development in the watershed. This report describes a new simulation tool, MINUHET (Minnesota Urban Heat Export Tool). MINUHET is a tool used to simulate the flow of stormwater surface runoff and its associated heat content through a small watershed for one or several rainfall event of interest. The tool includes hydrologic and thermal models for surface runoff, natural and man-made drainage networks, and stormwater treatment ponds. The main output of MINUHET is a time series of flow rates and temperatures at the outlet of the watershed, to enable prediction of thermal impact on receiving streams. MINUHET is event-based, i.e. it is designed primarily to simulate a single storm event. The MINUHET tool includes a database of observed and/or synthetic storm events that have the potential to produce high thermal loading in receiving streams. Verification of MINUHET has been performed at both the component level and the system level. The surface temperature model was verified for a number of different impervious and pervious land surfaces for continuous, multi-month simulations of wet and dry conditions. The runoff model has been compared to other models, including a more complex kinematic wave model and a commercial runoff model (EPA-SWMM), and to observed runoff time series from a parking lot. The pond model component was used successfully to simulate several months of observed water level and temperature data for a wet detention pond in Woodbury, MN. The MINUHET tool has been applied to a 12 acre residential development in Plymouth, MN that was instrumented in 2005. A common application of MINUHET may be to compare the thermal loading of stormwater runoff from an area of land before and after development, to quantify possible increases in thermal loading due to development. MINUHET includes components for developed and undeveloped land parcels (sub-watersheds), pervious and impervious open channels, storm sewer systems, and stormwater ponds. MINUHET does not include a stream temperature model, so that while MINUHET can be used to simulate the heat loading to a stream, it cannot be used, by itself, to simulate the resulting change in stream temperature.