The urban heat island (UHI) effect describes built up areas that are hotter than nearby rural areas. Large amounts of paved surfaces in urban areas absorb and retain solar radiation at a much greater rate than natural areas with exposed vegetation or soil. The result is the release of warmer air at the ground level. Another factor that contributes to warmer ground temperatures includes the inability of urban areas to absorb and naturally infiltrate rainwater, which means less moisture in the soil making the evapotranspiration process less intense. These processes are graphically depicted in the image shown below.
As part of our team’s assessment of downtown Sioux City, we created a unique dynamic systems model to predict the urban heat island effect as a result of the high imperviousness of downtown. The model was created in a systems modeling program called STELLA, and can predict local temperatures for any geography with local inputs for land cover units like roads, buildings, grassland, and trees.
Based on our model, we predicted the UHI effect in Sioux City is 4.53°C warmer than nearby rural areas. This prediction does hold true, within about 1 degree Celsius margin of error, when examining historical temperature trends.
Several additional degrees of heat does play a significant role in many factors of everyday life downtown. Some of these effects include hotter air temperatures in urban areas compared to adjacent rural areas, hotter surface temperatures, higher energy use, building and stormwater maintenance costs, sunlight-related health problems and air pollution. In addition, they have an adverse effect on wildlife habitat and plant growth in urban areas.
Our main goal when modeling the UHI effect in Sioux City was not simply to predict the magnitude of the problem, but also to present methods for reducing the additional heat. Our team came up with 4 scenarios, some of which closely resemble the recommendations that will be detailed in the final Downtown Greenspace Plan: (1) increase street trees, (2) convert several public surface parking lots into greenspace, (3) re-paint rooftops in white, light-reflecting paint, and (4) all the previous scenarios combined. After localizing the model with new inputs for each scenario, the resulting UHI magnitudes are displayed below.
As expected, the combined scenario produced the largest reduction in the UHI effect. Both the white-roofs and street trees provided additional reductions as well. The positive UHI change of 0.04 degrees Celsius is negligible. However, this may speak to the need to increase tree coverage on grassland parks in order to provide additional shade and cooling factors. The graph below shows these scenarios played out over the course of one day, with the blue line representing nearby rural temperatures. Here, a clear summer day in June is assumed.
As the results show, significant reductions in the UHI effect in downtown Sioux City will take significant investment in greenscaping as well as other less conventional means like white roofs, green roofs, cool pavement, and other building materials that properly reflect, absorb, and emit the sun’s solar radiation. In the near future, the development of the Pearl Street Park in downtown will certainly become a refuge for escaping the surrounding heat.